Java inner class and static nested class

Question

What is the main difference between an inner class and a static nested class in Java  Does design   implementation play a role in choosing one of these

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The instance of the inner class is created when instance of the outer class is created. Therefore the members and methods of the inner class have access to the members and methods of the instance (object) of the outer class. When the instance of the outer class goes out of scope, also the inner class instances cease to exist.

The static nested class doesn't have a concrete instance. It's just loaded when it's used for the first time (just like the static methods). It's a completely independent entity, whose methods and variables doesn't have any access to the instances of the outer class.

The static nested classes are not coupled with the outer object, they are faster, and they don't take heap/stack memory, because its not necessary to create instance of such class. Therefore the rule of thumb is to try to define static nested class, with as limited scope as possible (private >= class >= protected >= public), and then convert it to inner class (by removing "static" identifier) and loosen the scope, if it's really necessary.

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The following is an example of static nested class and inner class   OuterClass java  public class OuterClass        private String someVariable    Non Static         private static String anotherStaticVariable    Static           OuterClass                    Nested classes are static      static class StaticNestedClass          private static String privateStaticNestedClassVariable    Private Static Nested Class Variable               can access private variables declared in the outer class         public static void getPrivateVariableofOuterClass                System out println anotherStaticVariable                            non static      class InnerClass              can access private variables of outer class          public String getPrivateNonStaticVariableOfOuterClass                 return someVariable                          public static void accessStaticClass               can access any variable declared inside the Static Nested Class             even if it private          String var   OuterClass StaticNestedClass privateStaticNestedClassVariable            System out println var               OuterClassTest   public class OuterClassTest       public static void main String   args               access the Static Nested Class         OuterClass StaticNestedClass getPrivateVariableofOuterClass               test the private variable declared inside the static nested class         OuterClass accessStaticClass                          Inner Class Test                          Declaration            first instantiate the outer class         OuterClass outerClass   new OuterClass               then instantiate the inner class         OuterClass InnerClass innerClassExample    outerClass  new InnerClass               test the non static private variable         System out println innerClassExample getPrivateNonStaticVariableOfOuterClass

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I don t think the real difference became clear in the above answers    First to get the terms right     A nested class is a class which is contained in another class at the source code level  It is static if you declare it with the static modifier  A non-static nested class is called inner class   I stay with non-static nested class     Martin s answer is right so far  However  the actual question is  What is the purpose of declaring a nested class static or not   You use static nested classes if you just want to keep your classes together if they belong topically together or if the nested class is exclusively used in the enclosing class  There is no semantic difference between a static nested class and every other class   Non-static nested classes are a different beast  Similar to anonymous inner classes  such nested classes are actually closures  That means they capture their surrounding scope and their enclosing instance and make that accessible  Perhaps an example will clarify that  See this stub of a Container   public class Container       public class Item          Object data          public Container getContainer                return Container this                    public Item Object data                super                this data   data                        public static Item create Object data              does not compile since no instance of Container is available         return new Item data             public Item createSubItem Object data              compiles  since  this  Container is available         return new Item data             In this case you want to have a reference from a child item to the parent container  Using a non-static nested class  this works without some work  You can access the enclosing instance of Container with the syntax Container this   More hardcore explanations following   If you look at the Java bytecodes the compiler generates for an  non-static  nested class it might become even clearer      class version 49 0  49     access flags 33 public class Container Item         compiled from  Container java      access flags 1   public INNERCLASS Container Item Container Item       access flags 0   Object data       access flags 4112   final Container this 0       access flags 1   public getContainer     Container    L0     LINENUMBER 7 L0     ALOAD 0  this     GETFIELD Container Item this 0   Container     ARETURN    L1     LOCALVARIABLE this Container Item L0 L1 0     MAXSTACK   1     MAXLOCALS   1       access flags 1   public  lt init gt  Container Object    void    L0     LINENUMBER 12 L0     ALOAD 0  this     ALOAD 1     PUTFIELD Container Item this 0   Container    L1     LINENUMBER 10 L1     ALOAD 0  this     INVOKESPECIAL Object  lt init gt      void    L2     LINENUMBER 11 L2     ALOAD 0  this     ALOAD 2  data     PUTFIELD Container Item data   Object     RETURN    L3     LOCALVARIABLE this Container Item L0 L3 0     LOCALVARIABLE data Object L0 L3 2     MAXSTACK   2     MAXLOCALS   3     As you can see the compiler creates a hidden field Container this 0  This is set in the constructor which has an additional parameter of type Container to specify the enclosing instance  You can t see this parameter in the source but the compiler implicitly generates it for a nested class    Martin s example  OuterClass InnerClass innerObject   outerObject new InnerClass      would so be compiled to a call of something like  in bytecodes   new InnerClass outerObject    For the sake of completeness   An anonymous class is a perfect example of a non-static nested class which just has no name associated with it and can t be referenced later

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Nested class is a very general term  every class which is not top level is a nested class  An inner class is a non-static nested class  Joseph Darcy wrote a very nice explanation about Nested  Inner  Member  and Top-Level Classes

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The terms are used interchangeably   If you want to be really pedantic about it  then you could define  nested class  to refer to a static inner class  one which has no enclosing instance   In code  you might have something like this   public class Outer       public class Inner         public static class Nested        That s not really a widely accepted definition though

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I have illustrated various possible correct and error scenario which can occur in java code       class Outter1            String OutStr           Outter1 String str                OutStr   str                     public void NonStaticMethod String st                  String temp1    ashish               final String  tempFinal1    ashish                     below static attribute not permitted                static String tempStatic1    static                        below static with final attribute not permitted                         static final String  tempStatic1    ashish                     synchronized keyword is not permitted below                       class localInnerNonStatic1                                synchronized    public void innerMethod String str11                        str11   temp1    sharma                       System out println  innerMethod     gt    str11                                                      static method with final not permitted           public static void innerStaticMethod String str11                          str11   temp1    india                       System out println  innerMethod     gt    str11                                                     static class not permitted below                 static class localInnerStatic1                                                       public static  void StaticMethod String st                     String temp1    ashish               final String  tempFinal1    ashish                    static attribute not permitted below               static String tempStatic1    static                         static with final attribute not permitted below                static final String  tempStatic1    ashish                                         class localInnerNonStatic1                   public void innerMethod String str11                        str11   temp1    sharma                       System out println  innerMethod     gt    str11                                                static method with final not permitted     public static void innerStaticMethod String str11                          str11   temp1    india                       System out println  innerMethod     gt    str11                                                     static class not permitted below                 static class localInnerStatic1                                  synchronized keyword is not permitted         static  class inner1                          static String  temp1    ashish               String  tempNonStatic    ashish                  class localInner1                public void innerMethod String str11                    str11   temp1    sharma                   str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                              public static void innerStaticMethod String str11                        error in below step                 str11   temp1    india                         str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                                                      synchronized keyword is not permitted below         class innerNonStatic1                   This is important we have to keep final with static modifier in non    static innerclass below             static final String  temp1    ashish                 String  tempNonStatic    ashish                  class localInner1                synchronized    public void innerMethod String str11                    tempNonStatic   tempNonStatic                          str11   temp1    sharma                   str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                                                 error in below step             public static void innerStaticMethod String str11                                       error in below step                                str11   tempNonStatic    india                                                    str11   temp1    india                               System out println  innerMethod     gt    str11

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I think people here should notice to Poster that   Static Nest Class just only the first inner class  For example    public static class A      ERROR   public class A        public class B            public static class C      ERROR             public class A        public static class B      COMPILE           So  summarize  static class doesn t depend which class its contains  So  they cannot in normal class   because normal class need an instance

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I have illustrated various possible correct and error scenario which can occur in java code       class Outter1            String OutStr           Outter1 String str                OutStr   str                     public void NonStaticMethod String st                  String temp1    ashish               final String  tempFinal1    ashish                     below static attribute not permitted                static String tempStatic1    static                        below static with final attribute not permitted                         static final String  tempStatic1    ashish                     synchronized keyword is not permitted below                       class localInnerNonStatic1                                synchronized    public void innerMethod String str11                        str11   temp1    sharma                       System out println  innerMethod     gt    str11                                                      static method with final not permitted           public static void innerStaticMethod String str11                          str11   temp1    india                       System out println  innerMethod     gt    str11                                                     static class not permitted below                 static class localInnerStatic1                                                       public static  void StaticMethod String st                     String temp1    ashish               final String  tempFinal1    ashish                    static attribute not permitted below               static String tempStatic1    static                         static with final attribute not permitted below                static final String  tempStatic1    ashish                                         class localInnerNonStatic1                   public void innerMethod String str11                        str11   temp1    sharma                       System out println  innerMethod     gt    str11                                                static method with final not permitted     public static void innerStaticMethod String str11                          str11   temp1    india                       System out println  innerMethod     gt    str11                                                     static class not permitted below                 static class localInnerStatic1                                  synchronized keyword is not permitted         static  class inner1                          static String  temp1    ashish               String  tempNonStatic    ashish                  class localInner1                public void innerMethod String str11                    str11   temp1    sharma                   str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                              public static void innerStaticMethod String str11                        error in below step                 str11   temp1    india                         str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                                                      synchronized keyword is not permitted below         class innerNonStatic1                   This is important we have to keep final with static modifier in non    static innerclass below             static final String  temp1    ashish                 String  tempNonStatic    ashish                  class localInner1                synchronized    public void innerMethod String str11                    tempNonStatic   tempNonStatic                          str11   temp1    sharma                   str11   str11  tempNonStatic    sharma                   System out println  innerMethod     gt    str11                                                 error in below step             public static void innerStaticMethod String str11                                       error in below step                                str11   tempNonStatic    india                                                    str11   temp1    india                               System out println  innerMethod     gt    str11

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Nested class  class inside class  Types    Static nested class Non-static nested class  Inner class    Difference   Non-static nested class  Inner class   In non-static nested class object of inner class exist within object of outer class  So that data member of outer class is accessible to inner class  So to create object of inner class we must create object of outer class first   outerclass outerobject new outerobject    outerclass innerclass innerobjcet outerobject new innerclass       Static nested class  In static nested class object of inner class don t need object of outer class  because the word  static  indicate no need to create object   class outerclass A       static class nestedclass B           static int x   10            If you want to access x  then write the following inside method    outerclass nestedclass x   i e  System out prinltn  outerclass nestedclass x

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I don t think the real difference became clear in the above answers    First to get the terms right     A nested class is a class which is contained in another class at the source code level  It is static if you declare it with the static modifier  A non-static nested class is called inner class   I stay with non-static nested class     Martin s answer is right so far  However  the actual question is  What is the purpose of declaring a nested class static or not   You use static nested classes if you just want to keep your classes together if they belong topically together or if the nested class is exclusively used in the enclosing class  There is no semantic difference between a static nested class and every other class   Non-static nested classes are a different beast  Similar to anonymous inner classes  such nested classes are actually closures  That means they capture their surrounding scope and their enclosing instance and make that accessible  Perhaps an example will clarify that  See this stub of a Container   public class Container       public class Item          Object data          public Container getContainer                return Container this                    public Item Object data                super                this data   data                        public static Item create Object data              does not compile since no instance of Container is available         return new Item data             public Item createSubItem Object data              compiles  since  this  Container is available         return new Item data             In this case you want to have a reference from a child item to the parent container  Using a non-static nested class  this works without some work  You can access the enclosing instance of Container with the syntax Container this   More hardcore explanations following   If you look at the Java bytecodes the compiler generates for an  non-static  nested class it might become even clearer      class version 49 0  49     access flags 33 public class Container Item         compiled from  Container java      access flags 1   public INNERCLASS Container Item Container Item       access flags 0   Object data       access flags 4112   final Container this 0       access flags 1   public getContainer     Container    L0     LINENUMBER 7 L0     ALOAD 0  this     GETFIELD Container Item this 0   Container     ARETURN    L1     LOCALVARIABLE this Container Item L0 L1 0     MAXSTACK   1     MAXLOCALS   1       access flags 1   public  lt init gt  Container Object    void    L0     LINENUMBER 12 L0     ALOAD 0  this     ALOAD 1     PUTFIELD Container Item this 0   Container    L1     LINENUMBER 10 L1     ALOAD 0  this     INVOKESPECIAL Object  lt init gt      void    L2     LINENUMBER 11 L2     ALOAD 0  this     ALOAD 2  data     PUTFIELD Container Item data   Object     RETURN    L3     LOCALVARIABLE this Container Item L0 L3 0     LOCALVARIABLE data Object L0 L3 2     MAXSTACK   2     MAXLOCALS   3     As you can see the compiler creates a hidden field Container this 0  This is set in the constructor which has an additional parameter of type Container to specify the enclosing instance  You can t see this parameter in the source but the compiler implicitly generates it for a nested class    Martin s example  OuterClass InnerClass innerObject   outerObject new InnerClass      would so be compiled to a call of something like  in bytecodes   new InnerClass outerObject    For the sake of completeness   An anonymous class is a perfect example of a non-static nested class which just has no name associated with it and can t be referenced later

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From the Java Tutorial      Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are simply called static nested classes  Non-static nested classes are called inner classes     Static nested classes are accessed using the enclosing class name   OuterClass StaticNestedClass   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass      Objects that are instances of an inner class exist within an instance of the outer class  Consider the following classes   class OuterClass               class InnerClass                         An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance   To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass outerObject   new OuterClass   OuterClass InnerClass innerObject   outerObject new InnerClass      see  Java Tutorial - Nested Classes  For completeness note that there is also such a thing as an inner class without an enclosing instance   class A     int t     return 1      static A a    new A     int t     return 2           Here  new A           is an inner class defined in a static context and does not have an enclosing instance

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There is a subtlety about the use of nested static classes that might be useful in certain situations   Whereas static attributes get instantiated before the class gets instantiated via its constructor  static attributes inside of nested static classes don t seem to get instantiated until after the class s constructor gets invoked  or at least not until after the attributes are first referenced  even if they are marked as  final    Consider this example   public class C0        static C0 instance   null          Uncomment the following line and a null pointer exception will be        generated before anything gets printed        public static final String outerItem   instance makeString 98 6        public C0             instance   this             public String makeString int i            return   new Integer i   toString                public String makeString double d            return   new Double d   toString                public static final class nested           public static final String innerItem   instance makeString 42              static public void main String   argv            System out println  start               Comment out this line and a null pointer exception will be            generated after  start  prints and before the following            try catch block even gets entered          new C0            try               System out println  retrieve item      nested innerItem                     catch  Exception e                System out println  failed to retrieve item      e toString                       System out println  finish              Even though  nested  and  innerItem  are both declared as  static final   the setting of nested innerItem doesn t take place until after the class is instantiated  or at least not until after the nested static item is first referenced   as you can see for yourself by commenting and uncommenting the lines that I refer to  above  The same does not hold true for  outerItem    At least this is what I m seeing in Java 6 0

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The terms are used interchangeably   If you want to be really pedantic about it  then you could define  nested class  to refer to a static inner class  one which has no enclosing instance   In code  you might have something like this   public class Outer       public class Inner         public static class Nested        That s not really a widely accepted definition though

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Ummm    an inner class IS a nested class    do you mean anonymous class and inner class   Edit   If you actually meant inner vs anonymous    an inner class is just a class defined within a class such as   public class A       public class B             Whereas an anonymous class is an extension of a class defined anonymously  so no actual  class is defined  as in   public class A      A anon   new A        you could change behavior of A here         Further Edit   Wikipedia claims there is a difference in Java  but I ve been working with Java for 8 years  and it s the first I heard such a distinction    not to mention there are no references there to back up the claim    bottom line  an inner class is a class defined within a class  static or not   and nested is just another term to mean the same thing   There is a subtle difference between static and non-static nested class    basically non-static inner classes have implicit access to instance fields and methods of the enclosing class  thus they cannot be constructed in a static context  it will be a compiler error    Static nested classes  on the other hand  don t have implicit access to instance fields and methods  and CAN be constructed in a static context

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I don t think the real difference became clear in the above answers    First to get the terms right     A nested class is a class which is contained in another class at the source code level  It is static if you declare it with the static modifier  A non-static nested class is called inner class   I stay with non-static nested class     Martin s answer is right so far  However  the actual question is  What is the purpose of declaring a nested class static or not   You use static nested classes if you just want to keep your classes together if they belong topically together or if the nested class is exclusively used in the enclosing class  There is no semantic difference between a static nested class and every other class   Non-static nested classes are a different beast  Similar to anonymous inner classes  such nested classes are actually closures  That means they capture their surrounding scope and their enclosing instance and make that accessible  Perhaps an example will clarify that  See this stub of a Container   public class Container       public class Item          Object data          public Container getContainer                return Container this                    public Item Object data                super                this data   data                        public static Item create Object data              does not compile since no instance of Container is available         return new Item data             public Item createSubItem Object data              compiles  since  this  Container is available         return new Item data             In this case you want to have a reference from a child item to the parent container  Using a non-static nested class  this works without some work  You can access the enclosing instance of Container with the syntax Container this   More hardcore explanations following   If you look at the Java bytecodes the compiler generates for an  non-static  nested class it might become even clearer      class version 49 0  49     access flags 33 public class Container Item         compiled from  Container java      access flags 1   public INNERCLASS Container Item Container Item       access flags 0   Object data       access flags 4112   final Container this 0       access flags 1   public getContainer     Container    L0     LINENUMBER 7 L0     ALOAD 0  this     GETFIELD Container Item this 0   Container     ARETURN    L1     LOCALVARIABLE this Container Item L0 L1 0     MAXSTACK   1     MAXLOCALS   1       access flags 1   public  lt init gt  Container Object    void    L0     LINENUMBER 12 L0     ALOAD 0  this     ALOAD 1     PUTFIELD Container Item this 0   Container    L1     LINENUMBER 10 L1     ALOAD 0  this     INVOKESPECIAL Object  lt init gt      void    L2     LINENUMBER 11 L2     ALOAD 0  this     ALOAD 2  data     PUTFIELD Container Item data   Object     RETURN    L3     LOCALVARIABLE this Container Item L0 L3 0     LOCALVARIABLE data Object L0 L3 2     MAXSTACK   2     MAXLOCALS   3     As you can see the compiler creates a hidden field Container this 0  This is set in the constructor which has an additional parameter of type Container to specify the enclosing instance  You can t see this parameter in the source but the compiler implicitly generates it for a nested class    Martin s example  OuterClass InnerClass innerObject   outerObject new InnerClass      would so be compiled to a call of something like  in bytecodes   new InnerClass outerObject    For the sake of completeness   An anonymous class is a perfect example of a non-static nested class which just has no name associated with it and can t be referenced later

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From the Java Tutorial      Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are simply called static nested classes  Non-static nested classes are called inner classes     Static nested classes are accessed using the enclosing class name   OuterClass StaticNestedClass   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass      Objects that are instances of an inner class exist within an instance of the outer class  Consider the following classes   class OuterClass               class InnerClass                         An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance   To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass outerObject   new OuterClass   OuterClass InnerClass innerObject   outerObject new InnerClass      see  Java Tutorial - Nested Classes  For completeness note that there is also such a thing as an inner class without an enclosing instance   class A     int t     return 1      static A a    new A     int t     return 2           Here  new A           is an inner class defined in a static context and does not have an enclosing instance

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First of all There is no such class called Static class The Static modifier use with inner class  called as Nested Class  says that it is a static member of Outer Class which means we can access it as with other static members and without having any instance of Outer class   Which is benefit of static originally     Difference between using Nested class and regular Inner class is   OuterClass InnerClass inner   new OuterClass   new InnerClass      First We can to instantiate Outerclass then we Can access Inner   But if Class is Nested then syntax is   OuterClass InnerClass inner   new OuterClass InnerClass      Which uses the static Syntax as normal implementation of static keyword

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Nested class is a very general term  every class which is not top level is a nested class  An inner class is a non-static nested class  Joseph Darcy wrote a very nice explanation about Nested  Inner  Member  and Top-Level Classes

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I think that none of the above answers explain to you the real difference between a nested class and a static nested class in term of application design     OverView  A nested class could be nonstatic or static and in each case is a class defined within another class  A nested class should exist only to serve is enclosing class  if a nested class is useful by other classes  not only the enclosing   should be declared as a top level class   Difference  Nonstatic Nested class   is implicitly associated with the enclosing instance of the containing class  this means that it is possible to invoke methods and access variables of the enclosing instance  One common use of a nonstatic nested class is to define an Adapter class   Static Nested Class   can t access enclosing class instance and invoke methods on it  so should be used when the nested class doesn t require access to an instance of the enclosing class   A common use of static nested class is to implement a components of the outer object   Conclusion  So the main difference between the two from a design standpoint is   nonstatic nested class can access instance of the container class  while static can t

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The Java tutorial says      Terminology  Nested classes are   divided into two categories  static   and non-static  Nested classes that   are declared static are simply called   static nested classes  Non-static   nested classes are called inner   classes    In common parlance  the terms  nested  and  inner  are used interchangeably by most programmers  but I ll use the correct term  nested class  which covers both inner and static   Classes can be nested ad infinitum  e g  class A can contain class B which contains class C which contains class D  etc  However  more than one level of class nesting is rare  as it is generally bad design   There are three reasons you might create a nested class    organization  sometimes it seems most sensible to sort a class into the namespace of another class  especially when it won t be used in any other context access  nested classes have special access to the variables fields of their containing classes  precisely which variables fields depends on the kind of nested class  whether inner or static   convenience  having to create a new file for every new type is bothersome  again  especially when the type will only be used in one context   There are four kinds of nested class in Java  In brief  they are    static class  declared as a static member of another class inner class  declared as an instance member of another class local inner class  declared inside an instance method of another class anonymous inner class  like a local inner class  but written as an expression which returns a one-off object   Let me elaborate in more details     Static Classes  Static classes are the easiest kind to understand because they have nothing to do with instances of the containing class   A static class is a class declared as a static member of another class  Just like other static members  such a class is really just a hanger on that uses the containing class as its namespace  e g  the class Goat declared as a static member of class Rhino in the package pizza is known by the name pizza Rhino Goat   package pizza   public class Rhino                 public static class Goat                         Frankly  static classes are a pretty worthless feature because classes are already divided into namespaces by packages  The only real conceivable reason to create a static class is that such a class has access to its containing class s private static members  but I find this to be a pretty lame justification for the static class feature to exist     Inner Classes  An inner class is a class declared as a non-static member of another class   package pizza   public class Rhino        public class Goat                          private void jerry             Goat g   new Goat              Like with a static class  the inner class is known as qualified by its containing class name  pizza Rhino Goat  but inside the containing class  it can be known by its simple name  However  every instance of an inner class is tied to a particular instance of its containing class  above  the Goat created in jerry  is implicitly tied to the Rhino instance this in jerry  Otherwise  we make the associated Rhino instance explicit when we instantiate Goat   Rhino rhino   new Rhino    Rhino Goat goat   rhino new Goat       Notice you refer to the inner type as just Goat in the weird new syntax  Java infers the containing type from the rhino part  And  yes new rhino Goat   would have made more sense to me too    So what does this gain us  Well  the inner class instance has access to the instance members of the containing class instance  These enclosing instance members are referred to inside the inner class via just their simple names  not via this  this in the inner class refers to the inner class instance  not the associated containing class instance     public class Rhino        private String barry       public class Goat           public void colin                 System out println barry                       In the inner class  you can refer to this of the containing class as Rhino this  and you can use this to refer to its members  e g  Rhino this barry     Local Inner Classes  A local inner class is a class declared in the body of a method  Such a class is only known within its containing method  so it can only be instantiated and have its members accessed within its containing method  The gain is that a local inner class instance is tied to and can access the final local variables of its containing method  When the instance uses a final local of its containing method  the variable retains the value it held at the time of the instance s creation  even if the variable has gone out of scope  this is effectively Java s crude  limited version of closures    Because a local inner class is neither the member of a class or package  it is not declared with an access level   Be clear  however  that its own members have access levels like in a normal class    If a local inner class is declared in an instance method  an instantiation of the inner class is tied to the instance held by the containing method s this at the time of the instance s creation  and so the containing class s instance members are accessible like in an instance inner class  A local inner class is instantiated simply via its name  e g  local inner class Cat is instantiated as new Cat    not new this Cat   as you might expect     Anonymous Inner Classes  An anonymous inner class is a syntactically convenient way of writing a local inner class  Most commonly  a local inner class is instantiated at most just once each time its containing method is run  It would be nice  then  if we could combine the local inner class definition and its single instantiation into one convenient syntax form  and it would also be nice if we didn t have to think up a name for the class  the fewer unhelpful names your code contains  the better   An anonymous inner class allows both these things   new  ParentClassName   constructorArgs     members     This is an expression returning a new instance of an unnamed class which extends ParentClassName  You cannot supply your own constructor  rather  one is implicitly supplied which simply calls the super constructor  so the arguments supplied must fit the super constructor   If the parent contains multiple constructors  the    simplest    one is called     simplest    as determined by a rather complex set of rules not worth bothering to learn in detail--just pay attention to what NetBeans or Eclipse tell you    Alternatively  you can specify an interface to implement   new  InterfaceName      members     Such a declaration creates a new instance of an unnamed class which extends Object and implements InterfaceName  Again  you cannot supply your own constructor  in this case  Java implicitly supplies a no-arg  do-nothing constructor  so there will never be constructor arguments in this case    Even though you can t give an anonymous inner class a constructor  you can still do any setup you want using an initializer block  a    block placed outside any method    Be clear that an anonymous inner class is simply a less flexible way of creating a local inner class with one instance  If you want a local inner class which implements multiple interfaces or which implements interfaces while extending some class other than Object or which specifies its own constructor  you re stuck creating a regular named local inner class

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The instance of the inner class is created when instance of the outer class is created. Therefore the members and methods of the inner class have access to the members and methods of the instance (object) of the outer class. When the instance of the outer class goes out of scope, also the inner class instances cease to exist.

The static nested class doesn't have a concrete instance. It's just loaded when it's used for the first time (just like the static methods). It's a completely independent entity, whose methods and variables doesn't have any access to the instances of the outer class.

The static nested classes are not coupled with the outer object, they are faster, and they don't take heap/stack memory, because its not necessary to create instance of such class. Therefore the rule of thumb is to try to define static nested class, with as limited scope as possible (private >= class >= protected >= public), and then convert it to inner class (by removing "static" identifier) and loosen the scope, if it's really necessary.

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The Java programming language allows you to define a class within another class  Such a class is called a nested class and is illustrated here   class OuterClass       class NestedClass                     Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are called static nested classes  Non-static nested classes are called inner classes  One thing that we should keep in mind is Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes only have access to other members of the enclosing class if those are static  It can not access non static members of the outer class  As with class methods and variables  a static nested class is associated with its outer class   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject    new OuterClass StaticNestedClass       To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass InnerClass innerObject   new OuterClass   new InnerClass      Why we use nested classes   It is a way of logically grouping classes that are only used in one place  It increases encapsulation  It can lead to more readable and maintainable code    Source  The Java    Tutorials - Nested Classes

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When we declare static member class inside a class  it is known as top level nested class or a static nested class  It can be demonstrated as below     class Test      private static int x   1          static class A          private static int y   2          public static int getZ                return B z x                      static class B          private static int z   3          public static int getY                return A y                     class TestDemo       public static void main String   args           Test t   new Test            System out println Test A getZ             System out println Test B getY               When we declare non-static member class inside a class it is known as inner class  Inner class can be demonstrated as below         class Test          private int i   10          class A              private int i  20              void display                int i   30              System out println i               System out println this i               System out println Test this i

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Ummm    an inner class IS a nested class    do you mean anonymous class and inner class   Edit   If you actually meant inner vs anonymous    an inner class is just a class defined within a class such as   public class A       public class B             Whereas an anonymous class is an extension of a class defined anonymously  so no actual  class is defined  as in   public class A      A anon   new A        you could change behavior of A here         Further Edit   Wikipedia claims there is a difference in Java  but I ve been working with Java for 8 years  and it s the first I heard such a distinction    not to mention there are no references there to back up the claim    bottom line  an inner class is a class defined within a class  static or not   and nested is just another term to mean the same thing   There is a subtle difference between static and non-static nested class    basically non-static inner classes have implicit access to instance fields and methods of the enclosing class  thus they cannot be constructed in a static context  it will be a compiler error    Static nested classes  on the other hand  don t have implicit access to instance fields and methods  and CAN be constructed in a static context

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Ummm    an inner class IS a nested class    do you mean anonymous class and inner class   Edit   If you actually meant inner vs anonymous    an inner class is just a class defined within a class such as   public class A       public class B             Whereas an anonymous class is an extension of a class defined anonymously  so no actual  class is defined  as in   public class A      A anon   new A        you could change behavior of A here         Further Edit   Wikipedia claims there is a difference in Java  but I ve been working with Java for 8 years  and it s the first I heard such a distinction    not to mention there are no references there to back up the claim    bottom line  an inner class is a class defined within a class  static or not   and nested is just another term to mean the same thing   There is a subtle difference between static and non-static nested class    basically non-static inner classes have implicit access to instance fields and methods of the enclosing class  thus they cannot be constructed in a static context  it will be a compiler error    Static nested classes  on the other hand  don t have implicit access to instance fields and methods  and CAN be constructed in a static context

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In simple terms we need nested classes primarily because Java does not provide closures   Nested Classes are classes defined inside the body of another enclosing class  They are of two types - static and non-static   They are treated as members of the enclosing class  hence you can specify any of the four access specifiers - private  package  protected  public  We don t have this luxury with top-level classes  which can only be declared public or package-private   Inner classes aka Non-stack classes have access to other members of the top class  even if they are declared private while Static nested classes do not have access to other members of the top class   public class OuterClass       public static class Inner1             public class Inner2             Inner1 is our static inner class and Inner2 is our inner class which is not static  The key difference between them  you can t create an Inner2 instance without an Outer where as you can create an Inner1 object independently   When would you use Inner class   Think of a situation where Class A and Class B are related  Class B needs to access Class A members  and Class B is related only to Class A  Inner classes comes into the picture   For creating an instance of inner class  you need to create an instance of your outer class   OuterClass outer   new OuterClass    OuterClass Inner2 inner   outer new Inner2      or  OuterClass Inner2 inner   new OuterClass   new Inner2      When would you use static Inner class   You would define a static inner class when you know that it does not have any relationship with the instance of the enclosing class top class  If your inner class doesn t use methods or fields of the outer class  it s just a waste of space  so make it static   For example  to create an object for the static nested class  use this syntax   OuterClass Inner1 nestedObject   new OuterClass Inner1      The advantage of a static nested class is that it doesn t need an object of the containing class top class to work  This can help you to reduce the number of objects your application creates at runtime

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I don t think there is much to add here  most of the answers perfectly explain the differences between static nested class and Inner classes  However  consider the following issue when using nested classes vs inner classes   As mention in a couple of answers inner classes can not be instantiated without and instance of their enclosing class which mean that they HOLD a pointer to the instance of their enclosing class which can lead to memory overflow or stack overflow exception due to the fact the GC will not be able to garbage collect the enclosing classes even if they are not used any more  To make this clear check the following code out    public class Outer         public  class Inner                public Inner inner            return new Inner                Override     protected void finalize   throws Throwable          as you know finalize is called by the garbage collector due to destroying an object instance         System out println  I am destroyed                public static void main String arg           Outer outer   new Outer        Outer Inner inner   outer new Inner            out instance is no more used and should be garbage collected            However this will not happen as inner instance is still alive i e used  not null          and outer will be kept in memory until inner is destroyed     outer   null                 inner   null         kick out garbage collector     System gc         If you remove the comment on    inner   null  The program will out put   I am destroyed     but keeping this commented it will not  The reason is that white inner instance is still referenced GC cannot collect it and because it references  has a pointer to  the outer instance it is not collected too  Having enough of these objects in your project and can run out of memory  Compared to static inner classes which does not hold a point to inner class instance because it is not instance related but class related   The above program can print  I am destroyed    if you make Inner class static and instantiated with Outer Inner i   new Outer Inner

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The Java programming language allows you to define a class within another class  Such a class is called a nested class and is illustrated here   class OuterClass       class NestedClass                     Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are called static nested classes  Non-static nested classes are called inner classes  One thing that we should keep in mind is Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes only have access to other members of the enclosing class if those are static  It can not access non static members of the outer class  As with class methods and variables  a static nested class is associated with its outer class   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject    new OuterClass StaticNestedClass       To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass InnerClass innerObject   new OuterClass   new InnerClass      Why we use nested classes   It is a way of logically grouping classes that are only used in one place  It increases encapsulation  It can lead to more readable and maintainable code    Source  The Java    Tutorials - Nested Classes

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The instance of the inner class is created when instance of the outer class is created. Therefore the members and methods of the inner class have access to the members and methods of the instance (object) of the outer class. When the instance of the outer class goes out of scope, also the inner class instances cease to exist.

The static nested class doesn't have a concrete instance. It's just loaded when it's used for the first time (just like the static methods). It's a completely independent entity, whose methods and variables doesn't have any access to the instances of the outer class.

The static nested classes are not coupled with the outer object, they are faster, and they don't take heap/stack memory, because its not necessary to create instance of such class. Therefore the rule of thumb is to try to define static nested class, with as limited scope as possible (private >= class >= protected >= public), and then convert it to inner class (by removing "static" identifier) and loosen the scope, if it's really necessary.

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I don t think the real difference became clear in the above answers    First to get the terms right     A nested class is a class which is contained in another class at the source code level  It is static if you declare it with the static modifier  A non-static nested class is called inner class   I stay with non-static nested class     Martin s answer is right so far  However  the actual question is  What is the purpose of declaring a nested class static or not   You use static nested classes if you just want to keep your classes together if they belong topically together or if the nested class is exclusively used in the enclosing class  There is no semantic difference between a static nested class and every other class   Non-static nested classes are a different beast  Similar to anonymous inner classes  such nested classes are actually closures  That means they capture their surrounding scope and their enclosing instance and make that accessible  Perhaps an example will clarify that  See this stub of a Container   public class Container       public class Item          Object data          public Container getContainer                return Container this                    public Item Object data                super                this data   data                        public static Item create Object data              does not compile since no instance of Container is available         return new Item data             public Item createSubItem Object data              compiles  since  this  Container is available         return new Item data             In this case you want to have a reference from a child item to the parent container  Using a non-static nested class  this works without some work  You can access the enclosing instance of Container with the syntax Container this   More hardcore explanations following   If you look at the Java bytecodes the compiler generates for an  non-static  nested class it might become even clearer      class version 49 0  49     access flags 33 public class Container Item         compiled from  Container java      access flags 1   public INNERCLASS Container Item Container Item       access flags 0   Object data       access flags 4112   final Container this 0       access flags 1   public getContainer     Container    L0     LINENUMBER 7 L0     ALOAD 0  this     GETFIELD Container Item this 0   Container     ARETURN    L1     LOCALVARIABLE this Container Item L0 L1 0     MAXSTACK   1     MAXLOCALS   1       access flags 1   public  lt init gt  Container Object    void    L0     LINENUMBER 12 L0     ALOAD 0  this     ALOAD 1     PUTFIELD Container Item this 0   Container    L1     LINENUMBER 10 L1     ALOAD 0  this     INVOKESPECIAL Object  lt init gt      void    L2     LINENUMBER 11 L2     ALOAD 0  this     ALOAD 2  data     PUTFIELD Container Item data   Object     RETURN    L3     LOCALVARIABLE this Container Item L0 L3 0     LOCALVARIABLE data Object L0 L3 2     MAXSTACK   2     MAXLOCALS   3     As you can see the compiler creates a hidden field Container this 0  This is set in the constructor which has an additional parameter of type Container to specify the enclosing instance  You can t see this parameter in the source but the compiler implicitly generates it for a nested class    Martin s example  OuterClass InnerClass innerObject   outerObject new InnerClass      would so be compiled to a call of something like  in bytecodes   new InnerClass outerObject    For the sake of completeness   An anonymous class is a perfect example of a non-static nested class which just has no name associated with it and can t be referenced later

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Here is key differences and similarities between Java inner class and static nested class   Hope it helps   Inner class   Can access to outer class both instance and static methods and fields Associated with instance of enclosing class so to instantiate it first needs an instance of outer class  note new keyword place    Outerclass InnerClass innerObject   outerObject new Innerclass     Cannot define any static members itself Cannot have Class or Interface declaration   Static nested class   Cannot access outer class instance methods or fields Not associated with any instance of enclosing class So to instantiate it   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass       Similarities   Both Inner classes can access even private fields and methods of outer class Also the Outer class have access to private fields and methods of inner classes Both classes can have private  protected or public access modifier   Why Use Nested Classes   According to Oracle documentation there re several reasons  full documentation          It is a way of logically grouping classes that are only used in one place  If a class is useful to only one other class  then it is logical to embed it in that class and keep the two together  Nesting such  helper classes  makes their package more streamlined    It increases encapsulation  Consider two top-level classes  A and B  where B needs access to members of A that would otherwise be declared private  By hiding class B within class A  A s members can be declared private and B can access them  In addition  B itself can be hidden from the outside world    It can lead to more readable and maintainable code  Nesting small classes within top-level classes places the code closer to where it is used

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I think people here should notice to Poster that   Static Nest Class just only the first inner class  For example    public static class A      ERROR   public class A        public class B            public static class C      ERROR             public class A        public static class B      COMPILE           So  summarize  static class doesn t depend which class its contains  So  they cannot in normal class   because normal class need an instance

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In simple terms we need nested classes primarily because Java does not provide closures   Nested Classes are classes defined inside the body of another enclosing class  They are of two types - static and non-static   They are treated as members of the enclosing class  hence you can specify any of the four access specifiers - private  package  protected  public  We don t have this luxury with top-level classes  which can only be declared public or package-private   Inner classes aka Non-stack classes have access to other members of the top class  even if they are declared private while Static nested classes do not have access to other members of the top class   public class OuterClass       public static class Inner1             public class Inner2             Inner1 is our static inner class and Inner2 is our inner class which is not static  The key difference between them  you can t create an Inner2 instance without an Outer where as you can create an Inner1 object independently   When would you use Inner class   Think of a situation where Class A and Class B are related  Class B needs to access Class A members  and Class B is related only to Class A  Inner classes comes into the picture   For creating an instance of inner class  you need to create an instance of your outer class   OuterClass outer   new OuterClass    OuterClass Inner2 inner   outer new Inner2      or  OuterClass Inner2 inner   new OuterClass   new Inner2      When would you use static Inner class   You would define a static inner class when you know that it does not have any relationship with the instance of the enclosing class top class  If your inner class doesn t use methods or fields of the outer class  it s just a waste of space  so make it static   For example  to create an object for the static nested class  use this syntax   OuterClass Inner1 nestedObject   new OuterClass Inner1      The advantage of a static nested class is that it doesn t need an object of the containing class top class to work  This can help you to reduce the number of objects your application creates at runtime

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Ummm    an inner class IS a nested class    do you mean anonymous class and inner class   Edit   If you actually meant inner vs anonymous    an inner class is just a class defined within a class such as   public class A       public class B             Whereas an anonymous class is an extension of a class defined anonymously  so no actual  class is defined  as in   public class A      A anon   new A        you could change behavior of A here         Further Edit   Wikipedia claims there is a difference in Java  but I ve been working with Java for 8 years  and it s the first I heard such a distinction    not to mention there are no references there to back up the claim    bottom line  an inner class is a class defined within a class  static or not   and nested is just another term to mean the same thing   There is a subtle difference between static and non-static nested class    basically non-static inner classes have implicit access to instance fields and methods of the enclosing class  thus they cannot be constructed in a static context  it will be a compiler error    Static nested classes  on the other hand  don t have implicit access to instance fields and methods  and CAN be constructed in a static context

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In the case of creating instance  the instance of non  static inner class is created with the reference of object of outer class in which it is defined  This means it have inclosing instance  But the instance of static inner class is created with the reference of Outer class  not with the reference of object of outer class  This means it have not inclosing instance   For example   class A     class B            static int x  not allowed here                static class C         static int x     allowed here        class Test     public static void main String    str          A o new A        A B obj1  o new B     need of inclosing instance      A C obj2  new A C            not need of reference of object of outer class

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The following is an example of static nested class and inner class   OuterClass java  public class OuterClass        private String someVariable    Non Static         private static String anotherStaticVariable    Static           OuterClass                    Nested classes are static      static class StaticNestedClass          private static String privateStaticNestedClassVariable    Private Static Nested Class Variable               can access private variables declared in the outer class         public static void getPrivateVariableofOuterClass                System out println anotherStaticVariable                            non static      class InnerClass              can access private variables of outer class          public String getPrivateNonStaticVariableOfOuterClass                 return someVariable                          public static void accessStaticClass               can access any variable declared inside the Static Nested Class             even if it private          String var   OuterClass StaticNestedClass privateStaticNestedClassVariable            System out println var               OuterClassTest   public class OuterClassTest       public static void main String   args               access the Static Nested Class         OuterClass StaticNestedClass getPrivateVariableofOuterClass               test the private variable declared inside the static nested class         OuterClass accessStaticClass                          Inner Class Test                          Declaration            first instantiate the outer class         OuterClass outerClass   new OuterClass               then instantiate the inner class         OuterClass InnerClass innerClassExample    outerClass  new InnerClass               test the non static private variable         System out println innerClassExample getPrivateNonStaticVariableOfOuterClass

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Targeting learner  who are novice to Java and or Nested Classes    Nested classes can be either   1  Static Nested classes   2  Non Static Nested classes   also known as Inner classes    Please remember this   1 Inner classes Example   class OuterClass        some code here           class InnerClass           some code here          Inner classes are subsets of nested classes    inner class is a specific type of nested class inner classes are subsets of nested classes You can say that an inner class is also a nested class  but you can NOT say that a nested class is also an inner class      Specialty of Inner class    instance of an inner class has access to all of the members of the outer class  even those that are marked    private       2 Static Nested Classes  Example   class EnclosingClass     static class Nested       void someMethod     System out println  hello SO              Case 1 Instantiating a static nested class from a non-enclosing class  class NonEnclosingClass      public static void main String   args          instantiate the Nested class that is a static       member of the EnclosingClass class              EnclosingClass Nested n   new EnclosingClass Nested         n someMethod       prints out  hello          Case 2 Instantiating a static nested class from an enclosing class  class EnclosingClass      static class Nested       void anotherMethod     System out println  hi again              public static void main String   args          access enclosed class       Nested n   new Nested         n anotherMethod       prints out  hi again           Specialty of Static classes    Static inner class would only have access to the static members of the outer class  and have no access to non-static members    Conclusion  Question  What is the main difference between a inner class and a static nested class in Java  Answer  just go through specifics of each class mentioned above

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In the case of creating instance  the instance of non  static inner class is created with the reference of object of outer class in which it is defined  This means it have inclosing instance  But the instance of static inner class is created with the reference of Outer class  not with the reference of object of outer class  This means it have not inclosing instance   For example   class A     class B            static int x  not allowed here                static class C         static int x     allowed here        class Test     public static void main String    str          A o new A        A B obj1  o new B     need of inclosing instance      A C obj2  new A C            not need of reference of object of outer class

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I think that none of the above answers give the real example to you the difference between a nested class and a static nested class in term of application design  And the main difference between static nested class and inner class is the ability to access the outer class instance field    Let us take a look at the two following examples   Static nest class  An good example of using static nested classes is builder pattern  https   dzone com articles design-patterns-the-builder-pattern    For BankAccount we use a static nested class  mainly because    Static nest class instance could be created before the outer class  In the builder pattern  the builder is a helper class which is used to create the BankAccount  BankAccount Builder is only associated with BankAccount  No other classes are related to BankAccount Builder  so it is better to organize them together without using name convention    public class BankAccount        private long accountNumber      private String owner               public static class Builder        private long accountNumber      private String owner               static public Builder long accountNumber            this accountNumber   accountNumber             public Builder withOwner String owner           this owner   owner          return this                      public BankAccount build                BankAccount account   new BankAccount                 account accountNumber   this accountNumber              account owner   this owner                              return account                       Inner class  A common use of inner classes is to define an event handler  https   docs oracle com javase tutorial uiswing events generalrules html  For MyClass  we use the inner class  mainly because     Inner class MyAdapter need to access the outer class member  In the example  MyAdapter is only associated with MyClass  No other classes are related to MyAdapter  so it is better to organize them together without using a name convention   public class MyClass extends Applet                   someObject addMouseListener new MyAdapter                 class MyAdapter extends MouseAdapter           public void mouseClicked MouseEvent e                      Event listener implementation goes here                      change some outer class instance property depend on the event

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I think  the convention that is generally followed is this    static class within a top level class is a nested class non static class within a top level  class is a inner class  which further has two more form   local class - named classes declared inside of a block like a method or constructor body anonymous class - unnamed classes whose instances are created in expressions and statements    However  few other points to remembers are    Top level classes and static nested class are semantically same except that in case of static nested class it can make static reference to private static fields methods of its Outer  parent  class and vice versa  Inner classes have access to instance variables of the enclosing instance of the Outer  parent  class  However  not all inner classes have enclosing instances  for example inner classes in static contexts  like an anonymous class used in a static initializer block  do not  Anonymous class by default extends the parent class or implements the parent interface and there is no further clause to extend any other class or implement any more interfaces  So    new YourClass            means   class  Anonymous  extends YourClass    new YourInterface        means   class  Anonymous  implements YourInterface         I feel that the bigger question that remains open which one to use and when  Well that mostly depends on what scenario you are dealing with but reading the reply given by  jrudolph may help you making some decision

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From the Java Tutorial      Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are simply called static nested classes  Non-static nested classes are called inner classes     Static nested classes are accessed using the enclosing class name   OuterClass StaticNestedClass   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass      Objects that are instances of an inner class exist within an instance of the outer class  Consider the following classes   class OuterClass               class InnerClass                         An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance   To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass outerObject   new OuterClass   OuterClass InnerClass innerObject   outerObject new InnerClass      see  Java Tutorial - Nested Classes  For completeness note that there is also such a thing as an inner class without an enclosing instance   class A     int t     return 1      static A a    new A     int t     return 2           Here  new A           is an inner class defined in a static context and does not have an enclosing instance

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A diagram    The main difference between static nested and non-static nested classes is that static nested does not have an access to non-static outer class members

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I don t think there is much to add here  most of the answers perfectly explain the differences between static nested class and Inner classes  However  consider the following issue when using nested classes vs inner classes   As mention in a couple of answers inner classes can not be instantiated without and instance of their enclosing class which mean that they HOLD a pointer to the instance of their enclosing class which can lead to memory overflow or stack overflow exception due to the fact the GC will not be able to garbage collect the enclosing classes even if they are not used any more  To make this clear check the following code out    public class Outer         public  class Inner                public Inner inner            return new Inner                Override     protected void finalize   throws Throwable          as you know finalize is called by the garbage collector due to destroying an object instance         System out println  I am destroyed                public static void main String arg           Outer outer   new Outer        Outer Inner inner   outer new Inner            out instance is no more used and should be garbage collected            However this will not happen as inner instance is still alive i e used  not null          and outer will be kept in memory until inner is destroyed     outer   null                 inner   null         kick out garbage collector     System gc         If you remove the comment on    inner   null  The program will out put   I am destroyed     but keeping this commented it will not  The reason is that white inner instance is still referenced GC cannot collect it and because it references  has a pointer to  the outer instance it is not collected too  Having enough of these objects in your project and can run out of memory  Compared to static inner classes which does not hold a point to inner class instance because it is not instance related but class related   The above program can print  I am destroyed    if you make Inner class static and instantiated with Outer Inner i   new Outer Inner

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Inner class and nested static class in Java both are classes declared inside another class  known as top level class in Java  In Java terminology  If you declare a nested class static  it will called nested static class in Java while non static nested class are simply referred as Inner Class    What is Inner Class in Java   Any class which is not a top level or declared inside another class is known as nested class and out of those nested classes  class which are declared non static are known as Inner class in Java  there are three kinds of Inner class in Java   1  Local inner class    - is declared inside a code block or method  2  Anonymous inner class - is a class which doesn t have name to reference and initialized at same place where it gets created  3  Member inner class - is declared as non static member of outer class   public class InnerClassTest       public static void main String args                      creating local inner class inside method i e  main            class Local               public void name                     System out println  Example of Local class in Java                                             creating instance of local inner class         Local local   new Local            local name      calling method from local inner class            Creating anonymous inner class in Java for implementing thread         Thread anonymous   new Thread                 Override             public void run                    System out println  Anonymous class example in java                                     anonymous start               example of creating instance of inner class         InnerClassTest test   new InnerClassTest            InnerClassTest Inner inner   test new Inner            inner name      calling method of inner class               Creating Inner class in Java     private class Inner          public void name                System out println  Inner class example in java                        What is nested static class in Java   Nested static class is another class which is declared inside a class as member and made static  Nested static class is also declared as member of outer class and can be make private  public or protected like any other member  One of the main benefit of nested static class over inner class is that instance of nested static class is not attached to any enclosing instance of Outer class  You also don t need any instance of Outer class to create instance of nested static class in Java   1  It can access static data members of outer class including private  2  Static nested class cannot access non-static  instance  data member or method   public class NestedStaticExample       public static void main String args               StaticNested nested   new StaticNested            nested name                  static nested class in java     private static class StaticNested          public void name                System out println  static nested class example in java                        Ref  Inner class and nested Static Class in Java with Example

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Here is key differences and similarities between Java inner class and static nested class   Hope it helps   Inner class   Can access to outer class both instance and static methods and fields Associated with instance of enclosing class so to instantiate it first needs an instance of outer class  note new keyword place    Outerclass InnerClass innerObject   outerObject new Innerclass     Cannot define any static members itself Cannot have Class or Interface declaration   Static nested class   Cannot access outer class instance methods or fields Not associated with any instance of enclosing class So to instantiate it   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass       Similarities   Both Inner classes can access even private fields and methods of outer class Also the Outer class have access to private fields and methods of inner classes Both classes can have private  protected or public access modifier   Why Use Nested Classes   According to Oracle documentation there re several reasons  full documentation          It is a way of logically grouping classes that are only used in one place  If a class is useful to only one other class  then it is logical to embed it in that class and keep the two together  Nesting such  helper classes  makes their package more streamlined    It increases encapsulation  Consider two top-level classes  A and B  where B needs access to members of A that would otherwise be declared private  By hiding class B within class A  A s members can be declared private and B can access them  In addition  B itself can be hidden from the outside world    It can lead to more readable and maintainable code  Nesting small classes within top-level classes places the code closer to where it is used

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The difference is that a nested class declaration that is also static can be instantiated outside of the enclosing class   When you have a nested class declaration that is not static  also known as an inner class  Java won t let you instantiate it except via the enclosing class  The object created out of the inner class is linked to the object created from the outer class  so the inner class can reference the fields of the outer   But if it s static  then the link does not exist  the outer fields cannot be accessed  except via an ordinary reference like any other object  and you can therefore instantiate the nested class by itself

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I think that none of the above answers give the real example to you the difference between a nested class and a static nested class in term of application design  And the main difference between static nested class and inner class is the ability to access the outer class instance field    Let us take a look at the two following examples   Static nest class  An good example of using static nested classes is builder pattern  https   dzone com articles design-patterns-the-builder-pattern    For BankAccount we use a static nested class  mainly because    Static nest class instance could be created before the outer class  In the builder pattern  the builder is a helper class which is used to create the BankAccount  BankAccount Builder is only associated with BankAccount  No other classes are related to BankAccount Builder  so it is better to organize them together without using name convention    public class BankAccount        private long accountNumber      private String owner               public static class Builder        private long accountNumber      private String owner               static public Builder long accountNumber            this accountNumber   accountNumber             public Builder withOwner String owner           this owner   owner          return this                      public BankAccount build                BankAccount account   new BankAccount                 account accountNumber   this accountNumber              account owner   this owner                              return account                       Inner class  A common use of inner classes is to define an event handler  https   docs oracle com javase tutorial uiswing events generalrules html  For MyClass  we use the inner class  mainly because     Inner class MyAdapter need to access the outer class member  In the example  MyAdapter is only associated with MyClass  No other classes are related to MyAdapter  so it is better to organize them together without using a name convention   public class MyClass extends Applet                   someObject addMouseListener new MyAdapter                 class MyAdapter extends MouseAdapter           public void mouseClicked MouseEvent e                      Event listener implementation goes here                      change some outer class instance property depend on the event

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From the Java Tutorial      Nested classes are divided into two categories  static and non-static  Nested classes that are declared static are simply called static nested classes  Non-static nested classes are called inner classes     Static nested classes are accessed using the enclosing class name   OuterClass StaticNestedClass   For example  to create an object for the static nested class  use this syntax   OuterClass StaticNestedClass nestedObject   new OuterClass StaticNestedClass      Objects that are instances of an inner class exist within an instance of the outer class  Consider the following classes   class OuterClass               class InnerClass                         An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance   To instantiate an inner class  you must first instantiate the outer class  Then  create the inner object within the outer object with this syntax   OuterClass outerObject   new OuterClass   OuterClass InnerClass innerObject   outerObject new InnerClass      see  Java Tutorial - Nested Classes  For completeness note that there is also such a thing as an inner class without an enclosing instance   class A     int t     return 1      static A a    new A     int t     return 2           Here  new A           is an inner class defined in a static context and does not have an enclosing instance

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Targeting learner  who are novice to Java and or Nested Classes    Nested classes can be either   1  Static Nested classes   2  Non Static Nested classes   also known as Inner classes    Please remember this   1 Inner classes Example   class OuterClass        some code here           class InnerClass           some code here          Inner classes are subsets of nested classes    inner class is a specific type of nested class inner classes are subsets of nested classes You can say that an inner class is also a nested class  but you can NOT say that a nested class is also an inner class      Specialty of Inner class    instance of an inner class has access to all of the members of the outer class  even those that are marked    private       2 Static Nested Classes  Example   class EnclosingClass     static class Nested       void someMethod     System out println  hello SO              Case 1 Instantiating a static nested class from a non-enclosing class  class NonEnclosingClass      public static void main String   args          instantiate the Nested class that is a static       member of the EnclosingClass class              EnclosingClass Nested n   new EnclosingClass Nested         n someMethod       prints out  hello          Case 2 Instantiating a static nested class from an enclosing class  class EnclosingClass      static class Nested       void anotherMethod     System out println  hi again              public static void main String   args          access enclosed class       Nested n   new Nested         n anotherMethod       prints out  hi again           Specialty of Static classes    Static inner class would only have access to the static members of the outer class  and have no access to non-static members    Conclusion  Question  What is the main difference between a inner class and a static nested class in Java  Answer  just go through specifics of each class mentioned above

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The terms are used interchangeably   If you want to be really pedantic about it  then you could define  nested class  to refer to a static inner class  one which has no enclosing instance   In code  you might have something like this   public class Outer       public class Inner         public static class Nested        That s not really a widely accepted definition though

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First of all There is no such class called Static class The Static modifier use with inner class  called as Nested Class  says that it is a static member of Outer Class which means we can access it as with other static members and without having any instance of Outer class   Which is benefit of static originally     Difference between using Nested class and regular Inner class is   OuterClass InnerClass inner   new OuterClass   new InnerClass      First We can to instantiate Outerclass then we Can access Inner   But if Class is Nested then syntax is   OuterClass InnerClass inner   new OuterClass InnerClass      Which uses the static Syntax as normal implementation of static keyword

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The Java tutorial says      Terminology  Nested classes are   divided into two categories  static   and non-static  Nested classes that   are declared static are simply called   static nested classes  Non-static   nested classes are called inner   classes    In common parlance  the terms  nested  and  inner  are used interchangeably by most programmers  but I ll use the correct term  nested class  which covers both inner and static   Classes can be nested ad infinitum  e g  class A can contain class B which contains class C which contains class D  etc  However  more than one level of class nesting is rare  as it is generally bad design   There are three reasons you might create a nested class    organization  sometimes it seems most sensible to sort a class into the namespace of another class  especially when it won t be used in any other context access  nested classes have special access to the variables fields of their containing classes  precisely which variables fields depends on the kind of nested class  whether inner or static   convenience  having to create a new file for every new type is bothersome  again  especially when the type will only be used in one context   There are four kinds of nested class in Java  In brief  they are    static class  declared as a static member of another class inner class  declared as an instance member of another class local inner class  declared inside an instance method of another class anonymous inner class  like a local inner class  but written as an expression which returns a one-off object   Let me elaborate in more details     Static Classes  Static classes are the easiest kind to understand because they have nothing to do with instances of the containing class   A static class is a class declared as a static member of another class  Just like other static members  such a class is really just a hanger on that uses the containing class as its namespace  e g  the class Goat declared as a static member of class Rhino in the package pizza is known by the name pizza Rhino Goat   package pizza   public class Rhino                 public static class Goat                         Frankly  static classes are a pretty worthless feature because classes are already divided into namespaces by packages  The only real conceivable reason to create a static class is that such a class has access to its containing class s private static members  but I find this to be a pretty lame justification for the static class feature to exist     Inner Classes  An inner class is a class declared as a non-static member of another class   package pizza   public class Rhino        public class Goat                          private void jerry             Goat g   new Goat              Like with a static class  the inner class is known as qualified by its containing class name  pizza Rhino Goat  but inside the containing class  it can be known by its simple name  However  every instance of an inner class is tied to a particular instance of its containing class  above  the Goat created in jerry  is implicitly tied to the Rhino instance this in jerry  Otherwise  we make the associated Rhino instance explicit when we instantiate Goat   Rhino rhino   new Rhino    Rhino Goat goat   rhino new Goat       Notice you refer to the inner type as just Goat in the weird new syntax  Java infers the containing type from the rhino part  And  yes new rhino Goat   would have made more sense to me too    So what does this gain us  Well  the inner class instance has access to the instance members of the containing class instance  These enclosing instance members are referred to inside the inner class via just their simple names  not via this  this in the inner class refers to the inner class instance  not the associated containing class instance     public class Rhino        private String barry       public class Goat           public void colin                 System out println barry                       In the inner class  you can refer to this of the containing class as Rhino this  and you can use this to refer to its members  e g  Rhino this barry     Local Inner Classes  A local inner class is a class declared in the body of a method  Such a class is only known within its containing method  so it can only be instantiated and have its members accessed within its containing method  The gain is that a local inner class instance is tied to and can access the final local variables of its containing method  When the instance uses a final local of its containing method  the variable retains the value it held at the time of the instance s creation  even if the variable has gone out of scope  this is effectively Java s crude  limited version of closures    Because a local inner class is neither the member of a class or package  it is not declared with an access level   Be clear  however  that its own members have access levels like in a normal class    If a local inner class is declared in an instance method  an instantiation of the inner class is tied to the instance held by the containing method s this at the time of the instance s creation  and so the containing class s instance members are accessible like in an instance inner class  A local inner class is instantiated simply via its name  e g  local inner class Cat is instantiated as new Cat    not new this Cat   as you might expect     Anonymous Inner Classes  An anonymous inner class is a syntactically convenient way of writing a local inner class  Most commonly  a local inner class is instantiated at most just once each time its containing method is run  It would be nice  then  if we could combine the local inner class definition and its single instantiation into one convenient syntax form  and it would also be nice if we didn t have to think up a name for the class  the fewer unhelpful names your code contains  the better   An anonymous inner class allows both these things   new  ParentClassName   constructorArgs     members     This is an expression returning a new instance of an unnamed class which extends ParentClassName  You cannot supply your own constructor  rather  one is implicitly supplied which simply calls the super constructor  so the arguments supplied must fit the super constructor   If the parent contains multiple constructors  the    simplest    one is called     simplest    as determined by a rather complex set of rules not worth bothering to learn in detail--just pay attention to what NetBeans or Eclipse tell you    Alternatively  you can specify an interface to implement   new  InterfaceName      members     Such a declaration creates a new instance of an unnamed class which extends Object and implements InterfaceName  Again  you cannot supply your own constructor  in this case  Java implicitly supplies a no-arg  do-nothing constructor  so there will never be constructor arguments in this case    Even though you can t give an anonymous inner class a constructor  you can still do any setup you want using an initializer block  a    block placed outside any method    Be clear that an anonymous inner class is simply a less flexible way of creating a local inner class with one instance  If you want a local inner class which implements multiple interfaces or which implements interfaces while extending some class other than Object or which specifies its own constructor  you re stuck creating a regular named local inner class

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The difference is that a nested class declaration that is also static can be instantiated outside of the enclosing class   When you have a nested class declaration that is not static  also known as an inner class  Java won t let you instantiate it except via the enclosing class  The object created out of the inner class is linked to the object created from the outer class  so the inner class can reference the fields of the outer   But if it s static  then the link does not exist  the outer fields cannot be accessed  except via an ordinary reference like any other object  and you can therefore instantiate the nested class by itself

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Nested class  class inside class  Types    Static nested class Non-static nested class  Inner class    Difference   Non-static nested class  Inner class   In non-static nested class object of inner class exist within object of outer class  So that data member of outer class is accessible to inner class  So to create object of inner class we must create object of outer class first   outerclass outerobject new outerobject    outerclass innerclass innerobjcet outerobject new innerclass       Static nested class  In static nested class object of inner class don t need object of outer class  because the word  static  indicate no need to create object   class outerclass A       static class nestedclass B           static int x   10            If you want to access x  then write the following inside method    outerclass nestedclass x   i e  System out prinltn  outerclass nestedclass x

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A diagram    The main difference between static nested and non-static nested classes is that static nested does not have an access to non-static outer class members

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The Java tutorial says      Terminology  Nested classes are   divided into two categories  static   and non-static  Nested classes that   are declared static are simply called   static nested classes  Non-static   nested classes are called inner   classes    In common parlance  the terms  nested  and  inner  are used interchangeably by most programmers  but I ll use the correct term  nested class  which covers both inner and static   Classes can be nested ad infinitum  e g  class A can contain class B which contains class C which contains class D  etc  However  more than one level of class nesting is rare  as it is generally bad design   There are three reasons you might create a nested class    organization  sometimes it seems most sensible to sort a class into the namespace of another class  especially when it won t be used in any other context access  nested classes have special access to the variables fields of their containing classes  precisely which variables fields depends on the kind of nested class  whether inner or static   convenience  having to create a new file for every new type is bothersome  again  especially when the type will only be used in one context   There are four kinds of nested class in Java  In brief  they are    static class  declared as a static member of another class inner class  declared as an instance member of another class local inner class  declared inside an instance method of another class anonymous inner class  like a local inner class  but written as an expression which returns a one-off object   Let me elaborate in more details     Static Classes  Static classes are the easiest kind to understand because they have nothing to do with instances of the containing class   A static class is a class declared as a static member of another class  Just like other static members  such a class is really just a hanger on that uses the containing class as its namespace  e g  the class Goat declared as a static member of class Rhino in the package pizza is known by the name pizza Rhino Goat   package pizza   public class Rhino                 public static class Goat                         Frankly  static classes are a pretty worthless feature because classes are already divided into namespaces by packages  The only real conceivable reason to create a static class is that such a class has access to its containing class s private static members  but I find this to be a pretty lame justification for the static class feature to exist     Inner Classes  An inner class is a class declared as a non-static member of another class   package pizza   public class Rhino        public class Goat                          private void jerry             Goat g   new Goat              Like with a static class  the inner class is known as qualified by its containing class name  pizza Rhino Goat  but inside the containing class  it can be known by its simple name  However  every instance of an inner class is tied to a particular instance of its containing class  above  the Goat created in jerry  is implicitly tied to the Rhino instance this in jerry  Otherwise  we make the associated Rhino instance explicit when we instantiate Goat   Rhino rhino   new Rhino    Rhino Goat goat   rhino new Goat       Notice you refer to the inner type as just Goat in the weird new syntax  Java infers the containing type from the rhino part  And  yes new rhino Goat   would have made more sense to me too    So what does this gain us  Well  the inner class instance has access to the instance members of the containing class instance  These enclosing instance members are referred to inside the inner class via just their simple names  not via this  this in the inner class refers to the inner class instance  not the associated containing class instance     public class Rhino        private String barry       public class Goat           public void colin                 System out println barry                       In the inner class  you can refer to this of the containing class as Rhino this  and you can use this to refer to its members  e g  Rhino this barry     Local Inner Classes  A local inner class is a class declared in the body of a method  Such a class is only known within its containing method  so it can only be instantiated and have its members accessed within its containing method  The gain is that a local inner class instance is tied to and can access the final local variables of its containing method  When the instance uses a final local of its containing method  the variable retains the value it held at the time of the instance s creation  even if the variable has gone out of scope  this is effectively Java s crude  limited version of closures    Because a local inner class is neither the member of a class or package  it is not declared with an access level   Be clear  however  that its own members have access levels like in a normal class    If a local inner class is declared in an instance method  an instantiation of the inner class is tied to the instance held by the containing method s this at the time of the instance s creation  and so the containing class s instance members are accessible like in an instance inner class  A local inner class is instantiated simply via its name  e g  local inner class Cat is instantiated as new Cat    not new this Cat   as you might expect     Anonymous Inner Classes  An anonymous inner class is a syntactically convenient way of writing a local inner class  Most commonly  a local inner class is instantiated at most just once each time its containing method is run  It would be nice  then  if we could combine the local inner class definition and its single instantiation into one convenient syntax form  and it would also be nice if we didn t have to think up a name for the class  the fewer unhelpful names your code contains  the better   An anonymous inner class allows both these things   new  ParentClassName   constructorArgs     members     This is an expression returning a new instance of an unnamed class which extends ParentClassName  You cannot supply your own constructor  rather  one is implicitly supplied which simply calls the super constructor  so the arguments supplied must fit the super constructor   If the parent contains multiple constructors  the    simplest    one is called     simplest    as determined by a rather complex set of rules not worth bothering to learn in detail--just pay attention to what NetBeans or Eclipse tell you    Alternatively  you can specify an interface to implement   new  InterfaceName      members     Such a declaration creates a new instance of an unnamed class which extends Object and implements InterfaceName  Again  you cannot supply your own constructor  in this case  Java implicitly supplies a no-arg  do-nothing constructor  so there will never be constructor arguments in this case    Even though you can t give an anonymous inner class a constructor  you can still do any setup you want using an initializer block  a    block placed outside any method    Be clear that an anonymous inner class is simply a less flexible way of creating a local inner class with one instance  If you want a local inner class which implements multiple interfaces or which implements interfaces while extending some class other than Object or which specifies its own constructor  you re stuck creating a regular named local inner class

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I think  the convention that is generally followed is this    static class within a top level class is a nested class non static class within a top level  class is a inner class  which further has two more form   local class - named classes declared inside of a block like a method or constructor body anonymous class - unnamed classes whose instances are created in expressions and statements    However  few other points to remembers are    Top level classes and static nested class are semantically same except that in case of static nested class it can make static reference to private static fields methods of its Outer  parent  class and vice versa  Inner classes have access to instance variables of the enclosing instance of the Outer  parent  class  However  not all inner classes have enclosing instances  for example inner classes in static contexts  like an anonymous class used in a static initializer block  do not  Anonymous class by default extends the parent class or implements the parent interface and there is no further clause to extend any other class or implement any more interfaces  So    new YourClass            means   class  Anonymous  extends YourClass    new YourInterface        means   class  Anonymous  implements YourInterface         I feel that the bigger question that remains open which one to use and when  Well that mostly depends on what scenario you are dealing with but reading the reply given by  jrudolph may help you making some decision

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The Java tutorial says      Terminology  Nested classes are   divided into two categories  static   and non-static  Nested classes that   are declared static are simply called   static nested classes  Non-static   nested classes are called inner   classes    In common parlance  the terms  nested  and  inner  are used interchangeably by most programmers  but I ll use the correct term  nested class  which covers both inner and static   Classes can be nested ad infinitum  e g  class A can contain class B which contains class C which contains class D  etc  However  more than one level of class nesting is rare  as it is generally bad design   There are three reasons you might create a nested class    organization  sometimes it seems most sensible to sort a class into the namespace of another class  especially when it won t be used in any other context access  nested classes have special access to the variables fields of their containing classes  precisely which variables fields depends on the kind of nested class  whether inner or static   convenience  having to create a new file for every new type is bothersome  again  especially when the type will only be used in one context   There are four kinds of nested class in Java  In brief  they are    static class  declared as a static member of another class inner class  declared as an instance member of another class local inner class  declared inside an instance method of another class anonymous inner class  like a local inner class  but written as an expression which returns a one-off object   Let me elaborate in more details     Static Classes  Static classes are the easiest kind to understand because they have nothing to do with instances of the containing class   A static class is a class declared as a static member of another class  Just like other static members  such a class is really just a hanger on that uses the containing class as its namespace  e g  the class Goat declared as a static member of class Rhino in the package pizza is known by the name pizza Rhino Goat   package pizza   public class Rhino                 public static class Goat                         Frankly  static classes are a pretty worthless feature because classes are already divided into namespaces by packages  The only real conceivable reason to create a static class is that such a class has access to its containing class s private static members  but I find this to be a pretty lame justification for the static class feature to exist     Inner Classes  An inner class is a class declared as a non-static member of another class   package pizza   public class Rhino        public class Goat                          private void jerry             Goat g   new Goat              Like with a static class  the inner class is known as qualified by its containing class name  pizza Rhino Goat  but inside the containing class  it can be known by its simple name  However  every instance of an inner class is tied to a particular instance of its containing class  above  the Goat created in jerry  is implicitly tied to the Rhino instance this in jerry  Otherwise  we make the associated Rhino instance explicit when we instantiate Goat   Rhino rhino   new Rhino    Rhino Goat goat   rhino new Goat       Notice you refer to the inner type as just Goat in the weird new syntax  Java infers the containing type from the rhino part  And  yes new rhino Goat   would have made more sense to me too    So what does this gain us  Well  the inner class instance has access to the instance members of the containing class instance  These enclosing instance members are referred to inside the inner class via just their simple names  not via this  this in the inner class refers to the inner class instance  not the associated containing class instance     public class Rhino        private String barry       public class Goat           public void colin                 System out println barry                       In the inner class  you can refer to this of the containing class as Rhino this  and you can use this to refer to its members  e g  Rhino this barry     Local Inner Classes  A local inner class is a class declared in the body of a method  Such a class is only known within its containing method  so it can only be instantiated and have its members accessed within its containing method  The gain is that a local inner class instance is tied to and can access the final local variables of its containing method  When the instance uses a final local of its containing method  the variable retains the value it held at the time of the instance s creation  even if the variable has gone out of scope  this is effectively Java s crude  limited version of closures    Because a local inner class is neither the member of a class or package  it is not declared with an access level   Be clear  however  that its own members have access levels like in a normal class    If a local inner class is declared in an instance method  an instantiation of the inner class is tied to the instance held by the containing method s this at the time of the instance s creation  and so the containing class s instance members are accessible like in an instance inner class  A local inner class is instantiated simply via its name  e g  local inner class Cat is instantiated as new Cat    not new this Cat   as you might expect     Anonymous Inner Classes  An anonymous inner class is a syntactically convenient way of writing a local inner class  Most commonly  a local inner class is instantiated at most just once each time its containing method is run  It would be nice  then  if we could combine the local inner class definition and its single instantiation into one convenient syntax form  and it would also be nice if we didn t have to think up a name for the class  the fewer unhelpful names your code contains  the better   An anonymous inner class allows both these things   new  ParentClassName   constructorArgs     members     This is an expression returning a new instance of an unnamed class which extends ParentClassName  You cannot supply your own constructor  rather  one is implicitly supplied which simply calls the super constructor  so the arguments supplied must fit the super constructor   If the parent contains multiple constructors  the    simplest    one is called     simplest    as determined by a rather complex set of rules not worth bothering to learn in detail--just pay attention to what NetBeans or Eclipse tell you    Alternatively  you can specify an interface to implement   new  InterfaceName      members     Such a declaration creates a new instance of an unnamed class which extends Object and implements InterfaceName  Again  you cannot supply your own constructor  in this case  Java implicitly supplies a no-arg  do-nothing constructor  so there will never be constructor arguments in this case    Even though you can t give an anonymous inner class a constructor  you can still do any setup you want using an initializer block  a    block placed outside any method    Be clear that an anonymous inner class is simply a less flexible way of creating a local inner class with one instance  If you want a local inner class which implements multiple interfaces or which implements interfaces while extending some class other than Object or which specifies its own constructor  you re stuck creating a regular named local inner class

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Nested class is a very general term  every class which is not top level is a nested class  An inner class is a non-static nested class  Joseph Darcy wrote a very nice explanation about Nested  Inner  Member  and Top-Level Classes

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I think that none of the above answers explain to you the real difference between a nested class and a static nested class in term of application design     OverView  A nested class could be nonstatic or static and in each case is a class defined within another class  A nested class should exist only to serve is enclosing class  if a nested class is useful by other classes  not only the enclosing   should be declared as a top level class   Difference  Nonstatic Nested class   is implicitly associated with the enclosing instance of the containing class  this means that it is possible to invoke methods and access variables of the enclosing instance  One common use of a nonstatic nested class is to define an Adapter class   Static Nested Class   can t access enclosing class instance and invoke methods on it  so should be used when the nested class doesn t require access to an instance of the enclosing class   A common use of static nested class is to implement a components of the outer object   Conclusion  So the main difference between the two from a design standpoint is   nonstatic nested class can access instance of the container class  while static can t

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Inner class and nested static class in Java both are classes declared inside another class  known as top level class in Java  In Java terminology  If you declare a nested class static  it will called nested static class in Java while non static nested class are simply referred as Inner Class    What is Inner Class in Java   Any class which is not a top level or declared inside another class is known as nested class and out of those nested classes  class which are declared non static are known as Inner class in Java  there are three kinds of Inner class in Java   1  Local inner class    - is declared inside a code block or method  2  Anonymous inner class - is a class which doesn t have name to reference and initialized at same place where it gets created  3  Member inner class - is declared as non static member of outer class   public class InnerClassTest       public static void main String args                      creating local inner class inside method i e  main            class Local               public void name                     System out println  Example of Local class in Java                                             creating instance of local inner class         Local local   new Local            local name      calling method from local inner class            Creating anonymous inner class in Java for implementing thread         Thread anonymous   new Thread                 Override             public void run                    System out println  Anonymous class example in java                                     anonymous start               example of creating instance of inner class         InnerClassTest test   new InnerClassTest            InnerClassTest Inner inner   test new Inner            inner name      calling method of inner class               Creating Inner class in Java     private class Inner          public void name                System out println  Inner class example in java                        What is nested static class in Java   Nested static class is another class which is declared inside a class as member and made static  Nested static class is also declared as member of outer class and can be make private  public or protected like any other member  One of the main benefit of nested static class over inner class is that instance of nested static class is not attached to any enclosing instance of Outer class  You also don t need any instance of Outer class to create instance of nested static class in Java   1  It can access static data members of outer class including private  2  Static nested class cannot access non-static  instance  data member or method   public class NestedStaticExample       public static void main String args               StaticNested nested   new StaticNested            nested name                  static nested class in java     private static class StaticNested          public void name                System out println  static nested class example in java                        Ref  Inner class and nested Static Class in Java with Example

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When we declare static member class inside a class  it is known as top level nested class or a static nested class  It can be demonstrated as below     class Test      private static int x   1          static class A          private static int y   2          public static int getZ                return B z x                      static class B          private static int z   3          public static int getY                return A y                     class TestDemo       public static void main String   args           Test t   new Test            System out println Test A getZ             System out println Test B getY               When we declare non-static member class inside a class it is known as inner class  Inner class can be demonstrated as below         class Test          private int i   10          class A              private int i  20              void display                int i   30              System out println i               System out println this i               System out println Test this i

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The instance of the inner class is created when instance of the outer class is created. Therefore the members and methods of the inner class have access to the members and methods of the instance (object) of the outer class. When the instance of the outer class goes out of scope, also the inner class instances cease to exist.

The static nested class doesn't have a concrete instance. It's just loaded when it's used for the first time (just like the static methods). It's a completely independent entity, whose methods and variables doesn't have any access to the instances of the outer class.

The static nested classes are not coupled with the outer object, they are faster, and they don't take heap/stack memory, because its not necessary to create instance of such class. Therefore the rule of thumb is to try to define static nested class, with as limited scope as possible (private >= class >= protected >= public), and then convert it to inner class (by removing "static" identifier) and loosen the scope, if it's really necessary.

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The terms are used interchangeably   If you want to be really pedantic about it  then you could define  nested class  to refer to a static inner class  one which has no enclosing instance   In code  you might have something like this   public class Outer       public class Inner         public static class Nested        That s not really a widely accepted definition though

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There is a subtlety about the use of nested static classes that might be useful in certain situations   Whereas static attributes get instantiated before the class gets instantiated via its constructor  static attributes inside of nested static classes don t seem to get instantiated until after the class s constructor gets invoked  or at least not until after the attributes are first referenced  even if they are marked as  final    Consider this example   public class C0        static C0 instance   null          Uncomment the following line and a null pointer exception will be        generated before anything gets printed        public static final String outerItem   instance makeString 98 6        public C0             instance   this             public String makeString int i            return   new Integer i   toString                public String makeString double d            return   new Double d   toString                public static final class nested           public static final String innerItem   instance makeString 42              static public void main String   argv            System out println  start               Comment out this line and a null pointer exception will be            generated after  start  prints and before the following            try catch block even gets entered          new C0            try               System out println  retrieve item      nested innerItem                     catch  Exception e                System out println  failed to retrieve item      e toString                       System out println  finish              Even though  nested  and  innerItem  are both declared as  static final   the setting of nested innerItem doesn t take place until after the class is instantiated  or at least not until after the nested static item is first referenced   as you can see for yourself by commenting and uncommenting the lines that I refer to  above  The same does not hold true for  outerItem    At least this is what I m seeing in Java 6 0

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Nested class is a very general term  every class which is not top level is a nested class  An inner class is a non-static nested class  Joseph Darcy wrote a very nice explanation about Nested  Inner  Member  and Top-Level Classes

[java] Java inner class and static nested class

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