Static nested class in Java why

Question

I was looking at the Java code for LinkedList and noticed that it made use of a static nested class  Entry   public class LinkedList lt E gt              private static class Entry lt E gt               What is the reason for using a static nested class  rather than an normal inner class   The only reason I could think of  was that Entry doesn t have access to instance variables  so from an OOP point of view it has better encapsulation    But I thought there might be other reasons  maybe performance  What might it be   Note  I hope I have got my terms correct  I would have called it a static inner class  but I think this is wrong  http   java sun com docs books tutorial java javaOO nested html

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Simple example     package test   public class UpperClass   public static class StaticInnerClass     public class InnerClass     public static void main String   args           works     StaticInnerClass stat   new StaticInnerClass           doesn t compile     InnerClass inner   new InnerClass          If non-static the class cannot be instantiated exept in an instance of the upper class  so not in the example where main is a static function

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Static inner class is used in the builder pattern  Static inner class can instantiate it s outer class which has only private constructor  You can not do the same with the inner class as you need to have object of the outer class created prior to accessing the inner class  class OuterClass       private OuterClass int x            System out println  quot x   quot    x                  static class InnerClass           public static void test                 OuterClass outer   new OuterClass 1                      public class Test       public static void main String   args            OuterClass InnerClass test               OuterClass outer   new OuterClass 1      It is not possible to create outer instance from outside           This will output x  1

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There are non-obvious memory retention issues to take into account here  Since a non-static inner class maintains an implicit reference to it s  outer  class  if an instance of the inner class is strongly referenced  then the outer instance is strongly referenced too  This can lead to some head-scratching when the outer class is not garbage collected  even though it appears that nothing references it

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The Sun page you link to has some key differences between the two      A nested class is a member of its enclosing class  Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes do not have access to other members of the enclosing class             Note  A static nested class interacts with the instance members of its outer class  and other classes  just like any other top-level class  In effect  a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience    There is no need for LinkedList Entry to be  top-level class as it is only used by LinkedList  there are some other interfaces that also have static nested classes named Entry  such as Map Entry - same concept   And since it does not need access to LinkedList s members  it makes sense for it to be static - it s a much cleaner approach   As Jon Skeet points out  I think it is a better idea if you are using a nested class is to start off with it being static  and then decide if it really needs to be non-static based on your usage

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There are non-obvious memory retention issues to take into account here  Since a non-static inner class maintains an implicit reference to it s  outer  class  if an instance of the inner class is strongly referenced  then the outer instance is strongly referenced too  This can lead to some head-scratching when the outer class is not garbage collected  even though it appears that nothing references it

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From http   docs oracle com javase tutorial java javaOO whentouse html      Use a non-static nested class  or inner class  if you require access   to an enclosing instance s non-public fields and methods  Use a static   nested class if you don t require this access

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One of the reasons for static vs  normal have to do with classloading  You cannot instantiate an inner class in the constructor of it s parent   PS  I ve always understood  nested  and  inner  to be interchangeable  There may be subtle nuances in the terms but most Java developers would understand either

User · Answer

The Sun page you link to has some key differences between the two      A nested class is a member of its enclosing class  Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes do not have access to other members of the enclosing class             Note  A static nested class interacts with the instance members of its outer class  and other classes  just like any other top-level class  In effect  a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience    There is no need for LinkedList Entry to be  top-level class as it is only used by LinkedList  there are some other interfaces that also have static nested classes named Entry  such as Map Entry - same concept   And since it does not need access to LinkedList s members  it makes sense for it to be static - it s a much cleaner approach   As Jon Skeet points out  I think it is a better idea if you are using a nested class is to start off with it being static  and then decide if it really needs to be non-static based on your usage

User · Answer

Static inner class is used in the builder pattern  Static inner class can instantiate it s outer class which has only private constructor  You can not do the same with the inner class as you need to have object of the outer class created prior to accessing the inner class  class OuterClass       private OuterClass int x            System out println  quot x   quot    x                  static class InnerClass           public static void test                 OuterClass outer   new OuterClass 1                      public class Test       public static void main String   args            OuterClass InnerClass test               OuterClass outer   new OuterClass 1      It is not possible to create outer instance from outside           This will output x  1

User · Answer

There are non-obvious memory retention issues to take into account here  Since a non-static inner class maintains an implicit reference to it s  outer  class  if an instance of the inner class is strongly referenced  then the outer instance is strongly referenced too  This can lead to some head-scratching when the outer class is not garbage collected  even though it appears that nothing references it

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I don t know about performance difference  but as you say  static nested class is not a part of an instance of the enclosing class  Seems just simpler to create a static nested class unless you really need it to be an inner class   It s a bit like why I always make my variables final in Java - if they re not final  I know there s something funny going on with them  If you use an inner class instead of a static nested class  there should be a good reason

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I don t know about performance difference  but as you say  static nested class is not a part of an instance of the enclosing class  Seems just simpler to create a static nested class unless you really need it to be an inner class   It s a bit like why I always make my variables final in Java - if they re not final  I know there s something funny going on with them  If you use an inner class instead of a static nested class  there should be a good reason

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To my mind  the question ought to be the other way round whenever you see an inner class - does it really need to be an inner class  with the extra complexity and the implicit  rather than explicit and clearer  IMO  reference to an instance of the containing class   Mind you  I m biased as a C  fan - C  doesn t have the equivalent of inner classes  although it does have nested types  I can t say I ve missed inner classes yet

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JVM knows no nested classes  Nesting is just syntactic sugar  Below images shows Java file   Below images show class files representation of the java file    Notice that 2 class files are generated  one for parent and another for nested class   Non-static nested class  objects have access to the enclosing scope  That access to the enclosing scope is maintained by holding an implicit reference of the enclosing scope object in the nested object  Nested class is a way to represent the intent that the nested class type represents a component of the parent class  public class Message    private MessageType messageType     component of parent class  public enum MessageType       SENT  RECEIVE         class Otherclass    public boolean isSent Message message        if  message getMessageType      MessageType SENT       accessible at other places as well         return true            return false        private static nested class represents Point 3  amp  the fact the nested type can only be the subcomponent to the parent class  It can t be used separately  public class Message     private Content content     Component of message class   private static class Content      can only be a component of message class    private String body    private int sentBy     public String getBody          return body         public int getSentBy          return sentBy            class Message2     private Message Content content     Not possible     More details here

User · Answer

One of the reasons for static vs  normal have to do with classloading  You cannot instantiate an inner class in the constructor of it s parent   PS  I ve always understood  nested  and  inner  to be interchangeable  There may be subtle nuances in the terms but most Java developers would understand either

User · Answer

To my mind  the question ought to be the other way round whenever you see an inner class - does it really need to be an inner class  with the extra complexity and the implicit  rather than explicit and clearer  IMO  reference to an instance of the containing class   Mind you  I m biased as a C  fan - C  doesn t have the equivalent of inner classes  although it does have nested types  I can t say I ve missed inner classes yet

User · Answer

JVM knows no nested classes  Nesting is just syntactic sugar  Below images shows Java file   Below images show class files representation of the java file    Notice that 2 class files are generated  one for parent and another for nested class   Non-static nested class  objects have access to the enclosing scope  That access to the enclosing scope is maintained by holding an implicit reference of the enclosing scope object in the nested object  Nested class is a way to represent the intent that the nested class type represents a component of the parent class  public class Message    private MessageType messageType     component of parent class  public enum MessageType       SENT  RECEIVE         class Otherclass    public boolean isSent Message message        if  message getMessageType      MessageType SENT       accessible at other places as well         return true            return false        private static nested class represents Point 3  amp  the fact the nested type can only be the subcomponent to the parent class  It can t be used separately  public class Message     private Content content     Component of message class   private static class Content      can only be a component of message class    private String body    private int sentBy     public String getBody          return body         public int getSentBy          return sentBy            class Message2     private Message Content content     Not possible     More details here

User · Answer

I don t know about performance difference  but as you say  static nested class is not a part of an instance of the enclosing class  Seems just simpler to create a static nested class unless you really need it to be an inner class   It s a bit like why I always make my variables final in Java - if they re not final  I know there s something funny going on with them  If you use an inner class instead of a static nested class  there should be a good reason

User · Answer

Well  for one thing  non-static inner classes have an extra  hidden field that points to the instance of the outer class   So if the Entry class weren t static  then besides having access that it doesn t need  it would carry around four pointers instead of three   As a rule  I would say  if you define a class that s basically there to act as a collection of data members  like a  struct  in C  consider making it static

User · Answer

One of the reasons for static vs  normal have to do with classloading  You cannot instantiate an inner class in the constructor of it s parent   PS  I ve always understood  nested  and  inner  to be interchangeable  There may be subtle nuances in the terms but most Java developers would understand either

User · Answer

static nested class is just like any other outer class  as it doesn t have access to outer class members   Just for packaging convenience we can club static nested classes into one outer class for readability purpose  Other than this there is no other use case of static nested class   Example for such kind of usage  you can find in Android R java  resources  file  Res folder of android contains layouts  containing screen designs   drawable folder  containing images used for project   values folder  which contains string constants   etc    Sine all the folders are part of Res folder  android tool generates a R java  resources  file which internally contains lot of static nested classes for each of their inner folders   Here is the look and feel of R java file generated in android  Here they are using only for packaging convenience       AUTO-GENERATED FILE   DO NOT MODIFY        This class was automatically generated by the    aapt tool from the resource data it found   It    should not be modified by hand       package com techpalle b17 testthird   public final class R       public static final class drawable           public static final int ic launcher 0x7f020000            public static final class layout           public static final int activity main 0x7f030000            public static final class menu           public static final int main 0x7f070000            public static final class string           public static final int action settings 0x7f050001          public static final int app name 0x7f050000          public static final int hello world 0x7f050002

User · Answer

Well  for one thing  non-static inner classes have an extra  hidden field that points to the instance of the outer class   So if the Entry class weren t static  then besides having access that it doesn t need  it would carry around four pointers instead of three   As a rule  I would say  if you define a class that s basically there to act as a collection of data members  like a  struct  in C  consider making it static

User · Answer

From http   docs oracle com javase tutorial java javaOO whentouse html      Use a non-static nested class  or inner class  if you require access   to an enclosing instance s non-public fields and methods  Use a static   nested class if you don t require this access

User · Answer

I don t know about performance difference  but as you say  static nested class is not a part of an instance of the enclosing class  Seems just simpler to create a static nested class unless you really need it to be an inner class   It s a bit like why I always make my variables final in Java - if they re not final  I know there s something funny going on with them  If you use an inner class instead of a static nested class  there should be a good reason

User · Answer

Simple example     package test   public class UpperClass   public static class StaticInnerClass     public class InnerClass     public static void main String   args           works     StaticInnerClass stat   new StaticInnerClass           doesn t compile     InnerClass inner   new InnerClass          If non-static the class cannot be instantiated exept in an instance of the upper class  so not in the example where main is a static function

User · Answer

Adavantage of inner class--   one time use supports and improves encapsulation readibility  private field access   Without existing of outer class inner class will not exist   class car      class wheel             There are four types of inner class    normal inner class Method Local Inner class Anonymous inner class static inner class   point ---    from static inner class  we can only access static member of outer class  Inside inner class we cananot declare static member   inorder to invoke normal inner class in static area of outer class   Outer 0 new Outer    Outer Inner i  O new Inner    inorder to invoke normal inner class in instance area of outer class   Inner i new Inner    inorder to invoke normal inner class in outside of outer class   Outer 0 new Outer    Outer Inner i  O new Inner    inside Inner class This pointer to inner class   this member-current inner class outerclassname this--outer class for inner class applicable modifier is -- public default    final abstract strictfp  private protected static outer inner is the name of inner class name  inner class inside instance method then we can acess static and instance field of outer class    10 inner class inside static method then we can access only static  field of   outer class   class outer       int x 10      static int y-20       public void m1             int i 30          final j 40           class inner               public void m2                        have accees x y and j

User · Answer

To my mind  the question ought to be the other way round whenever you see an inner class - does it really need to be an inner class  with the extra complexity and the implicit  rather than explicit and clearer  IMO  reference to an instance of the containing class   Mind you  I m biased as a C  fan - C  doesn t have the equivalent of inner classes  although it does have nested types  I can t say I ve missed inner classes yet

User · Answer

There are non-obvious memory retention issues to take into account here  Since a non-static inner class maintains an implicit reference to it s  outer  class  if an instance of the inner class is strongly referenced  then the outer instance is strongly referenced too  This can lead to some head-scratching when the outer class is not garbage collected  even though it appears that nothing references it

User · Answer

Well  for one thing  non-static inner classes have an extra  hidden field that points to the instance of the outer class   So if the Entry class weren t static  then besides having access that it doesn t need  it would carry around four pointers instead of three   As a rule  I would say  if you define a class that s basically there to act as a collection of data members  like a  struct  in C  consider making it static

User · Answer

Non static inner classes can result in memory leaks while static inner class will protect against them  If the outer class holds considerable data  it can lower the performance of the application

User · Answer

Adavantage of inner class--   one time use supports and improves encapsulation readibility  private field access   Without existing of outer class inner class will not exist   class car      class wheel             There are four types of inner class    normal inner class Method Local Inner class Anonymous inner class static inner class   point ---    from static inner class  we can only access static member of outer class  Inside inner class we cananot declare static member   inorder to invoke normal inner class in static area of outer class   Outer 0 new Outer    Outer Inner i  O new Inner    inorder to invoke normal inner class in instance area of outer class   Inner i new Inner    inorder to invoke normal inner class in outside of outer class   Outer 0 new Outer    Outer Inner i  O new Inner    inside Inner class This pointer to inner class   this member-current inner class outerclassname this--outer class for inner class applicable modifier is -- public default    final abstract strictfp  private protected static outer inner is the name of inner class name  inner class inside instance method then we can acess static and instance field of outer class    10 inner class inside static method then we can access only static  field of   outer class   class outer       int x 10      static int y-20       public void m1             int i 30          final j 40           class inner               public void m2                        have accees x y and j

User · Answer

Simple example     package test   public class UpperClass   public static class StaticInnerClass     public class InnerClass     public static void main String   args           works     StaticInnerClass stat   new StaticInnerClass           doesn t compile     InnerClass inner   new InnerClass          If non-static the class cannot be instantiated exept in an instance of the upper class  so not in the example where main is a static function

User · Answer

To my mind  the question ought to be the other way round whenever you see an inner class - does it really need to be an inner class  with the extra complexity and the implicit  rather than explicit and clearer  IMO  reference to an instance of the containing class   Mind you  I m biased as a C  fan - C  doesn t have the equivalent of inner classes  although it does have nested types  I can t say I ve missed inner classes yet

User · Answer

Simple example     package test   public class UpperClass   public static class StaticInnerClass     public class InnerClass     public static void main String   args           works     StaticInnerClass stat   new StaticInnerClass           doesn t compile     InnerClass inner   new InnerClass          If non-static the class cannot be instantiated exept in an instance of the upper class  so not in the example where main is a static function

User · Answer

Using a static nested class rather than non-static one may save spaces in some cases  For example  implementing a Comparator inside a class  say Student   public class Student     public static final Comparator lt Student gt  BY NAME   new ByName      private final String name          private static class ByName implements Comparator lt Student gt        public int compare                 Then the static ensures that the Student class has only one Comparator  rather than instantiate a new one every time a new student instance is created

User · Answer

One of the reasons for static vs  normal have to do with classloading  You cannot instantiate an inner class in the constructor of it s parent   PS  I ve always understood  nested  and  inner  to be interchangeable  There may be subtle nuances in the terms but most Java developers would understand either

User · Answer

The Sun page you link to has some key differences between the two      A nested class is a member of its enclosing class  Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes do not have access to other members of the enclosing class             Note  A static nested class interacts with the instance members of its outer class  and other classes  just like any other top-level class  In effect  a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience    There is no need for LinkedList Entry to be  top-level class as it is only used by LinkedList  there are some other interfaces that also have static nested classes named Entry  such as Map Entry - same concept   And since it does not need access to LinkedList s members  it makes sense for it to be static - it s a much cleaner approach   As Jon Skeet points out  I think it is a better idea if you are using a nested class is to start off with it being static  and then decide if it really needs to be non-static based on your usage

User · Answer

Using a static nested class rather than non-static one may save spaces in some cases  For example  implementing a Comparator inside a class  say Student   public class Student     public static final Comparator lt Student gt  BY NAME   new ByName      private final String name          private static class ByName implements Comparator lt Student gt        public int compare                 Then the static ensures that the Student class has only one Comparator  rather than instantiate a new one every time a new student instance is created

User · Answer

static nested class is just like any other outer class  as it doesn t have access to outer class members   Just for packaging convenience we can club static nested classes into one outer class for readability purpose  Other than this there is no other use case of static nested class   Example for such kind of usage  you can find in Android R java  resources  file  Res folder of android contains layouts  containing screen designs   drawable folder  containing images used for project   values folder  which contains string constants   etc    Sine all the folders are part of Res folder  android tool generates a R java  resources  file which internally contains lot of static nested classes for each of their inner folders   Here is the look and feel of R java file generated in android  Here they are using only for packaging convenience       AUTO-GENERATED FILE   DO NOT MODIFY        This class was automatically generated by the    aapt tool from the resource data it found   It    should not be modified by hand       package com techpalle b17 testthird   public final class R       public static final class drawable           public static final int ic launcher 0x7f020000            public static final class layout           public static final int activity main 0x7f030000            public static final class menu           public static final int main 0x7f070000            public static final class string           public static final int action settings 0x7f050001          public static final int app name 0x7f050000          public static final int hello world 0x7f050002

User · Answer

Non static inner classes can result in memory leaks while static inner class will protect against them  If the outer class holds considerable data  it can lower the performance of the application

User · Answer

The Sun page you link to has some key differences between the two      A nested class is a member of its enclosing class  Non-static nested classes  inner classes  have access to other members of the enclosing class  even if they are declared private  Static nested classes do not have access to other members of the enclosing class             Note  A static nested class interacts with the instance members of its outer class  and other classes  just like any other top-level class  In effect  a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience    There is no need for LinkedList Entry to be  top-level class as it is only used by LinkedList  there are some other interfaces that also have static nested classes named Entry  such as Map Entry - same concept   And since it does not need access to LinkedList s members  it makes sense for it to be static - it s a much cleaner approach   As Jon Skeet points out  I think it is a better idea if you are using a nested class is to start off with it being static  and then decide if it really needs to be non-static based on your usage

User · Answer

Well  for one thing  non-static inner classes have an extra  hidden field that points to the instance of the outer class   So if the Entry class weren t static  then besides having access that it doesn t need  it would carry around four pointers instead of three   As a rule  I would say  if you define a class that s basically there to act as a collection of data members  like a  struct  in C  consider making it static

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