Static Classes In Java

Question

Is there anything like static class in java   What is the meaning of such a class  Do all the methods of the static class need to be static too   Is it required the other way round  that if a class contains all the static methods  shall the class be static too   What are static classes good  for

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Well  Java has  static nested classes   but they re not at all the same as C  s static classes  if that s where you were coming from  A static nested class is just one which doesn t implicitly have a reference to an instance of the outer class   Static nested classes can have instance methods and static methods   There s no such thing as a top-level static class in Java

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There is a static nested class  this  static nested  class does not need an instance of the enclosing class in order to be instantiated itself    These classes  static nested ones  can access only the static members of the enclosing class  since it does not have any reference to instances of the enclosing class      code sample   public class Test      class A        static class B       public static void main String   args           will fail - compilation error  you need an instance of Test to instantiate A       A a   new A           will compile successfully  not instance of Test is needed to instantiate B        B b   new B

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All good answers  but I did not saw a reference to java util Collections which uses tons of static inner class for their static factor methods  So adding the same   Adding an example from java util Collections which has multiple static inner class  Inner classes are useful to group code which needs to be accessed via outer class           serial include     static class UnmodifiableSet lt E gt  extends UnmodifiableCollection lt E gt                               implements Set lt E gt   Serializable       private static final long serialVersionUID   -9215047833775013803L       UnmodifiableSet Set lt   extends E gt  s       super s        public boolean equals Object o   return o    this    c equals o        public int hashCode              return c hashCode         Here is the static factor method in the java util Collections class  public static  lt T gt  Set lt T gt  unmodifiableSet Set lt   extends T gt  s        return new UnmodifiableSet lt  gt  s

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Java has static methods that are associated with classes  e g  java lang Math has only static methods   but the class itself is not static

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Can a class be static in Java    The answer is YES  we can have static class in java  In java  we have static instance variables as well as static methods and also static block  Classes can also be made static in Java   In java  we can   t make Top-level  outer  class static  Only nested classes can be static   static nested class vs non-static nested class  1  Nested static class doesn   t need a reference of Outer class  but     Non-static nested class or Inner class requires Outer class     reference   2  Inner class or non-static nested class  can access both static     and non-static members of Outer class  A static class cannot access non-static members of the Outer class  It can access only static     members of Outer class   see here  https   docs oracle com javase tutorial java javaOO nested html

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Java has static nested classes but it sounds like you re looking for a top-level static class  Java has no way of making a top-level class static but you can simulate a static class like this    Declare your class final - Prevents extension of the class since extending a static class makes no sense Make the constructor private - Prevents instantiation by client code as it makes no sense to instantiate a static class Make all the members and functions of the class static -  Since the class cannot be instantiated no instance methods can be called or instance fields accessed Note that the compiler will not prevent you from declaring an instance  non-static  member  The issue will only show up if you attempt to call the instance member   Simple example per suggestions from above   public class TestMyStaticClass        public static void main String   args           MyStaticClass setMyStaticMember 5           System out println  Static value      MyStaticClass getMyStaticMember             System out println  Value squared      MyStaticClass squareMyStaticMember                MyStaticClass x   new MyStaticClass       results in compile time error              A top-level Java class mimicking static class behavior public final class MyStaticClass       private MyStaticClass         private constructor         myStaticMember   1            private static int myStaticMember      public static void setMyStaticMember int val            myStaticMember   val            public static int getMyStaticMember             return myStaticMember            public static int squareMyStaticMember             return myStaticMember   myStaticMember            What good are static classes  A good use of a static class is in defining one-off  utility and or library classes where instantiation would not make sense  A great example is the Math class that contains some mathematical constants such as PI and E and simply provides mathematical calculations  Requiring instantiation in such a case would be unnecessary and confusing  See the Math class and source code  Notice that it is final and all of its members are static  If Java allowed top-level classes to be declared static then the Math class would indeed be static

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In simple terms  Java supports the declaration of a class to be static only for the inner classes but not for the top level classes   top level classes  A java project can contain more than one top level classes in each java source file  one of the classes being named after the file name  There are only three options or keywords allowed in front of the top level classes  public  abstract and final    Inner classes  classes that are inside of a top level class are called inner classes  which is basically the concept of nested classes  Inner classes can be static  The idea making the inner classes static  is to take the advantage of instantiating the objects of inner classes without instantiating the object of the top level class  This is exactly the same way as the static methods and variables work inside of a top level class   Hence Java Supports Static Classes at Inner Class Level  in nested classes   And Java Does Not Support Static Classes at Top Level Classes   I hope this gives a simpler solution to the question for basic understanding of the static classes in Java

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A static method means that it can be accessed without creating an object of the class  unlike public   public class MyClass         Static method    static void myStaticMethod           System out println  Static methods can be called without creating objects               Public method   public void myPublicMethod           System out println  Public methods must be called by creating objects               Main method   public static void main String    args          myStaticMethod       Call the static method        myPublicMethod    This would output an error      MyClass myObj   new MyClass       Create an object of MyClass     myObj myPublicMethod       Call the public method

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Outer classes cannot be static  but nested inner classes can be  That basically helps you to use the nested inner class without creating an instance of the outer class

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What s happening when a members inside a class is declared as static    That members can be accessed without instantiating the class  Therefore making outer class top level class  static has no meaning  Therefore it is not allowed   But you can set inner classes as static  As it is a member of the top level class   Then that class can be accessed without instantiating the top level class  Consider the following example   public class A       public static class B              Now  inside a different class C  class B can be accessed without making an instance of class A   public class C       A B ab   new A B        static classes can have non-static members too  Only the class gets static   But if the static keyword is removed from class B  it cannot be accessed directly without making an instance of A   public class C       A a   new A        A B ab   a  new B        But we cannot have static members inside a non-static inner class

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Yes there is a static nested class in java  When you declare a nested class static  it automatically becomes a stand alone class which can be instantiated without having to instantiate the outer class it belongs to   Example   public class A     public static class B           Because class B is declared static you can explicitly instantiate as   B b   new B      Note if class B wasn t declared static to make it stand alone  an instance object call would ve looked like this   A a  new A    B b   a new B

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Seeing as this is the top result on Google for  static class java  and the best answer isn t here I figured I d add it   I m interpreting OP s question as concerning static classes in C   which are known as singletons in the Java world   For those unaware  in C  the  static  keyword can be applied to a class declaration which means the resulting class can never be instantiated   Excerpt from  Effective Java - Second Edition  by Joshua Bloch  widely considered to be one of the best Java style guides available       As of release 1 5  there is a third approach to implementing singletons  Simply make an enum type with one element      Enum singleton - the preferred approach public enum Elvis       INSTANCE      public void leaveTheBuilding                   This approach is functionally equivalent to the public field approach  except that it is more concise  provides the serialization machinery for free   and provides an ironclad guarantee against multiple instantiation  even in the face of sophisticated serialization or reflection attacks  While this approach has yet to be widely adopted  a single-element enum type is the best way to implement a singleton   emphasis author s    Bloch  Joshua  2008-05-08   Effective Java  Java Series   p  18   Pearson Education   I think the implementation and justification are pretty self explanatory

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You cannot use the static keyword with a class unless it is an inner class  A static inner class is a nested class which is a static member of the outer class  It can be accessed without instantiating the outer class  using other static members  Just like static members  a static nested class does not have access to the instance variables and methods of the outer class   public class Outer      static class Nested Demo         public void my method               System out println  This is my nested class                 public static void main String args            Outer Nested Demo nested   new Outer Nested Demo          nested my method

[java] Static Classes In Java

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