11. Usage of Static class

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.

Java allows us to define a class within another class. Such a class is called a nested class. The class which enclosed nested class is known as Outer class. In java, we can’t make Top level class static. Only nested classes can be static.

What are the differences between static and non-static nested classes? 

Following are major differences between static nested class and non-static nested class. Non-static nested class is also called Inner Class.

1.  Nested static class doesn’t need 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

3. An instance of Inner class cannot be created without an instance of outer class and an Inner class can reference data and methods defined in Outer class in which it nests, so we don’t need to pass reference of an object to the constructor of the Inner class. For this reason Inner classes can make program simple and concise.

Static Class

A Class can be made static only if it is a nested Class. The nested static class can be accessed without having an object of outer class.

Example 1:

class Example1{

  //Static class

  static class X{

      static String str="Inside Class X";

  }

  public static void main(String args[])

  {

      X.str="Inside Class Example1";

      System.out.println("String stored in str is- "+ X.str);

  }

}

Output:

String stored in str is- Inside Class Example1

Example 2: Compile time Error!!

class Example2{

   int num;

   //Static class

   static class X{

      static String str="Inside Class X";

      num=99;

   }

   public static void main(String args[])

   {

      Example2.X obj = new Example2.X();

      System.out.println("Value of num="+obj.str);

   }

}

Output:
Compile time error. Static inner class cannot access instance data of outer class.