Java: Class Variable/Static Variable

1. Class Variable/Static Variable:

• Class variable is also known as static variable with "static" keyword inside the class but outside the methods.
• There is only one copy of class variable is no matter how many objects are initiated from this class.
• Class variable is accessed as: className.classVariableName.
• Static variable is pretty like constant, declared with key word "static", stored in static memory, created when program begins and destroyed when program ends.

2. Java Static Method:

• A static method belongs to a class rather than a object.
• A static method could be invoked without creating an object.
• Static method could access and change static data value.

3. Static Block:

• It is used to initialize static data member.
• It is executed before main method at the time of class loading.

4. Static Class:

• There is inner class nested in outer class, inner class could be static class, out class can't be static class.
• Inner static class doesn't need reference of outer class, but inner non-static class need reference of outer class.
• Inner static class could only access outer static members of outer class.

•  class OuterClass{
      private static String msg = "GeeksForGeeks";
  
      // Static nested class
      public static class NestedStaticClass{
  
          // Only static members of Outer class is directly accessible in nested
          // static class
          public void printMessage() {
  
            // Try making 'message' a non-static variable, there will be
            // compiler error
            System.out.println("Message from nested static class: " + msg);
          }
       }
  
       // non-static nested class - also called Inner class
       public class InnerClass{
  
          // Both static and non-static members of Outer class are accessible in
          // this Inner class
          public void display(){
             System.out.println("Message from non-static nested class: "+ msg);
          }
       }
   }
   class Main
   {
       // How to create instance of static and non static nested class?
       public static void main(String args[]){
  
          // create instance of nested Static class
          OuterClass.NestedStaticClass printer = new OuterClass.NestedStaticClass();
  
          // call non static method of nested static class
          printer.printMessage();   
  
          // In order to create instance of Inner class we need an Outer class
          // instance. Let us create Outer class instance for creating
          // non-static nested class
          OuterClass outer = new OuterClass();
          OuterClass.InnerClass inner  = outer.new InnerClass();
  
          // calling non-static method of Inner class
          inner.display();
  
          // we can also combine above steps in one step to create instance of
          // Inner class
          OuterClass.InnerClass innerObject = new OuterClass().new InnerClass();
  
          // similarly we can now call Inner class method
          innerObject.display();
       }
   }