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();
      }
  }