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.

 1 class OuterClass{
 2    private static String msg = "GeeksForGeeks";
 3     
 4    // Static nested class
 5    public static class NestedStaticClass{
 6       
 7        // Only static members of Outer class is directly accessible in nested 
 8        // static class 
 9        public void printMessage() {
10  
11          // Try making 'message' a non-static variable, there will be 
12          // compiler error  
13          System.out.println("Message from nested static class: " + msg); 
14        }
15     }
16     
17     // non-static nested class - also called Inner class
18     public class InnerClass{
19         
20        // Both static and non-static members of Outer class are accessible in 
21        // this Inner class
22        public void display(){
23           System.out.println("Message from non-static nested class: "+ msg);
24        }
25     }
26 } 
27 class Main
28 {
29     // How to create instance of static and non static nested class?
30     public static void main(String args[]){
31         
32        // create instance of nested Static class
33        OuterClass.NestedStaticClass printer = new OuterClass.NestedStaticClass();
34         
35        // call non static method of nested static class
36        printer.printMessage();   
37   
38        // In order to create instance of Inner class we need an Outer class 
39        // instance. Let us create Outer class instance for creating 
40        // non-static nested class
41        OuterClass outer = new OuterClass();        
42        OuterClass.InnerClass inner  = outer.new InnerClass();
43         
44        // calling non-static method of Inner class
45        inner.display();
46         
47        // we can also combine above steps in one step to create instance of 
48        // Inner class
49        OuterClass.InnerClass innerObject = new OuterClass().new InnerClass();
50         
51        // similarly we can now call Inner class method
52        innerObject.display();
53     }
54 }