static variable

• It is a variable which belongs to the class and not to object(instance)
• Static variables are initialized only once , at the start of the execution . These variables will be initialized first, before the initialization of any instance variables
• A single copy to be shared by all instances of the class
• A static variable can be accessed directly by the class name and doesn’t need any object
• Syntax : <class-name>.<variable-name>

static method

• It is a method which belongs to the class and not to the object(instance)
• A static method can access only static data. It can not access non-static data (instance variables)
• A static method can  call only  other static methods and can not call a non-static method from it.
• A static method  can be accessed directly by the class name and doesn’t need any object
• Syntax : <class-name>.<method-name>
• A static method cannot refer to "this" or "super"  keywords in anyway

Side Note:

main method is static , since it must be be accessible for an application to run , before any instantiation takes place.

Lets learn the nuances of the static keywords by doing some excercises!

Assignment: To Learn working of static variables & methods

Step 1) Copy the following code into a editor

class Student {
int a; //initialized to zero
static int b; //initialized to zero only when class is loaded not for each object created.

  Student(){
   //Constructor incrementing static variable b
   b++;
  }


   public void showData(){
      System.out.println("Value of a = "+a);
      System.out.println("Value of b = "+b);
   }
//public static void increment(){
//a++;
//}

}

class Demo{
   public static void main(String args[]){
     Student s1 = new Student();
     s1.showData();
     Student s2 = new Student();
     s2.showData();
     //Student.b++;
     //s1.showData();
  }
}

Step 2) Save & Compile the code. Run the code as, java Demo.

Step 3) Expected output show below

Following diagram shows , how reference variables & objects are created and static variables are accessed by the different instances.

Step 4) It is possible to access a static  variable from outside the class using the syntax ClassName.Variable_Name. Uncomment line # 27 & 28 .   Save , Compile & Run . Observe the output.

Step 5)  Uncomment line 15,16 & 17  . Save , Compile & Run.

Step 5)  Error = ? This is because it is not possible to access instance variable "a" from static method "increment".

static block

The static block, is a block of statement inside a Java class that will be executed when a class is first loaded in to the JVM

class Test{

  static {

     //Code goes here

  }

}

A static block helps to initialize the static data members, just like constructors help to initialize instance members

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