Types of Variable

  1. static variables
  2. static methods
  3. static blocks of code.

Lets look at static variables and static methods first.

Java 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>

Java 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

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

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++;
//}

}

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

Step 3) Expected output show below

Java Static Methods and Variables


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

Java Static Methods and Variables

Step 4) It is possible to access a static variable from outside the class using the syntax ClassName.Variable_Name. Uncomment line # 7 & 8 . Save , Compile & Run . Observe the output.

Step 5) Uncomment line 25,26 & 27 . Save , Compile & Run.

Step 6) Error = ? This is because it is not possible to access instance variable "a" from java static class 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 in java programming language.

 

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