1. 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>

2. 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) Check output.
Step 4) It is possible to access a static variable from outside the class using the syntaxClassName.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“.

3. 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.

References

Duc Nguyen Dev Note

Friday, October 5, 2012

All about “static” Keyword

1. Static variable