difference between Instance variable & class variable?

[jami siva]

Hi,

I read a sentance in K & B, Instance variables Cannot be marked static, because then they'd become class variables.

As of now i know the variables declared as static are called class variables,

Here in below i wrote

   
class Const4
{
	public String name; // this is an instance variable
                static public String addr;  //this is an instance variable class variable i make it as static by adding static
}
public class ch1Ex3 extends Const4 {
	public static void main(String[] args) {
	
                }
}
 

This code is compiled , what is it mean by instance variable can not be marked static


Thanks
Sivva

[Jelle Klap]

The variable declared at line 4 is an instance variable. You can certainly declare this variable static, but then it becomes a class variable.
So, the reason you can't declare an instance variable static, is because as soon as you do, it becomes a class variable. There's no such thing as a static instance variable.

[John de Michele]

Jami:

An instance variable belongs to an individual class object. So in this class:

public final class Example {

    private static int counter;
    private String name;

    static {
        counter = 0;
    }

    public Example(final String name) {
        this.name = name;
        counter++;
    }

    public int getCounter() {
        return counter;
    }

    public String getName() {
        return name;
    }

}

The String 'name' is an instance variable. Every object of this class has its own copy of name (which is set when the class is instantiated). The int 'counter' is a static variable, which means that it is shared by all objects of type Example. So, if you create three objects of type Example, counter will end up with a value of 3 (ignoring thread-safety issues).

John.

[jami siva]

i understand guys, thanks

[John de Michele]

Jami:

Glad I could help .

John.