Object Variables vs Class Variables in Java

Question

I am in the process of learning Java and I don't understand the difference between Object Variables and Class Variable. All I know is that in order for it to be a Class Variable you must declare it first with the static statement. Thanks!

Answer 1

In Java (and in OOP in general) the objects have two kinds of fields(variable).

Instance variables(or object variable) are fields that belong to a particular instance of an object.

Static variables (or class variable) are common to all the instances of the same class.

Here's an example:

public class Foobar{
    static int counter = 0 ; //static variable..all instances of Foobar will share the same counter and will change if such is done
    public int id; //instance variable. Each instance has its own id
    public Foobar(){
        this.id = counter++;
    }
}

usage:

Foobar obj1 = new Foobar();
Foobar obj2 = new Foobar();
System.out.println("obj1 id : " + obj1.id + " obj2.id "+ obj2.id + " id count " + Foobar.counter);

Answer 2

An object variable or instance member belongs to a specific instance of a class. That is to say that every instance has its own copy of that piece of data. A class variable or static member is shared by every instance of the class. That is to say that there is only one copy of that piece of data no matter how many class instances there are.