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 Var

Answer 1

An object variable is state dependent on a specific instance of a class, whereas a class variable is globally accessible through the class itself. That might be a little fuzzy, so here are some examples:

class Muffin {

    private static final int calories = 9320;

    public String flavor;

    public Muffin( String flavor ){
        this.flavor = flavor;
    }

}

In this class, calories is a class variable. In any other piece of code, you can get the number of calories in any kind of muffin by calling Muffin.calories. In this case, the final keyword is also used to make the number of calories constant.

In the same class, we have an object variable, flavor. This is dependent on the instance of the class, and is set in the constructor.

Muffin myMuffin = new Muffin( "blueberry" );

So now you can access this specific muffin's flavor by calling myMuffin.flavor. Notice how we need to instantiate a Muffin object before we can access its flavor.

Changing static (class) variables

The above example is a bit of a stretch, since different types of muffins would have different calorie counts. They are useful for constants, but here's a case where the value of the static variable changes:

class Muffin {

    private static int next_id = 1;

    public int id;
    public String flavor;

    public Muffin( String flavor ){
        this.flavor = flavor;
        id = next_id++;
    }

}

In the second example, we need to have a unique ID number for every muffin we create, so we can have a static variable that gets incremented every time a Muffin is instantiated. The static keyword makes the value of next_id persist through every call to the constructor, so the id will be different and continue to increase for every new muffin.