Question

I have a class, Super:

public class Super {
    public static String foo = "foo";
}

I also have another class, Sub that extends Super:

public class Sub extends Super {
    static {
        foo = "bar";
    }

    public static void main (String[] args) {
        System.out.println(Super.foo);
    }
}

When I run it, it prints out bar.
My third (and last) class is Testing:

public class Testing {
    public static void main (String[] args) {
        System.out.println(Super.foo);
        System.out.println(Sub.foo);
        System.out.println(Super.foo);
    }
}

This prints:

foo
foo
foo

I don’t understand why the contents of foo vary depending on what class you’re accessing it from. Can anyone explain?

Answer 1

I don’t understand why the contents of foo vary depending on what class you’re accessing it from.

Basically it’s a matter of type initialization. The value of foo is set to "bar" when Sub is initialized. However, in your Testing class, the reference to Sub.foo is actually compiled into a reference to Super.foo, so it doesn’t end up initializing Sub, so foo never becomes "bar".

If you change your Testing code to:

public class Testing {
    public static void main (String[] args) {
        Sub.main(args);
        System.out.println(Super.foo);
        System.out.println(Sub.foo);
        System.out.println(Super.foo);
    }
}

Then it would print out “bar” four times, because the first statement would force Sub to be initialized, which would change the value of foo. It’s not a matter of where it’s accessed from at all.

Note that this isn’t just about class loading – it’s about class initialization. Classes can be loaded without being initialized. For example:

public class Testing {
    public static void main (String[] args) {
        System.out.println(Super.foo);
        System.out.println(Sub.class);
        System.out.println(Super.foo);
    }
}

That still prints “foo” twice, showing that Sub isn’t initialized – but it’s definitely loaded, and the program will fail if you delete the Sub.class file before running it, for example.