Final keyword in method signatures

Possible Duplicate:
Final arguments in interface methods - what’s the point?

While trying to experiment a few things, I've ran into a problem that it's described in this page.

interface B {
    public int something(final int a);
}

abstract class C {
    public int other(final int b);
}

class A extends C implements B {

    public int something(int a) {
        return a++;
    }

    public int other(int b) {
        return b++
    }
}

Why is such feature possible? I don't know why it's possible to to make a final parameter into a non-final one by just overriding the method. Why is the final keyword ignored in a method signature? And how do I obligate sub-classes to use in their methods final variables?