Java Polymorphism : How can I avoid type casting input parameters?

Question

Assume we have a Parent interface with compare() function.

public interface Parent {
    public int compare(Parent otherParent);
}

Suppose that children Child1, Child2, Child3 implement this interface Parent

public class Child1 implements Parent {
    @Override
    public int compare(Parent other) {
        Child1 otherChild = (Child1)other;
    }
}

Also, I am using generics <T extends Parent> everywhere else in the code. So I need to compare two objects of type T from other parts of code.

I understand this is a bad design as I am typecasting the Parent object in compare() function and I don't even know whether the input is of type Child1.

How can I avoid this type casting while using generics?

Answer 1

Why not this?

interface Parent<T extends Parent<?>> {
    int compare(T type);
}

class Child1 implements Parent<Child1>{

    @Override
    public int compare(Child1 type) {
        return 0;
    }
}

Edit: To insure correct usage you can use

interface Parent<T extends Parent<T>>{ /* ... */ }   //instead of wildcard

But to be honest that "loop" doesn't look pretty, and since Generics in Java don't work at RunTime (more information), they're essentially syntactic-sugar for that same cast you called "bad design" so I don't think your current approach is bad.