定义实现可比的接口

Question

I'm trying to define an interface such that all implementing classes must be Comparable to themselves and their subclasses.

For example, consider:

public interface Rating implements Comparable<Rating> {}

This means that implementing classes must be Comparable to all Ratings:

public class A implements Rating {
    public int compareTo(Rating r) {return 0;}
}

I'd like to loosen that requirement so that I can define a class like:

public class A implements Rating {
    public int compareTo(A a) {return 0;}
}

Answer 1

I expect you're trying to get at something like this:

public class Rating implements Comparable<? extends Rating> {
   ...
}

or maybe (although I'm not sure you can do this):

public interface Rating extends Comparable<? extends Rating> {
   ...
}

See this page at Oracle for more information on bounded wildcards.

Answer 2

1. I imagine you probably want your "compareTo()" to return something besides "0" ;)

2. You can use "instanceof" to see if an arbitrary object is some particular class (or a subclass)

3. I'm not sure overloading "compareTo()" is necessarily a good idea. You might even want to make the input argument "Object o".

IMHO .. pSM