Java通用继承/通用可比

Question

Say that I have a function that does things with a list of shapes. I can define a few useful classes:

public abstract class Shape implements Comparable<Shape> { }

public abstract class FourSidedShapes extends Shape { }

public class Rectangle extends FourSidedShapes{
   ...
   @Override
   public int compareTo(Shape o) {
    return 0;
   }
}

public class Square extends FourSidedShapes {... }

I have a helper container:

//Implements doubly-linked sorted list
public class SortedLinkedListNode<T extends Shape> {
   public SortedLinkedListNode<T> add(T e) { }
   public <E extends Shape> T get(E target) { }
   ...
}

I would like to do something like this: TreeSet<FourSidedShapes> myshapes;

If I do that, I have two problems. First, I want to keep the SortedLinkedList helper collection as generic as possible. So I would like to write the implementation assuming that it will hold Shape or some subclass of shape (including objects that have not been considered / implemented such as circles / triangles, etc. But if I do that, then I have to implement public int compareTo(Shape o) for both rectangle and square even though I really only want to implement public int compareTo(FourSidedShapes o) because I'm really only storing subclasses of FourSidedShapes in my collection. So my question is, how do I modify my SortedLinkedListNode so that it extends a compareTo that is no lower than the base class that it is storing? For example, if I am storing shapes, then it expects shapes to implement Comparable<Shapes> . If I'm storing FourSidedShapes, then it expects FourSidedShapes to implement Comparable<FourSidedShapes> . I suspect that the right answer involves making the parameter of Comparable generic, but I'm not sure what that look like. For example, something like

public class ShapeComparable<T extends Comparable<T>> {
   public abstract int compareTo(T other);
}

Answer 1

I am not completely clear on your question but I am assuming that you are asking how to make Rectangle or FourSidedShapes implement Comparable<FourSidedShapes> . Here is the code showing how to do that:

public abstract class Shape< T extends Shape<T> > implements Comparable<T> { }

public abstract class FourSidedShapes extends Shape<FourSidedShapes> {

    @Override
    public int compareTo(FourSidedShapes o) {
        return 0;
    }        
}

public class Rectangle extends FourSidedShapes{

    @Override
    public int compareTo(FourSidedShapes o) {
        return 0;
    }

}

public class Square extends FourSidedShapes{

    @Override
    public int compareTo(FourSidedShapes o) {
        return 0;
    }

}