Java可比和接口

Question

Lets Say you have ClassA, ClassB, ClassC, and interfaceA.

ClassA and ClassB implements interfaceA, The classC contains a List< interface >

Now lets say we add some values from ClassA and ClassB into ClassC List

What happens if ClassA implement different comparable method CompareTo than ClassB, and we call Collection.sort(list< interface >)

which CompareTo will be applied to sort the list ? or we must have the same CompareTo method in every Class that implement interfaceA

Answer 1

The Comparable interface is not a best approach. I would recommend using a Comparator passed into the Collection sort method.

Answer 2

I think you are not able to run such code. Imagine you have an abstract class:

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

and two classes which extend it:

public class One extends Number<One> {
    @Override
    public int compareTo(One o) {
        return -1;
    }
}

public class Two extends Number<Two> {
    @Override
    public int compareTo(Two o) {
        return 0;
    }
}

Then execution of the following snippet:

List<Number> list = new ArrayList<>();
list.add(new One());
list.add(new Two());
Collections.sort((List<Number>) list);

will throw a ClassCastException

 Exception in thread "main" java.lang.ClassCastException: comparable.One cannot be cast to comparable.Two 

In general your idea breaks the contract of the compareTo method (See Joshua Bloch Effective Java , Item 12):

• The implementor must ensure sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) for all x and y .
• The implementor must also ensure that the relation is transitive: (x.com- pareTo(y) > 0 && y.compareTo(z) > 0) implies x.compareTo(z) > 0.
• Finally, the implementor must ensure that x.compareTo(y) == 0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)) , for all z .
• It is strongly recommended, but not strictly required, that (x.compareTo(y) == 0) == (x.equals(y)).