Java Comparable - 自定义 compareTo 结果

Question

I have a question regarding a specific overriding of compareTo.

class Sjavac {
static class A implements Comparable<A> {
    @Override
    public int compareTo(A a) {
        return 1;
    }

    @Override
    public String toString() {
        return this.getClass().toString();
    }
}

static class B extends A {
    @Override
    public int compareTo(A a) {
        return 1;
    }
}

public static void main(String[] args) {
    List<A> list = new ArrayList<>();
    list.add(new A());
    list.add(new B());
    Collections.sort(list);
    for(A a : list){
        System.out.println(a);
    }
}
}

The result I get is that first A's string is printed, and then B's.

My question is, why is this the result? What exactly happends in this case? When A is compared to B, it is considered bigger, and when B is compared to A it is considered bigger. Than what determines that order?

Thanks

Answer 1

In this case the result is undefined because you can't sort a list where the elements are changing. So, if you want to know why this is the result you have to check how the sorting algorithm works.

Answer 2

You are comparing B to A and return B should come after A (in B's compare to method). Also, you are comparing A with A and saying the first one should come after the second one (in A's compare to method). Then, you have a list with A object and B object. Sorting this, A is compared to B, B is greater (comes later), and that is returned.

The trick happened here, why do Collections.sort method choose B's implementation of compareTo? Because it's late binding! It's inheritance. Check this implementation, with console printing...

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Sjavac {
static class A implements Comparable<A> {
    @Override
    public int compareTo(A a) {
        System.out.println("A");
        return 1;
    }

    @Override
    public String toString() {
        return this.getClass().toString();
    }
}

static class B extends A {
    @Override
    public int compareTo(A a) {
        System.out.println("Here");
        return 1;
    }
}

public static void main(String[] args) {
    List<A> list = new ArrayList<>();
    list.add(new A());
    list.add(new B());
    Collections.sort(list);
    for(A a : list){
        System.out.println(a);
    }
}
}