I need to overwrite CompareTo method in a subclass

Question

I have a class A which implements Comparable interface, and a class B which extends A. I need to find a way to overwrite compareTo in class B.

I found a similar question Cannot use comparable with father-son-grandson inheritance And tried to implement offered solution

Class A:

public class A<E extends A> implements Comparable<E>{
    String surname;
    int age;
      A(String surname, int age){
        this.surname = surname;
        this.age = age;
    }

   public  int compareTo(E o) {
        int res=0;
        res =this.surname.compareTo(o.surname);
        if(res==0)
            res = this.age-o.age;
        return res;
}

Class B:

public class B extends A<B> {
    int num;

    Student(String surname, int age, int num){
        super(surname,age);
        this.num = num;
    }
@Override
   public int compareTo(B o){
        int res=0;
        res =this.surname.compareTo(o.surname);
        if(res==0)
            res = this.age-o.age;
        if(res ==0){
            res= this.num-o.num;
        }
        return res;
    }

Exception:

Exception in thread "main" java.lang.ClassCastException: class T4.A cannot be cast to class T4.B (T4.A and T4.B are in unnamed module of loader 'app')

What code snippet do you get the classcastexception? This example works fine without such exception. Are you trying to cast like `B b = (A) a; — Laksitha Ranasingha, Mar 29 '19 at 18:41
check [java.lang.ClassCastException ](https://stackoverflow.com/questions/3511169/java-lang-classcastexception), your code seems to work fine for me too, its maybe the way you are calling the classes that is wrong — Mischiefz, Mar 29 '19 at 18:42

score 1 · Answer 1 · answered Mar 29 '19 at 18:45

Can you post the code which fails?

I just tried it on my own machine and it seems to work. The only time I could recreate the error was with following code: b.compareTo((B) a); Which can't work since you can't cast a parent type to its child type.

Also, I think it is just a copy error, but you are missing brackets in the provided code and constructor of class B does not match the class name.

score 0 · Answer 2 · answered Mar 29 '19 at 18:26

0

Your method in class B should be written as this instead:

@Override
public int compareTo(B o){
    int res=0;
    res =this.surname.compareTo(o.surname);
    if(res==0)
        res = this.age-o.age;
    if(res ==0){
        res= this.num-o.num;
    }
    return res;
}

answered Mar 29 '19 at 18:26

Connor Ruggles

41
1
7

Thanks for your comment, I added @Override but it didn't help – Sam Mar 29 '19 at 18:31
@Connor Ruggles, This is not going to help. The annotation has nothing to do with overriding in Java. That method has already been overridden. – Laksitha Ranasingha Mar 29 '19 at 18:37
@LaksithaRanasingha It won't solve the problem, but of course the annotation has something to do with overriding :/ – Dave Newton Mar 29 '19 at 18:48
@DaveNewton I don't think so. I was referring to the concept of override. It's a runtime operation and with or without an annotation, it stays the same. As long as method signature & return type are the same override works. So it doesn't. Of course, annotation provides compile time safety. – Laksitha Ranasingha Mar 29 '19 at 18:56
So it's related then. Saying it's "unrelated" is nonsense; it's *directly* related, as you stated. As i said, it's not *necessary*, but it's clearly related to overriding by definition. – Dave Newton Mar 29 '19 at 20:45
Picking words without understanding the whole doesn’t help @DaveNewton. – Laksitha Ranasingha Mar 29 '19 at 22:15
@LaksithaRanasingha Which is why I take issue with saying the Override annotation doesn't have anything to do with overriding. Saying an annotation explicitly about overriding isn't related is wrong, because it's imprecise (and because it's *only* about overriding), even if it isn't *required* to override a method. – Dave Newton Mar 29 '19 at 23:09
@LaksithaRanasingha ah my bad, I haven’t written java in a couple years and thought that was required for an override. – Connor Ruggles Mar 30 '19 at 19:28

score 0 · Answer 3 · answered Mar 29 '19 at 18:49

0

Student != B

The class B need to have a contructor called B

answered Mar 29 '19 at 18:49

FrV

260
2
9

I need to overwrite CompareTo method in a subclass

3 Answers3