Implementing Comparable interface for a generic class with Type parameter

Question

public class ClassX<T> implements Comparable<ClassX<T>> {

   private T o;

   public ClassX(T o) {
       this.o = o;
   }

   public T getObject() {
       return o;
   }

   public void setObject(T o) {
       this.o = o;
   }

   @Override
   public int compareTo(ClassX<T> arg0) {
       return o.compareTo(arg0.o);
   }
}

If I have a class like this and I want to implement the Comparable interface, I read that I have to change ClassX<T> to ClassX<T extends Comparable<T>>. But what if I also want to use my ClassX for objects that don't implement the Comparable interface, knowing that I would not be able to use the method compareTo() for those objects?

"what if I also want to use my ClassX for objects that don't implement the Comparable interface" - Then you will be basically in violation of the Comparable interface contract. — hfontanez
– hfontanez, Commented Nov 7, 2020 at 2:38
So you want something like "conditional implementation of Comparable"? That is, ClassX<T> will implement Comparable if and only if T implements Comparable? Did I understand correctly? — Sweeper
– Sweeper, Commented Nov 7, 2020 at 2:38
One (ugly) way to do this is to try to cast T to Comparable, and if it fails, throw a UnsupportedOperationException. Conditional implementation does not exist in Java. — Sweeper
– Sweeper, Commented Nov 7, 2020 at 2:52
@hfontanez Well, something similar to that happens when you try to sort a Stream of non-Comparable objects. If Stream uses this approach, I'd say it's fine to use it in our code too, albeit ugly. — Sweeper
– Sweeper, Commented Nov 7, 2020 at 2:57
OP, now that you said you are making a Pair class, I suggest not doing that. Write more specific classes instead. They are more future-proof, among other things. — Sweeper
– Sweeper, Commented Nov 7, 2020 at 3:01

newacct · Accepted Answer · 2020-11-08 07:40:55Z

1

Another option is, instead of implementing Comparable, have a static method that returns a Comparator, and this method can only be called when the constraint on T is met. So then your ClassX can be used with T that do not implement Comparable, but you can only obtain Comparators for ClassXs with T that do implement Comparable.

public class ClassX<T> {
    private T o;

    // ...

    public static <T extends Comparable<? super T>> Comparator<ClassX<T>> getComparator() {
        return (x, y) -> x.o.compareTo(y.o);
    }
}

answered Nov 8, 2020 at 7:40

newacct

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Comments

Serial Lazer · Accepted Answer · 2020-11-07 03:57:23Z

0

How about creating another layer of hierarchy to support this:

public class NonComparableClassX<T> implements Comparable<NonComparableClassX<T>> {
 // ....

  @Override
   public int compareTo(ClassX<T> arg0) {
     // throw Exception;
   }
}

public class ClassX<T> extends NonComparableClassX<T> {
 // ....

  @Override
   public int compareTo(ClassX<T> arg0) {
     // Implement comparables here
   }
}

This way you can encapsulate all objects as ClassX, which can also be of the type NonComparableClassX if need be

answered Nov 7, 2020 at 3:57

Serial Lazer

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