Java Generics - Can not cast K to Long

Question

I have below code which compiles perfectly fine with maven but Intellij keeps on giving me error saying Inconvertible types can not cast 'K' to java.lang.Long.

public class BPlusTree<K extends Comparable<K>, T> {

    public void debug(K time) {
      Long l = (Long) time;
    }
}

When I remove the extends Comparable<K> part from the class definition, intellij stops giving this error. What is happening here? I know this casting can throw a runtime ClassCastException but why the compilation error?

What did you actually mean when you wrote `K extends Comparable`? — Mike 'Pomax' Kamermans, Oct 05 '17 at 15:35
Probably not a compilation error. IntelliJ is probably just letting you know that your code is broken. Why the error goes away when you remove the constraint, I don't know. — Carcigenicate, Oct 05 '17 at 15:36
@Mike'Pomax'Kamermans I want to make sure that my 'K' is comparable (it provides compareTo method)? — Nullpointer, Oct 05 '17 at 15:38
@Nullpointer The fact that K extends Comparable says nothing about whether or not it necessarily can be cast to a long. "Broken" might not have been the right word. "Just asking for a runtime exception" would be better. — Carcigenicate, Oct 05 '17 at 15:41
@Nullpointer So then the question becomes: what is `K` going to be in your code, concretely. — Mike 'Pomax' Kamermans, Oct 05 '17 at 15:44

score 1 · Accepted Answer · answered Oct 05 '17 at 15:51

Comparable is a bit weird when dealing with generics. I believe the correct definition is below:

class BPlusTree<K extends Comparable<? super K>, T> {

    public void debug(K time) {
      Long l = (Long) time;
    }

}

There's a good explanation of why it needs to be this way here: Explanation of generic <T extends Comparable<? super T>> in collection.sort/ comparable code?

Java Generics - Can not cast K to Long

1 Answers1