Java Type Erasure Problem

Question

I\'ve made an example to demonstrate my problem:

Metrical.java

public interface Metrical
{
    double distance(T other);
}

Answer 1

Sorry, the bad news is that you cannot do this:

public class Algorithm<T extends Metrical<T>> {
    public void compute(Set<T> objects) {
    }

    public void compute(Set<Pair<T>> pairs) {
    }
}

Due to erasure, both will erase to the same signature. There is no way around this short of renaming one of the methods.

Answer 2

Use public class Widget<K, P> implements Metrical<K extends Widget<P>>.

public double distance(Widget other) {} becomes public double distance(Widget<P> other) {}

Answer 3

Sadly, this is the major area where Java Generics falls down... there is just no good solution.

I've generally resorted to making a new class, with the interface as Set<Pair<T>>, but that wraps Set<Pair<T>> (without extending it, which would cause the same problem).

Answer 4

No there isn't.

You have to remember that someone could call your method with just a vanilla Set, in which case which one would be called?

That's why you can't do it. Just like you can't do:

interface A {
  void blah(Set set);
  void blah(Set<T> set);
}

Same problem.

The type information isn't available at runtime (ie type erasure).

Answer 5

I've written an article about type erasure which can be of your interest. It gives the common widely known solution and also a tricky way to circumvent the problem. I don't know if it will be relevant for you. Anyway, it contains some techniques which may be useful under certain circumstances.

See also: Using TypeTokens to retrieve generic parameters

I hope it helps.