Question

Given a KeyHolder interface such as the following:

public interface KeyHolder<K extends Key> {
    K getKey();
}

I’d like to create a class like this:

public KeyHolderSet<H extends KeyHolder<K extends Key>> extends HashSet<H> {
   public Set<K> getKeySet() {
       Set<K> keySet = new HashSet<K>();
       for (H keyHolder : this) {
           keySet.add(keyHolder.getKey());
       }
       return keySet;
   }
}

But that doesn’t work, the closest I can get is this:

public KeyHolderSet<H extends KeyHolder<? extends Key>> extends HashSet<H> {
   public <K extends Key> Set<K> getKeySet() {
       Set<K> keySet = new HashSet<K>();
       for (H keyHolder : this) {
           // Explicit cast to K
           keySet.add((K)keyHolder.getKey());
       }
       return keySet;
   }
}

Any way around this?

Answer 1

You need to write it like this:

public KeyHolderSet<K extends Key, H extends KeyHolder<K>> extends HashSet<H> {
    public Set<K> getKeySet() {
        ...
    }
}

Unfortunately you will have to declare the type of K first and it cannot be inferred.