Question

Could anybody explain, why next code isn’t compiling?
I’m creating a partially-specialized Map and Map.Entry for it:

public class Trie<T> implements Map<String, T> {
    private class TrieEntry<S> implements Map.Entry<String, S> {
        // stupid implementation here
    }
    // uninterested code here
}

Here everything is ok, but then i’m implementing entrySet() method:

public Set<java.util.Map.Entry<String, T>> entrySet() {
    Set<java.util.Map.Entry<String, T>> x = new HashSet<TrieEntry<T>>();
    // some uninterested code here
}

Eclipse says

“Type mismatch: cannot convert from HashSet<Trie<T>.TrieEntry<T>> to Set<Map.Entry<String,T>>“

So, in my mind TrieEntry<T> should be unrolled to Map.Entry<String, T> and it accords to expression in definition.

Where am I wrong?

Answer 1

You are right that a TrieEntry<T> is a Map.Entry<String, T>. A HashSet<TrieEntry<T>> is also a Set<TrieEntry<T>>, but it is not a Set<Map.Entry<String, T>>.

If it were you could do this:

Set<TrieEntry<T>> trieSet = ...;
Set<Map.Entry<String, T>> mapSet = trieSet;
mapSet.add(mapEntry);

So now trieSet would now contain a Map.Entry<String, T>! This would break generics.

So how to solve this particular problem? Easy – use a bounded wildcard:

Set<? extends Map.Entry<String, T>> x = new HashSet<TrieEntry<T>>();

You can read ? extends Map.Entry<String, T> as "anything which is at least a Map.Entry<String, T>".

---

Okay, now to your implementation problem. I actually believe, that this is not solveable in this way because of the current definition of the method entrySet(). It should return ? extends Map.Entry<String, T>, but it does return Map.Entry<String, T>.

There is actually a bug report (or feature request) for exactly this problem. Looking at the submit date and the priority, a fix can’t be expected soon.

So you have two options:

1. Drop your TrieEntry class and try to use Map.Entry instead, storing information in your keys and/or values.

2. Drop the Map interface and let your custom entrySet method return ? extends Map.Entry<String, T>.

Both solutions may not be ideal, and there may be better solutions, but this is all what I currently can tell you.