Why doesn’t Map> match Map>?

Question

I have a method with the following signature:

public <T> int numberOfValues(Map<T, Set<?>> map)

However I can’t call it passing in a Map<String, Set<String>>. For instance, the following doesn’t compile:

Map<String, Set<String>> map = new HashMap<>();
numberOfValues(map);

The error message being that:

numberOfValues (java.util.Map<java.lang.String,java.util.Set<?>>) in class cannot be applied to (java.util.Map<java.lang.String,java.util.Set<java.lang.String>>)

However, if I change to the following all is fine:

public <T, V> int numberOfValues(Map<T, Set<V>> map)

However I’m not at all interested in V, as I just want to know the size of each of the sets.

For completeness sake, this is the whole method:

public <T, V> int numberOfValues(Map<T, Set<V>> map) {
    int n = 0;
    for (T key : map.keySet()) {
        n += map.get(key).size();
    }
    return n;
}

Which I’m aware it can also be accomplished like this, but isn’t the point of the question :)

public <T> int numberOfValues(Map<?, Set<T>> map) {
    int n = 0;
    for (Set<T> value : map.values()) {
        n += value.size();
    }
    return n;
}

Update: yet another way of achieving the same

public <T> int numberOfValues(Map<?, Set<T>> map) {
    int n = 0;
    for (Object key : map.keySet()) {
        n += map.get(key).size();
    }

    return n;
}

Final update: Thanks to Jorn’s answer, this is the final implementation...

public int numberOfValues(Map<?, ? extends Set<?>> map) {
    int n = 0;
    for (Set<?> value : map.values()) {
        n += value.size();
    }
    return n;
}

You haven't shown how you're trying to call the method, which makes it hard to help you. Please provide a [mcve]. — Jon Skeet, Nov 08 '16 at 14:31
Fixed, I added a code snippet of how the method is called with an empty map. — Alf, Nov 08 '16 at 14:34
See this thread : http://stackoverflow.com/questions/40149591/why-is-this-conversion-not-valid — jaudo, Nov 08 '16 at 14:47

score 9 · Accepted Answer · answered Nov 08 '16 at 14:43

9

You're missing the fact that Set<?> is also used as a generic parameter. And generics are invariant. i.e. when the parameter is Map<String, Set<?>>, the passed argument must be exactly Map<String, Set<?> (or a subtype of). Whereas with Set<V> the type argument is inferred.

You can solve this by using a bounded wildcard:

public <T> int numberOfValues(Map<T, ? extends Set<?>> map) {
    ...
}

answered Nov 08 '16 at 14:43

Jorn Vernee

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Thanks, that makes sense. I edited the question and added an implementation with no types at all. – Alf Nov 08 '16 at 15:50

score 0 · Answer 2 · answered Nov 08 '16 at 14:45

Hint:

In your numberOfValues method, it is completely legal to write something like

Set<Object> set = new HashSet<>();
set.add(1);
set.add("Powned");
map.put(null, set);

You can replace the null with a suitable key already in the map to make a valid mapping, utterly breaking the type safety in the surrounding code.

Either infer or explicitly define the type of the set, or add a wildcard bound preventing mutating map access (like ? extends Set<?>)

Why doesn’t Map> match Map>?

2 Answers2