无法从Guava MultiMap获取密钥

Question

I have the following code:

@Override
public boolean putAll(Multimap<? extends Index, ? extends V> multimap) {
    for (Index index : multimap.keySet()) {
        putAll(index, multimap.get(index));
    }
}

Where multimap.get(index) is a compilation error:

The method get(capture#5-of ? extends Index) in the type Multimap is not applicable for the arguments (Index)

Have I stumbled upon a famous generics gotcha? I don't quiet see what the problem can be.

Side note: I'm building a class that extends SetMultiMap because I have specific key matching requirements

Answer 1

Suppose you put in a Multimap<FooIndex, Integer> . Then you've got:

Multimap<FooIndex, Integer> map = ...;
Index plainIndex = ...;
Integer value = map.get(plainIndex);

That's a type failure, because Multimap.get takes a Key . I suspect you need to make this method generic:

@Override
public <Key extends Index> boolean putAll(Multimap<Key, ? extends V> multimap) {
    for (Key index : multimap.keySet()) {
        putAll(index, multimap.get(index));
    }
}

(I haven't tested it, but that makes more sense, IMO.)

Answer 2

The signature of the get method is

get(K key) 

Your Multimap is declared as

Multimap<? extends Index, ? extends V> multimap

So you don't know the type of the key. You know that it is or that it extends Index , but you don't know its type. So passing it an instance of Index is invalid.

Answer 3

您是否尝试过仅使用常规的Multimap和Equivalence.wrap -ing键？