Combine two Maps into a MultiMap

Question

What is the best way to combine two Maps into a single Guava MultiMap in Java?

For example:

• Map1 contains (1, a) and (2, b)
• Map2 contains (2, c) and (3, d)

Then the resulting combined multimap would contain

• (1, {a}), (2, {b, c}), and (3, {d})

This is my current solution:

Multimap<T, K> combineMaps(Map<T, K> map1, Map<T, K> map2) {
    Multimap<T, K> multimap = new MultiMap();
    for (final Map.Entry<T, K> entry : map1.entrySet()) {
        multimap.put(entry.getKey(), entry.getValue());
    }
    for (final Map.Entry<T, K> entry : map2.entrySet()) {
        multimap.put(entry.getKey(), entry.getValue());
    }
    return multimap;
}

Answer 1

...What sort of multimaps are these? Are they from Guava, or some other library?

In Guava, you could do

multimap.putAll(Multimaps.forMap(map1));
multimap.putAll(Multimaps.forMap(map2));

Answer 2

Your solution looks fine. You could initialize like this:

Multimap<T, K> multimap = new MultiMap(map1);

and then only iterate through the second map, however the complexity/speed is the same.