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;
}

Well, there often is no _best_ way and your way seems sensible to me. Do you have any problems with this? If not, I'd just stick with it (except that I'd add the generics to `new MultiMap()` too :) ). — Thomas, Feb 17 '12 at 07:01

score 24 · Accepted Answer · answered Feb 17 '12 at 07:04

24

...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));

answered Feb 17 '12 at 07:04

Louis Wasserman

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They are indeed Guava and this is what I was looking for. My google skills seem to have failed me this time. Thanks – Jake Walsh Feb 17 '12 at 07:39

score 1 · Answer 2 · answered Feb 17 '12 at 07:04

1

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.

answered Feb 17 '12 at 07:04

Bernd Elkemann

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Combine two Maps into a MultiMap

2 Answers2

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