guava: Best way to iterate over the key->collection entries of a Multimap?

Question

I'm looking for the corresponding way, for Multimap, to iterate over entries of a Map, namely:

Map<K,V> map = ...;
for (Map.Entry<K,V> entry : map.entrySet())
{
    K k = entry.getKey();
    V v = entry.getValue();
}

Which of the following is better? (or perhaps more importantly, what are the differences?)

Multimap<K,V> mmap = ...;
for (Map.Entry<K,Collection<V>> entry : mmap.asMap().entrySet())
{
    K k = entry.getKey();
    Collection<V> v = entry.getValue();
}

or

Multimap<K,V> mmap = ...;
for (K k : mmap.keySet())
{
    Collection<V> v = mmap.get(k);
}

score 14 · Accepted Answer · answered Jul 14 '11 at 01:47

14

They're both valid; the second tends to be a lot easier to read (especially as you can get an actual List out of a ListMultimap and so on), but the first might be more efficient (to a degree that may or may not matter to you).

answered Jul 14 '11 at 01:47

Kevin Bourrillion

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1

Could you explain why first approach might be more efficient? – user482594 Nov 05 '16 at 21:44

score 3 · Answer 2 · answered Jul 12 '11 at 18:38

I would say that the Guava folks were considerate enough to allow you to do it either way, depending on your needs. For example, if the structure does is important, and you wish to view your multimap in a grouped manner, the latter would be preferential. OTOH, if you wish to view the multimap as a set of key-value pairs, where duplicate keys are insignificant, you might prefer to use mmap.entries. It's really a matter of what is most appropriate in the context that your multimap is being used.

Actually, I don't think that's the distinction that the OP is asking about. — Kevin Bourrillion, Jul 14 '11 at 01:46

guava: Best way to iterate over the key->collection entries of a Multimap?

2 Answers2