Compact way to create Guava Multimaps?

Question

If I want to create a new Multimap with simple defaults, I curently need to do something like:

private final Multimap<Key, Value> providersToClasses = Multimaps
        .newListMultimap(
                new HashMap<Key, Collection<Value>>(),
                new Supplier<List<Value>>() {
                    @Override
                    public List<Value> get() {
                        return Lists.newArrayList();
                    }
                });

...because Java can't infer the correct types if Maps.newHashMap is used for the backing map. Of course, this can be refactored into a separate method, but is there already a way to write it more concisely?

score 62 · Answer 1 · edited May 24 '16 at 14:19

62

Why aren't you using ArrayListMultimap.create() for such a simple case? It's the default way to create the simple HashMap/ArrayList that is probably the most common used multimap.

edited May 24 '16 at 14:19

CubeJockey

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answered May 17 '12 at 14:25

Daniel Teply

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2

I think you mean `ArrayListMultimap`. `HashMultimap` is for `HashMap`/`HashSet`. – Louis Wasserman May 17 '12 at 15:13
14

We were concerned about this, so we put this doc on newListMultimap(): "Call this method only when the simpler methods ArrayListMultimap.create() and LinkedListMultimap.create() won't suffice." – Kevin Bourrillion May 17 '12 at 18:03
Hi, @KevinBourrillion. Forcing that factory parameter creates lots of noise for sometimes very simple usecases. What would be very useful to have simplified version, something like that NewListMultimap(Map> map) which would create multiMaps using some default factory. I have lot's MultivaluedMap classes from Jersey to convert to multimaps and that conversion code makes it very difficult to read the code. – husayt Dec 27 '12 at 19:51
2

As of version 21.0, the documentation states that this method will be deprecated, in favor of ```MultimapBuilder.hashKeys().arrayListValues().build()```. – Arve Feb 09 '17 at 10:18

Sam Berry · Answer 2 · 2016-08-06T16:03:56.650

24

I run into this problem when writing clients and building up maps of query params. A nice succinct pattern I like for constructing multi-maps is to use ImmutableMultiMap#builder

Multimap<String, String> queryParams = 
  ImmutableMultimap.<String, String>builder()
    .put("key-1", "value-1")
    .put("key-1", "value-2")
    .build();

edited Aug 06 '16 at 16:03

answered Aug 05 '16 at 19:14

Sam Berry

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Pyves · Accepted Answer · 2018-04-14T17:41:36.573

The Guava documentation states that the create method advocated by some other answers "will soon be deprecated" in favour of the different forms presented below, and should therefore be avoided.

From Guava 21.0 onwards, the recommended way of creating a Multimap object where values are stored in ArrayList collections is the following:

MultimapBuilder.hashKeys().arrayListValues().build();

You can also use parameters if you want to specify the expected number of keys in your map and the expected number of values per key:

MultimapBuilder.hashKeys(expectedKeys).arrayListValues(expectedValuesPerKey).build();

Finally, you can create a new Multimap from an existing one using this construct:

MultimapBuilder.hashKeys().arrayListValues().build(multimap);

If you want to use data structures other than ArrayLists in your Multimap, you can replace the call to arrayListValues() by a number of other ones, listed here.

In 28.0/28.1 it isn't (but `create` still "will soon be deprecated") — Alexey Romanov, Sep 19 '19 at 13:52

score 6 · Answer 4 · answered Oct 28 '16 at 05:44

6

Here is compact solution:

Multimap<Integer, String> multi = HashMultimap.create();

answered Oct 28 '16 at 05:44

Алексей Мальченко

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Lii · Answer 5 · 2016-09-16T08:34:46.560

In Java 8 this is much nicer, for all kinds of multimaps. This is for two reasons:

Because the compiler uses target types for type inference, you don't have to provide explicit type arguments for the HashMap contructor.
Constructor method references can be used to create the ArrayList factory.

It looks like this:

Multimap<Key, Value> providersToClasses =
    Multimaps.newListMultimap(new HashMap<>(), ArrayList::new);

score 2 · Answer 6 · answered May 21 '12 at 09:13

To answer the original type inference problem, though, you can also specify the generic types on a static method using Maps.<Key, Collection<Value>>newHashMap(), but it's certainly not more concise than new HashMap<Key, Collection<Value>>() (it may be more consistent).

Compact way to create Guava Multimaps?

6 Answers6