Cleanest way to create a Guava Multimap from a Java 8 stream

Question

I have a List<Foo> and want a Multimap<String, Foo> where we've grouped the Foo's by their getId() function.

I am using Java 8 and its almost awesome in that you can do:

List<Foo> foos = ... Map<String, List<Foo>> foosById = foos.stream().collect(groupingBy(Foo::getId)); 

However, I have a good amount of code that wants a MultiMap<String, Foo> so this doesnt save me anything and I'm back to using a for-loop to create my Multimap. Is there a nice "functional" way that I am missing?

Answer 1

You can just use the Guava Multimaps factory:

ImmutableMultimap<String, Foo> foosById = Multimaps.index(foos, Foo::getId); 

or wrap a call to Multimaps.index with a Collector<T, A, R> interface (shown below, in an unoptimized naive implementation).

Multimap<String, Foo> collect = foos.stream()         .collect(MultimapCollector.toMultimap(Foo::getId)); 

and the Collector:

public class MultimapCollector<T, K, V> implements Collector<T, Multimap<K, V>, Multimap<K, V>> {      private final Function<T, K> keyGetter;     private final Function<T, V> valueGetter;      public MultimapCollector(Function<T, K> keyGetter, Function<T, V> valueGetter) {         this.keyGetter = keyGetter;         this.valueGetter = valueGetter;     }      public static <T, K, V> MultimapCollector<T, K, V> toMultimap(Function<T, K> keyGetter, Function<T, V> valueGetter) {         return new MultimapCollector<>(keyGetter, valueGetter);     }      public static <T, K, V> MultimapCollector<T, K, T> toMultimap(Function<T, K> keyGetter) {         return new MultimapCollector<>(keyGetter, v -> v);     }      @Override     public Supplier<Multimap<K, V>> supplier() {         return ArrayListMultimap::create;     }      @Override     public BiConsumer<Multimap<K, V>, T> accumulator() {         return (map, element) -> map.put(keyGetter.apply(element), valueGetter.apply(element));     }      @Override     public BinaryOperator<Multimap<K, V>> combiner() {         return (map1, map2) -> {             map1.putAll(map2);             return map1;         };     }      @Override     public Function<Multimap<K, V>, Multimap<K, V>> finisher() {         return map -> map;     }      @Override     public Set<Characteristics> characteristics() {         return ImmutableSet.of(Characteristics.IDENTITY_FINISH);     } } 

Answer 2

Guava 21.0 introduced several methods that return Collector instances which will convert a Stream into a Multimap grouped by the result of applying a function to its elements. These methods are:

• ImmutableListMultimap.toImmutableListMultimap — creates an ImmutableListMultimap
• ImmutableSetMultimap.toImmutableSetMultimap — creates an ImmutableSetMultimap
• Multimaps.toMultimap — Creates a Multimap using the given Supplier

ImmutableListMultimap<String, Foo> foosById = foos.stream().collect(         ImmutableListMultimap.toImmutableListMultimap(                 Foo::getId, Function.identity())); 

ImmutableSetMultimap<String, Foo> foosById = foos.stream().collect(         ImmutableSetMultimap.toImmutableSetMultimap(                 Foo::getId, Function.identity())); 

HashMultimap<String, Foo> foosById = foos.stream().collect(         Multimaps.toMultimap(                 Foo::getId, Function.identity(), HashMultimap::create) );