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?
You can just use the Guava Multimaps factory:
Multimaps.index(foos, Foo::getId);
or wrap a call to Multimaps.index with a Collector 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);
}
}