Java - Are there any Stream Collectors that return ImmutableMap? [duplicate]

Question

I find myself wanting a variant of Collectors.toMap which returns an ImmutableMap, such that I can do:

ImmutableMap result = list.stream().collect(MyCollectors.toImmutableMap(
    tuple -> tuple._1(), tuple -> tuple._2());

(where tuple in this particular example is a Scala Tuple2)

I've just learned that such a method will be coming with Java-8 support in Guava 21 (yay!) but that sounds a good 6 months away. Does anyone know of any existing libraries (etc) which might implement this today?

ImmutableMap is not strictly required but seems the best choice as I require: lookup by key, and retaining original iteration order. Immutability is always preferred too.

Note that FluentIterable.toMap(Function) is not sufficient because I need both a key-mapping function as well as a value-mapping function.

Answer 1

You don't need to write an anonymous class for this collector. You can use Collector.of instead:

public static <T, K, V> Collector<T, ?, ImmutableMap<K,V>> toImmutableMap(
            Function<? super T, ? extends K> keyMapper,
            Function<? super T, ? extends V> valueMapper) {
    return Collector.of(
               ImmutableMap.Builder<K, V>::new,
               (b, e) -> b.put(keyMapper.apply(e), valueMapper.apply(e)),
               (b1, b2) -> b1.putAll(b2.build()),
               ImmutableMap.Builder::build);
}

Or if you don't mind collecting the results into a mutable map first and then copy the data into an immutable map, you can use the built-in toMap collector combined with collectingAndThen:

ImmutableMap<String, String> result = 
     list.stream()
         .collect(collectingAndThen(
             toMap(
                 tuple -> tuple._1(), 
                 tuple -> tuple._2()),
             ImmutableMap::copyOf));