CollectorUtils.java

/**
* Returns a collector that will collect the stream into an immutable map.
*
* @param keyMapper   A function that given a stream element returns a key
* @param valueMapper A function that given a stream element returns a value
* @param <T>         The type of element in the stream
* @param <K>         The type of keys used in the map
* @param <V>         the type of values contained in the map
* @return a collector that will collect the a stream into an immutable map
*/
public static <T, K, V> Collector<T, Builder<K, V>, Map<K, V>> toImmutableMap(
        Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends V> valueMapper) {
    return Collector.of(
            ImmutableMap::builder,
            (b, e) -> b.put(keyMapper.apply(e), valueMapper.apply(e)),
            (m1, m2) -> m1.putAll(m2.build()),
            Builder::build);
}