code: public static <T, K, V> ImmutableMapCollector<T, K, V> collector(Function<T, K> keyMapper, Function<T, V> valueMapper) {
          return new ImmutableMapCollector<>(keyMapper, valueMapper);
      }
docs: Returns a collector that accumulates elements into an immutable map.
  <p>
  Duplicate mappings are not merged - the old value is replaced.
  
  @param keyMapper
  @param valueMapper
  @return collector