Java 8 Collector for Guava's ImmutableListMultimap<String,T>

Question

Looking for an easy way to collect collection of KeyAndValues having fields - String and List<T> to ImmutableListMultimap<String,T>? Tried to do something like,

 Collector.of(
          ImmutableListMultimap.Builder::new,
          ImmutableListMultimap.Builder<String,List<T>>::putAll,
          (b1, b2) -> b1.putAll(b2.build()),
          (builder) -> builder.build());

putAll takes Key, List<T>. I am not sure how to achieve this in combiner.

EDIT :

 @Value(staticConstructor = "of")
 private static class KeyAndValues<T> {

    private final String key;
    private final List<T> values;
}

Answer 1

Not sure about the methods available on KeyAndValues<T>, but how about this?

  public static <T> Collector<KeyAndValues<T>,
                              ImmutableListMultimap.Builder<String, T>, 
                              ImmutableListMultimap<String, T>> toImmutableMultimap()
  {
    return Collector.of(
        ImmutableListMultimap.Builder::new,
        (b, kav) -> b.putAll(kav.getKey(), kav.getValues()),
        (builder1, builder2) -> builder1.putAll(builder2.build()),
        (builder) -> builder.build());
  }

Alternatively, if you would prefer to hide the accumulator details, you can use something like this:

public static <T> Collector<KeyAndValues<T>,
                            ?,
                            ImmutableListMultimap<String, T>> toImmutableMultimap()
{
  return Collector.of(
    ImmutableListMultimap.Builder<String, T>::new,
    (b, kav) -> b.putAll(kav.getKey(), kav.getValues()),
    (builder1, builder2) -> builder1.putAll(builder2.build()),
    (builder) -> builder.build());
}