guava/src/com/google/common/collect/ImmutableMultiset.java
public static <E> Collector<E, ?, ImmutableMultiset<E>> toImmutableMultiset() {
    return toImmutableMultiset(Function.identity(), e -> 1);
  }
public static <E> Collector<E, ?, ImmutableMultiset<E>> toImmutableMultiset() {
    return toImmutableMultiset(Function.identity(), e -> 1);
  }
Returns a {@code Collector} that accumulates the input elements into a new {@code
ImmutableMultiset}. Elements iterate in order by the <i>first</i> appearance of that element in
encounter order.

@since 21.0