public final @NotNull Collection<V> getOrPut(K key, Supplier<? extends V> defaultValue) {
    Collection<V> collection = getModifiable(key);
    if (collection.isEmpty()) {
      collection.add(defaultValue.get());
    }
    return collection;
  }
public final @NotNull Collection<V> getOrPut(K key, Supplier<? extends V> defaultValue)