private final BinaryOperator<V> mergeFunction;
    private @Nullable EnumMap<K, V> map = null;

    EnumMapAccumulator(BinaryOperator<V> mergeFunction) {
      this.mergeFunction = mergeFunction;
    }

    void put(K key, V value) {
      if (map == null) {
        map = new EnumMap<>(singletonMap(key, value));
      } else {
        map.merge(key, value, mergeFunction);
      }
    }

    private final BinaryOperator<V> mergeFunction;
    private @Nullable EnumMap<K, V> map = null;

    EnumMapAccumulator(BinaryOperator<V> mergeFunction) {
      this.mergeFunction = mergeFunction;
    }

    void put(K key, V value) {
      if (map == null) {
        map = new EnumMap<>(singletonMap(key, value));
      } else {
        map.merge(key, value, mergeFunction);
      }
    }

  // unboxed
  @J2ktIncompatible
  public void testToTableNullMerge() {
    // TODO github.com/google/guava/issues/6824 - the null merge feature is not compatible with the
    // current nullness annotation of the mergeFunction parameter. Work around with casts.
    BinaryOperator<@Nullable Integer> mergeFunction = (v1, v2) -> null;

          Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction) {
    checkNotNull(keyFunction);
    checkNotNull(valueFunction);
    checkNotNull(mergeFunction);
    return Collectors.collectingAndThen(
        Collectors.toMap(keyFunction, valueFunction, mergeFunction, LinkedHashMap::new),
        ImmutableMap::copyOf);
  }

          Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction) {
    checkNotNull(comparator);
    checkNotNull(keyFunction);
    checkNotNull(valueFunction);
    checkNotNull(mergeFunction);
    return Collectors.collectingAndThen(
        Collectors.toMap(
            keyFunction, valueFunction, mergeFunction, () -> new TreeMap<K, V>(comparator)),
        ImmutableSortedMap::copyOfSorted);
  }

          java.util.function.Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction,
          java.util.function.Supplier<I> tableSupplier) {
    return TableCollectors.<T, R, C, V, I>toTable(
        rowFunction, columnFunction, valueFunction, mergeFunction, tableSupplier);
  }

  /**
   * Returns an immutable cell with the specified row key, column key, and value.
   *

      Collector<T, ?, ImmutableMap<K, V>> toImmutableMap(
          Function<? super T, ? extends K> keyFunction,
          Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction) {
    throw new UnsupportedOperationException();
  }

  @GwtIncompatible @J2ktIncompatible private static final long serialVersionUID = 0xdecaf;

          Function<? super T, ? extends K> keyFunction,
          Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction) {
    return CollectCollectors.toImmutableMap(keyFunction, valueFunction, mergeFunction);
  }

  /**
   * Returns the empty map. This map behaves and performs comparably to {@link

          Function<? super T, ? extends K> keyFunction,
          Function<? super T, ? extends V> valueFunction,
          BinaryOperator<V> mergeFunction) {
    return CollectCollectors.toImmutableMap(keyFunction, valueFunction, mergeFunction);
  }

  /**
   * Returns the empty map. This map behaves and performs comparably to {@link