https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.RetainedWith;
import com.google.j2objc.annotations.Weak;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.Arrays;
import java.util.Collection;
import java.util.Comparator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
import org.checkerframework.checker.nullness.qual.Nullable;


A SetMultimap whose contents will never change, with many other important properties detailed at ImmutableCollection. 
See the Guava User Guide article on  immutable collections.

Author:Mike Ward
Since:2.0
/**
 * A {@link SetMultimap} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>.
 *
 * @author Mike Ward
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)
public class ImmutableSetMultimap<K, V> extends ImmutableMultimap<K, V>
    implements SetMultimap<K, V> {
  
Returns a Collector that accumulates elements into an ImmutableSetMultimap whose keys and values are the result of applying the provided mapping functions to the input elements. 
For streams with defined encounter order, that order is preserved, but entries are grouped by key.

Example:


static final Multimap<Character, String> FIRST_LETTER_MULTIMAP =
    Stream.of("banana", "apple", "carrot", "asparagus", "cherry")
        .collect(toImmutableSetMultimap(str -> str.charAt(0), str -> str.substring(1)));
// is equivalent to
static final Multimap<Character, String> FIRST_LETTER_MULTIMAP =
    new ImmutableSetMultimap.Builder<Character, String>()
        .put('b', "anana")
        .putAll('a', "pple", "sparagus")
        .putAll('c', "arrot", "herry")
        .build();



Since:21.0
/**
   * Returns a {@link Collector} that accumulates elements into an {@code ImmutableSetMultimap}
   * whose keys and values are the result of applying the provided mapping functions to the input
   * elements.
   *
   * <p>For streams with {@linkplain java.util.stream#Ordering defined encounter order}, that order
   * is preserved, but entries are <a href="ImmutableMultimap.html#iteration">grouped by key</a>.
   *
   * <p>Example:
   *
   * <pre>{@code
   * static final Multimap<Character, String> FIRST_LETTER_MULTIMAP =
   *     Stream.of("banana", "apple", "carrot", "asparagus", "cherry")
   *         .collect(toImmutableSetMultimap(str -> str.charAt(0), str -> str.substring(1)));
   *
   * // is equivalent to
   *
   * static final Multimap<Character, String> FIRST_LETTER_MULTIMAP =
   *     new ImmutableSetMultimap.Builder<Character, String>()
   *         .put('b', "anana")
   *         .putAll('a', "pple", "sparagus")
   *         .putAll('c', "arrot", "herry")
   *         .build();
   * }</pre>
   *
   * @since 21.0
   */
  @Beta
  public static <T, K, V> Collector<T, ?, ImmutableSetMultimap<K, V>> toImmutableSetMultimap(
      Function<? super T, ? extends K> keyFunction,
      Function<? super T, ? extends V> valueFunction) {
    checkNotNull(keyFunction, "keyFunction");
    checkNotNull(valueFunction, "valueFunction");
    return Collector.of(
        ImmutableSetMultimap::<K, V>builder,
        (builder, t) -> builder.put(keyFunction.apply(t), valueFunction.apply(t)),
        ImmutableSetMultimap.Builder::combine,
        ImmutableSetMultimap.Builder::build);
  }

  
Returns a Collector accumulating entries into an ImmutableSetMultimap. Each input element is mapped to a key and a stream of values, each of which are put into the resulting Multimap, in the encounter order of the stream and the encounter order of the streams of values. 
Example:


static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
    Stream.of("banana", "apple", "carrot", "asparagus", "cherry")
        .collect(
            flatteningToImmutableSetMultimap(
                 str -> str.charAt(0),
                 str -> str.substring(1).chars().mapToObj(c -> (char) c));
// is equivalent to
static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
    ImmutableSetMultimap.<Character, Character>builder()
        .putAll('b', Arrays.asList('a', 'n', 'a', 'n', 'a'))
        .putAll('a', Arrays.asList('p', 'p', 'l', 'e'))
        .putAll('c', Arrays.asList('a', 'r', 'r', 'o', 't'))
        .putAll('a', Arrays.asList('s', 'p', 'a', 'r', 'a', 'g', 'u', 's'))
        .putAll('c', Arrays.asList('h', 'e', 'r', 'r', 'y'))
        .build();
// after deduplication, the resulting multimap is equivalent to
static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
    ImmutableSetMultimap.<Character, Character>builder()
        .putAll('b', Arrays.asList('a', 'n'))
        .putAll('a', Arrays.asList('p', 'l', 'e', 's', 'a', 'r', 'g', 'u'))
        .putAll('c', Arrays.asList('a', 'r', 'o', 't', 'h', 'e', 'y'))
        .build();
 }


Since:21.0
/**
   * Returns a {@code Collector} accumulating entries into an {@code ImmutableSetMultimap}. Each
   * input element is mapped to a key and a stream of values, each of which are put into the
   * resulting {@code Multimap}, in the encounter order of the stream and the encounter order of the
   * streams of values.
   *
   * <p>Example:
   *
   * <pre>{@code
   * static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
   *     Stream.of("banana", "apple", "carrot", "asparagus", "cherry")
   *         .collect(
   *             flatteningToImmutableSetMultimap(
   *                  str -> str.charAt(0),
   *                  str -> str.substring(1).chars().mapToObj(c -> (char) c));
   *
   * // is equivalent to
   *
   * static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
   *     ImmutableSetMultimap.<Character, Character>builder()
   *         .putAll('b', Arrays.asList('a', 'n', 'a', 'n', 'a'))
   *         .putAll('a', Arrays.asList('p', 'p', 'l', 'e'))
   *         .putAll('c', Arrays.asList('a', 'r', 'r', 'o', 't'))
   *         .putAll('a', Arrays.asList('s', 'p', 'a', 'r', 'a', 'g', 'u', 's'))
   *         .putAll('c', Arrays.asList('h', 'e', 'r', 'r', 'y'))
   *         .build();
   *
   * // after deduplication, the resulting multimap is equivalent to
   *
   * static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
   *     ImmutableSetMultimap.<Character, Character>builder()
   *         .putAll('b', Arrays.asList('a', 'n'))
   *         .putAll('a', Arrays.asList('p', 'l', 'e', 's', 'a', 'r', 'g', 'u'))
   *         .putAll('c', Arrays.asList('a', 'r', 'o', 't', 'h', 'e', 'y'))
   *         .build();
   * }
   * }</pre>
   *
   * @since 21.0
   */
  @Beta
  public static <T, K, V>
      Collector<T, ?, ImmutableSetMultimap<K, V>> flatteningToImmutableSetMultimap(
          Function<? super T, ? extends K> keyFunction,
          Function<? super T, ? extends Stream<? extends V>> valuesFunction) {
    checkNotNull(keyFunction);
    checkNotNull(valuesFunction);
    return Collectors.collectingAndThen(
        Multimaps.flatteningToMultimap(
            input -> checkNotNull(keyFunction.apply(input)),
            input -> valuesFunction.apply(input).peek(Preconditions::checkNotNull),
            MultimapBuilder.linkedHashKeys().linkedHashSetValues()::<K, V>build),
        ImmutableSetMultimap::copyOf);
  }

  
Returns the empty multimap. 
/** Returns the empty multimap. */
  // Casting is safe because the multimap will never hold any elements.
  @SuppressWarnings("unchecked")
  public static <K, V> ImmutableSetMultimap<K, V> of() {
    return (ImmutableSetMultimap<K, V>) EmptyImmutableSetMultimap.INSTANCE;
  }

  
Returns an immutable multimap containing a single entry. 
/** Returns an immutable multimap containing a single entry. */
  public static <K, V> ImmutableSetMultimap<K, V> of(K k1, V v1) {
    ImmutableSetMultimap.Builder<K, V> builder = ImmutableSetMultimap.builder();
    builder.put(k1, v1);
    return builder.build();
  }

  
Returns an immutable multimap containing the given entries, in order. Repeated occurrences of an entry (according to Object.equals) after the first are ignored. 
/**
   * Returns an immutable multimap containing the given entries, in order. Repeated occurrences of
   * an entry (according to {@link Object#equals}) after the first are ignored.
   */
  public static <K, V> ImmutableSetMultimap<K, V> of(K k1, V v1, K k2, V v2) {
    ImmutableSetMultimap.Builder<K, V> builder = ImmutableSetMultimap.builder();
    builder.put(k1, v1);
    builder.put(k2, v2);
    return builder.build();
  }

  
Returns an immutable multimap containing the given entries, in order. Repeated occurrences of an entry (according to Object.equals) after the first are ignored. 
/**
   * Returns an immutable multimap containing the given entries, in order. Repeated occurrences of
   * an entry (according to {@link Object#equals}) after the first are ignored.
   */
  public static <K, V> ImmutableSetMultimap<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
    ImmutableSetMultimap.Builder<K, V> builder = ImmutableSetMultimap.builder();
    builder.put(k1, v1);
    builder.put(k2, v2);
    builder.put(k3, v3);
    return builder.build();
  }

  
Returns an immutable multimap containing the given entries, in order. Repeated occurrences of an entry (according to Object.equals) after the first are ignored. 
/**
   * Returns an immutable multimap containing the given entries, in order. Repeated occurrences of
   * an entry (according to {@link Object#equals}) after the first are ignored.
   */
  public static <K, V> ImmutableSetMultimap<K, V> of(
      K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
    ImmutableSetMultimap.Builder<K, V> builder = ImmutableSetMultimap.builder();
    builder.put(k1, v1);
    builder.put(k2, v2);
    builder.put(k3, v3);
    builder.put(k4, v4);
    return builder.build();
  }

  
Returns an immutable multimap containing the given entries, in order. Repeated occurrences of an entry (according to Object.equals) after the first are ignored. 
/**
   * Returns an immutable multimap containing the given entries, in order. Repeated occurrences of
   * an entry (according to {@link Object#equals}) after the first are ignored.
   */
  public static <K, V> ImmutableSetMultimap<K, V> of(
      K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
    ImmutableSetMultimap.Builder<K, V> builder = ImmutableSetMultimap.builder();
    builder.put(k1, v1);
    builder.put(k2, v2);
    builder.put(k3, v3);
    builder.put(k4, v4);
    builder.put(k5, v5);
    return builder.build();
  }

  // looking for of() with > 5 entries? Use the builder instead.

  
Returns a new Builder. 
/** Returns a new {@link Builder}. */
  public static <K, V> Builder<K, V> builder() {
    return new Builder<>();
  }

  
A builder for creating immutable SetMultimap instances, especially public static
final multimaps ("constant multimaps"). Example: 

static final Multimap<String, Integer> STRING_TO_INTEGER_MULTIMAP =
    new ImmutableSetMultimap.Builder<String, Integer>()
        .put("one", 1)
        .putAll("several", 1, 2, 3)
        .putAll("many", 1, 2, 3, 4, 5)
        .build();



Builder instances can be reused; it is safe to call build multiple times to build multiple multimaps in series. Each multimap contains the key-value mappings in the previously created multimaps. 
Since:2.0
/**
   * A builder for creating immutable {@code SetMultimap} instances, especially {@code public static
   * final} multimaps ("constant multimaps"). Example:
   *
   * <pre>{@code
   * static final Multimap<String, Integer> STRING_TO_INTEGER_MULTIMAP =
   *     new ImmutableSetMultimap.Builder<String, Integer>()
   *         .put("one", 1)
   *         .putAll("several", 1, 2, 3)
   *         .putAll("many", 1, 2, 3, 4, 5)
   *         .build();
   * }</pre>
   *
   * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
   * multiple multimaps in series. Each multimap contains the key-value mappings in the previously
   * created multimaps.
   *
   * @since 2.0
   */
  public static final class Builder<K, V> extends ImmutableMultimap.Builder<K, V> {
    
Creates a new builder. The returned builder is equivalent to the builder generated by ImmutableSetMultimap.builder. 
/**
     * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
     * ImmutableSetMultimap#builder}.
     */
    public Builder() {
      super();
    }

    @Override
    Collection<V> newMutableValueCollection() {
      return Platform.preservesInsertionOrderOnAddsSet();
    }

    
Adds a key-value mapping to the built multimap if it is not already present. 
/** Adds a key-value mapping to the built multimap if it is not already present. */
    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> put(K key, V value) {
      super.put(key, value);
      return this;
    }

    
Adds an entry to the built multimap if it is not already present.

Since:11.0
/**
     * Adds an entry to the built multimap if it is not already present.
     *
     * @since 11.0
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> put(Entry<? extends K, ? extends V> entry) {
      super.put(entry);
      return this;
    }

    
{@inheritDoc}

Since:19.0
/**
     * {@inheritDoc}
     *
     * @since 19.0
     */
    @CanIgnoreReturnValue
    @Beta
    @Override
    public Builder<K, V> putAll(Iterable<? extends Entry<? extends K, ? extends V>> entries) {
      super.putAll(entries);
      return this;
    }

    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> putAll(K key, Iterable<? extends V> values) {
      super.putAll(key, values);
      return this;
    }

    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> putAll(K key, V... values) {
      return putAll(key, Arrays.asList(values));
    }

    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> putAll(Multimap<? extends K, ? extends V> multimap) {
      for (Entry<? extends K, ? extends Collection<? extends V>> entry :
          multimap.asMap().entrySet()) {
        putAll(entry.getKey(), entry.getValue());
      }
      return this;
    }

    @CanIgnoreReturnValue
    @Override
    Builder<K, V> combine(ImmutableMultimap.Builder<K, V> other) {
      super.combine(other);
      return this;
    }

    
{@inheritDoc}

Since:8.0
/**
     * {@inheritDoc}
     *
     * @since 8.0
     */
    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> orderKeysBy(Comparator<? super K> keyComparator) {
      super.orderKeysBy(keyComparator);
      return this;
    }

    
Specifies the ordering of the generated multimap's values for each key.

If this method is called, the sets returned by the get() method of the generated multimap and its Multimap.asMap() view are ImmutableSortedSet instances. However, serialization does not preserve that property, though it does maintain the key and value ordering. 
Since:8.0
/**
     * Specifies the ordering of the generated multimap's values for each key.
     *
     * <p>If this method is called, the sets returned by the {@code get()} method of the generated
     * multimap and its {@link Multimap#asMap()} view are {@link ImmutableSortedSet} instances.
     * However, serialization does not preserve that property, though it does maintain the key and
     * value ordering.
     *
     * @since 8.0
     */
    // TODO: Make serialization behavior consistent.
    @CanIgnoreReturnValue
    @Override
    public Builder<K, V> orderValuesBy(Comparator<? super V> valueComparator) {
      super.orderValuesBy(valueComparator);
      return this;
    }

    
Returns a newly-created immutable set multimap. 
/** Returns a newly-created immutable set multimap. */
    @Override
    public ImmutableSetMultimap<K, V> build() {
      Collection<Map.Entry<K, Collection<V>>> mapEntries = builderMap.entrySet();
      if (keyComparator != null) {
        mapEntries = Ordering.from(keyComparator).<K>onKeys().immutableSortedCopy(mapEntries);
      }
      return fromMapEntries(mapEntries, valueComparator);
    }
  }

  
Returns an immutable set multimap containing the same mappings as multimap. The generated multimap's key and value orderings correspond to the iteration ordering of the 
multimap.asMap() view. Repeated occurrences of an entry in the multimap after the first are ignored. 
Despite the method name, this method attempts to avoid actually copying the data when it is
safe to do so. The exact circumstances under which a copy will or will not be performed are
undocumented and subject to change.

Throws:
/**
   * Returns an immutable set multimap containing the same mappings as {@code multimap}. The
   * generated multimap's key and value orderings correspond to the iteration ordering of the {@code
   * multimap.asMap()} view. Repeated occurrences of an entry in the multimap after the first are
   * ignored.
   *
   * <p>Despite the method name, this method attempts to avoid actually copying the data when it is
   * safe to do so. The exact circumstances under which a copy will or will not be performed are
   * undocumented and subject to change.
   *
   * @throws NullPointerException if any key or value in {@code multimap} is null
   */
  public static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap) {
    return copyOf(multimap, null);
  }

  private static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Multimap<? extends K, ? extends V> multimap, Comparator<? super V> valueComparator) {
    checkNotNull(multimap); // eager for GWT
    if (multimap.isEmpty() && valueComparator == null) {
      return of();
    }

    if (multimap instanceof ImmutableSetMultimap) {
      @SuppressWarnings("unchecked") // safe since multimap is not writable
      ImmutableSetMultimap<K, V> kvMultimap = (ImmutableSetMultimap<K, V>) multimap;
      if (!kvMultimap.isPartialView()) {
        return kvMultimap;
      }
    }

    return fromMapEntries(multimap.asMap().entrySet(), valueComparator);
  }

  
Returns an immutable multimap containing the specified entries. The returned multimap iterates over keys in the order they were first encountered in the input, and the values for each key are iterated in the order they were encountered. If two values for the same key are equal, the first value encountered is used. 
Throws:
Since:19.0
/**
   * Returns an immutable multimap containing the specified entries. The returned multimap iterates
   * over keys in the order they were first encountered in the input, and the values for each key
   * are iterated in the order they were encountered. If two values for the same key are {@linkplain
   * Object#equals equal}, the first value encountered is used.
   *
   * @throws NullPointerException if any key, value, or entry is null
   * @since 19.0
   */
  @Beta
  public static <K, V> ImmutableSetMultimap<K, V> copyOf(
      Iterable<? extends Entry<? extends K, ? extends V>> entries) {
    return new Builder<K, V>().putAll(entries).build();
  }

  
Creates an ImmutableSetMultimap from an asMap.entrySet. 
/** Creates an ImmutableSetMultimap from an asMap.entrySet. */
  static <K, V> ImmutableSetMultimap<K, V> fromMapEntries(
      Collection<? extends Map.Entry<? extends K, ? extends Collection<? extends V>>> mapEntries,
      @Nullable Comparator<? super V> valueComparator) {
    if (mapEntries.isEmpty()) {
      return of();
    }
    ImmutableMap.Builder<K, ImmutableSet<V>> builder =
        new ImmutableMap.Builder<>(mapEntries.size());
    int size = 0;

    for (Entry<? extends K, ? extends Collection<? extends V>> entry : mapEntries) {
      K key = entry.getKey();
      Collection<? extends V> values = entry.getValue();
      ImmutableSet<V> set = valueSet(valueComparator, values);
      if (!set.isEmpty()) {
        builder.put(key, set);
        size += set.size();
      }
    }

    return new ImmutableSetMultimap<>(builder.build(), size, valueComparator);
  }

  
Returned by get() when a missing key is provided. Also holds the comparator, if any, used for
values.

/**
   * Returned by get() when a missing key is provided. Also holds the comparator, if any, used for
   * values.
   */
  private final transient ImmutableSet<V> emptySet;

  ImmutableSetMultimap(
      ImmutableMap<K, ImmutableSet<V>> map,
      int size,
      @Nullable Comparator<? super V> valueComparator) {
    super(map, size);
    this.emptySet = emptySet(valueComparator);
  }

  // views

  
Returns an immutable set of the values for the given key. If no mappings in the multimap have
the provided key, an empty immutable set is returned. The values are in the same order as the
parameters used to build this multimap.

/**
   * Returns an immutable set of the values for the given key. If no mappings in the multimap have
   * the provided key, an empty immutable set is returned. The values are in the same order as the
   * parameters used to build this multimap.
   */
  @Override
  public ImmutableSet<V> get(@Nullable K key) {
    // This cast is safe as its type is known in constructor.
    ImmutableSet<V> set = (ImmutableSet<V>) map.get(key);
    return MoreObjects.firstNonNull(set, emptySet);
  }

  @LazyInit @MonotonicNonNull @RetainedWith private transient ImmutableSetMultimap<V, K> inverse;

  
{@inheritDoc}

Because an inverse of a set multimap cannot contain multiple pairs with the same key and value, this method returns an ImmutableSetMultimap rather than the 
ImmutableMultimap specified in the ImmutableMultimap class. 
Since:11.0
/**
   * {@inheritDoc}
   *
   * <p>Because an inverse of a set multimap cannot contain multiple pairs with the same key and
   * value, this method returns an {@code ImmutableSetMultimap} rather than the {@code
   * ImmutableMultimap} specified in the {@code ImmutableMultimap} class.
   *
   * @since 11.0
   */
  public ImmutableSetMultimap<V, K> inverse() {
    ImmutableSetMultimap<V, K> result = inverse;
    return (result == null) ? (inverse = invert()) : result;
  }

  private ImmutableSetMultimap<V, K> invert() {
    Builder<V, K> builder = builder();
    for (Entry<K, V> entry : entries()) {
      builder.put(entry.getValue(), entry.getKey());
    }
    ImmutableSetMultimap<V, K> invertedMultimap = builder.build();
    invertedMultimap.inverse = this;
    return invertedMultimap;
  }

  
Guaranteed to throw an exception and leave the multimap unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the multimap unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public ImmutableSet<V> removeAll(Object key) {
    throw new UnsupportedOperationException();
  }

  
Guaranteed to throw an exception and leave the multimap unmodified.

Throws:
  • UnsupportedOperationException – always
Deprecated:Unsupported operation.
/**
   * Guaranteed to throw an exception and leave the multimap unmodified.
   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  public ImmutableSet<V> replaceValues(K key, Iterable<? extends V> values) {
    throw new UnsupportedOperationException();
  }

  private transient @MonotonicNonNull ImmutableSet<Entry<K, V>> entries;

  
Returns an immutable collection of all key-value pairs in the multimap. Its iterator traverses
the values for the first key, the values for the second key, and so on.

/**
   * Returns an immutable collection of all key-value pairs in the multimap. Its iterator traverses
   * the values for the first key, the values for the second key, and so on.
   */
  @Override
  public ImmutableSet<Entry<K, V>> entries() {
    ImmutableSet<Entry<K, V>> result = entries;
    return result == null ? (entries = new EntrySet<>(this)) : result;
  }

  private static final class EntrySet<K, V> extends ImmutableSet<Entry<K, V>> {
    @Weak private final transient ImmutableSetMultimap<K, V> multimap;

    EntrySet(ImmutableSetMultimap<K, V> multimap) {
      this.multimap = multimap;
    }

    @Override
    public boolean contains(@Nullable Object object) {
      if (object instanceof Entry) {
        Entry<?, ?> entry = (Entry<?, ?>) object;
        return multimap.containsEntry(entry.getKey(), entry.getValue());
      }
      return false;
    }

    @Override
    public int size() {
      return multimap.size();
    }

    @Override
    public UnmodifiableIterator<Entry<K, V>> iterator() {
      return multimap.entryIterator();
    }

    @Override
    boolean isPartialView() {
      return false;
    }
  }

  private static <V> ImmutableSet<V> valueSet(
      @Nullable Comparator<? super V> valueComparator, Collection<? extends V> values) {
    return (valueComparator == null)
        ? ImmutableSet.copyOf(values)
        : ImmutableSortedSet.copyOf(valueComparator, values);
  }

  private static <V> ImmutableSet<V> emptySet(@Nullable Comparator<? super V> valueComparator) {
    return (valueComparator == null)
        ? ImmutableSet.<V>of()
        : ImmutableSortedSet.<V>emptySet(valueComparator);
  }

  private static <V> ImmutableSet.Builder<V> valuesBuilder(
      @Nullable Comparator<? super V> valueComparator) {
    return (valueComparator == null)
        ? new ImmutableSet.Builder<V>()
        : new ImmutableSortedSet.Builder<V>(valueComparator);
  }

  
@serialDatanumber of distinct keys, and then for each distinct key: the key, the number of
    values for that key, and the key's values
/**
   * @serialData number of distinct keys, and then for each distinct key: the key, the number of
   *     values for that key, and the key's values
   */
  @GwtIncompatible // java.io.ObjectOutputStream
  private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeObject(valueComparator());
    Serialization.writeMultimap(this, stream);
  }

  @Nullable
  Comparator<? super V> valueComparator() {
    return emptySet instanceof ImmutableSortedSet
        ? ((ImmutableSortedSet<V>) emptySet).comparator()
        : null;
  }
  
  @GwtIncompatible // java serialization
  private static final class SetFieldSettersHolder {
    static final Serialization.FieldSetter<ImmutableSetMultimap> EMPTY_SET_FIELD_SETTER =
        Serialization.getFieldSetter(ImmutableSetMultimap.class, "emptySet");
  }

  @GwtIncompatible // java.io.ObjectInputStream
  // Serialization type safety is at the caller's mercy.
  @SuppressWarnings("unchecked")
  private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();
    Comparator<Object> valueComparator = (Comparator<Object>) stream.readObject();
    int keyCount = stream.readInt();
    if (keyCount < 0) {
      throw new InvalidObjectException("Invalid key count " + keyCount);
    }
    ImmutableMap.Builder<Object, ImmutableSet<Object>> builder = ImmutableMap.builder();
    int tmpSize = 0;

    for (int i = 0; i < keyCount; i++) {
      Object key = stream.readObject();
      int valueCount = stream.readInt();
      if (valueCount <= 0) {
        throw new InvalidObjectException("Invalid value count " + valueCount);
      }

      ImmutableSet.Builder<Object> valuesBuilder = valuesBuilder(valueComparator);
      for (int j = 0; j < valueCount; j++) {
        valuesBuilder.add(stream.readObject());
      }
      ImmutableSet<Object> valueSet = valuesBuilder.build();
      if (valueSet.size() != valueCount) {
        throw new InvalidObjectException("Duplicate key-value pairs exist for key " + key);
      }
      builder.put(key, valueSet);
      tmpSize += valueCount;
    }

    ImmutableMap<Object, ImmutableSet<Object>> tmpMap;
    try {
      tmpMap = builder.build();
    } catch (IllegalArgumentException e) {
      throw (InvalidObjectException) new InvalidObjectException(e.getMessage()).initCause(e);
    }

    FieldSettersHolder.MAP_FIELD_SETTER.set(this, tmpMap);
    FieldSettersHolder.SIZE_FIELD_SETTER.set(this, tmpSize);
    SetFieldSettersHolder.EMPTY_SET_FIELD_SETTER.set(this, emptySet(valueComparator));
  }

  @GwtIncompatible // not needed in emulated source.
  private static final long serialVersionUID = 0;
}