* "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#maps">{@code Maps}</a>.
 *
 * @author Kevin Bourrillion
 * @author Mike Bostock
 * @author Isaac Shum
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
public final class Maps {
  private Maps() {}

  private enum EntryFunction implements Function<Entry<?, ?>, @Nullable Object> {

    assertTrue(entries.contains(Maps.immutableEntry("a", true)));
    assertTrue(entries.contains(Maps.<String, @Nullable Boolean>immutableEntry("b", null)));
    assertTrue(
        entries.contains(Maps.<@Nullable String, @Nullable Boolean>immutableEntry(null, null)));

    assertFalse(entries.contains(Maps.<String, @Nullable Boolean>immutableEntry("c", null)));

    @Override
    protected Map<AnEnum, String> create(Entry<AnEnum, String>[] entries) {
      Map<AnEnum, String> map = Maps.newHashMap();
      for (Entry<AnEnum, String> entry : entries) {
        map.put(entry.getKey(), entry.getValue());
      }
      return Maps.immutableEnumMap(map);
    }
  }

  @J2ktIncompatible
  @GwtIncompatible // suite
  public static Test suite() {

   * ExecutionSequencer objects interfere with each other, which would be undesirable, in addition
   * to increasing the memory footprint of every thread that interacted with it. In order to release
   * entries in thread-specific maps when the ThreadLocal object itself is no longer referenced,
   * ThreadLocal is usually implemented with a WeakReference, which can have negative performance

  abstract Map<String, Integer> createUnfiltered();

  public void testFilteredKeysIllegalPut() {
    Map<String, Integer> unfiltered = createUnfiltered();
    Map<String, Integer> filtered = Maps.filterKeys(unfiltered, NOT_LENGTH_3);
    filtered.put("a", 1);
    filtered.put("b", 2);
    assertEquals(ImmutableMap.of("a", 1, "b", 2), filtered);

    try {
      filtered.put("yyy", 3);
      fail();

  abstract Map<String, Integer> createUnfiltered();

  public void testFilteredKeysIllegalPut() {
    Map<String, Integer> unfiltered = createUnfiltered();
    Map<String, Integer> filtered = Maps.filterKeys(unfiltered, NOT_LENGTH_3);
    filtered.put("a", 1);
    filtered.put("b", 2);
    assertEquals(ImmutableMap.of("a", 1, "b", 2), filtered);

    try {
      filtered.put("yyy", 3);
      fail();

  public void testComposition() {
    Map<String, Integer> mJapaneseToInteger = Maps.newHashMap();
    mJapaneseToInteger.put("Ichi", 1);
    mJapaneseToInteger.put("Ni", 2);
    mJapaneseToInteger.put("San", 3);
    Function<String, Integer> japaneseToInteger = Functions.forMap(mJapaneseToInteger);

    Map<Integer, String> mIntegerToSpanish = Maps.newHashMap();
    mIntegerToSpanish.put(1, "Uno");
    mIntegerToSpanish.put(3, "Tres");

  public void testComposition() {
    Map<String, Integer> mJapaneseToInteger = Maps.newHashMap();
    mJapaneseToInteger.put("Ichi", 1);
    mJapaneseToInteger.put("Ni", 2);
    mJapaneseToInteger.put("San", 3);
    Function<String, Integer> japaneseToInteger = Functions.forMap(mJapaneseToInteger);

    Map<Integer, String> mIntegerToSpanish = Maps.newHashMap();
    mIntegerToSpanish.put(1, "Uno");
    mIntegerToSpanish.put(3, "Tres");

        .containsExactly(
            Maps.<@Nullable String, Integer>immutableEntry(null, 7),
            Maps.<@Nullable String, Integer>immutableEntry(null, 3),
            Maps.<@Nullable String, Integer>immutableEntry(null, 1),
            Maps.<String, @Nullable Integer>immutableEntry("tree", null),
            Maps.immutableEntry("tree", 0),
            Maps.immutableEntry("google", 6),

      ImmutableSet<R> rowSpace,
      ImmutableSet<C> columnSpace) {
    Map<R, Integer> rowIndex = Maps.indexMap(rowSpace);
    Map<R, Map<C, V>> rows = Maps.newLinkedHashMap();
    for (R row : rowSpace) {
      rows.put(row, new LinkedHashMap<C, V>());
    }
    Map<C, Map<R, V>> columns = Maps.newLinkedHashMap();
    for (C col : columnSpace) {
      columns.put(col, new LinkedHashMap<R, V>());
    }