* code.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings("unchecked")
  public static <K, V> ImmutableMap<K, V> of() {
    return (ImmutableMap<K, V>) RegularImmutableMap.EMPTY;
  }

  /**
   * Returns an immutable map containing a single entry. This map behaves and performs comparably to
   * {@link Collections#singletonMap} but will not accept a null key or value. It is preferable

    return RegularImmutableMap.fromEntries(entryOf(k1, v1), entryOf(k2, v2));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {

    for (int i = 0; i < 10; i++) {
      builder.put(i, i);
    }
    Entry<Integer, Integer>[] builderArrayAfterPuts = builder.entries;
    RegularImmutableMap<Integer, Integer> map =
        (RegularImmutableMap<Integer, Integer>) builder.buildOrThrow();
    Entry<Integer, Integer>[] mapInternalArray = map.entries;
    assertSame(builderArray, builderArrayAfterPuts);

    for (int i = 0; i < 10; i++) {
      builder.put(i, i);
    }
    Object[] builderArrayAfterPuts = builder.alternatingKeysAndValues;
    RegularImmutableMap<Integer, Integer> map =
        (RegularImmutableMap<Integer, Integer>) builder.buildOrThrow();
    Object[] mapInternalArray = map.alternatingKeysAndValues;
    assertSame(builderArray, builderArrayAfterPuts);