* @see ConcurrentHashMap#putIfAbsent(Object, Object)
     */
    public static <K, V> V putIfAbsent(final ConcurrentMap<K, V> map, final K key, final V value) {
        final V exists = map.putIfAbsent(key, value);
        if (exists != null) {
            return exists;
        }
        return value;
    }

      synchronized (mutex) {
        return delegate().put(key, value);
      }
    }

    @Override
    public @Nullable V putIfAbsent(K key, V value) {
      synchronized (mutex) {
        return delegate().putIfAbsent(key, value);
      }
    }

    @Override
    public boolean replace(K key, V oldValue, V newValue) {
      synchronized (mutex) {

    assertThrows(UnsupportedOperationException.class, () -> unmod.replaceAll((k, v) -> v));
    assertThrows(UnsupportedOperationException.class, () -> unmod.putIfAbsent(3, "three"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.replace(3, "three", "four"));
    assertThrows(UnsupportedOperationException.class, () -> unmod.replace(3, "four"));
    assertThrows(

    checkNotNull(key);
    checkNotNull(value);
    int hash = hash(key);
    return segmentFor(hash).put(key, hash, value, false);
  }

  @CanIgnoreReturnValue
  @Override
  public @Nullable V putIfAbsent(K key, V value) {
    checkNotNull(key);
    checkNotNull(value);
    int hash = hash(key);
    return segmentFor(hash).put(key, hash, value, true);
  }

  @Override