// to generate a UID for the same object from multiple threads, but
        // that shouldn't be a problem.
        uid = counter.getAndIncrement();
        Integer alreadySet = uids.putIfAbsent(obj, uid);
        if (alreadySet != null) {
          uid = alreadySet;
        }
      }
      return uid;
    }

    @Override

   * @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  @CheckForNull
  public final V putIfAbsent(K key, V value) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the map unmodified.
   *
   * @throws UnsupportedOperationException always

    @Override
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
      throw new UnsupportedOperationException();
    }

    @Override
    @CheckForNull
    public V putIfAbsent(K key, V value) {
      throw new UnsupportedOperationException();
    }

    @Override
    public boolean remove(@Nullable Object key, @Nullable Object value) {

            ValueReference<K, V> valueReference = e.getValueReference();
            V entryValue = valueReference.get();
            // replace the old LoadingValueReference if it's live, otherwise
            // perform a putIfAbsent
            if (oldValueReference == valueReference
                || (entryValue == null && valueReference != UNSET)) {
              ++modCount;
              if (oldValueReference.isActive()) {

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

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

  @Override

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

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

  @Override