001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtIncompatible;
021import com.google.errorprone.annotations.CanIgnoreReturnValue;
022
023/**
024 * Provides equivalent behavior to {@link String#intern} for other immutable types. Common
025 * implementations are available from the {@link Interners} class.
026 *
027 * @author Kevin Bourrillion
028 * @since 3.0
029 */
030@Beta
031@GwtIncompatible
032public interface Interner<E> {
033  /**
034   * Chooses and returns the representative instance for any of a collection of instances that are
035   * equal to each other. If two {@linkplain Object#equals equal} inputs are given to this method,
036   * both calls will return the same instance. That is, {@code intern(a).equals(a)} always holds,
037   * and {@code intern(a) == intern(b)} if and only if {@code a.equals(b)}. Note that {@code
038   * intern(a)} is permitted to return one instance now and a different instance later if the
039   * original interned instance was garbage-collected.
040   *
041   * <p><b>Warning:</b> do not use with mutable objects.
042   *
043   * @throws NullPointerException if {@code sample} is null
044   */
045  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
046  E intern(E sample);
047}