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.GwtIncompatible;
020import com.google.errorprone.annotations.DoNotMock;
021
022/**
023 * Provides similar behavior to {@link String#intern} for any immutable type. Common implementations
024 * are available from the {@link Interners} class.
025 *
026 * <p>Note that {@code String.intern()} has some well-known performance limitations, and should
027 * generally be avoided. Prefer {@link Interners#newWeakInterner} or another {@code Interner}
028 * implementation even for {@code String} interning.
029 *
030 * @author Kevin Bourrillion
031 * @since 3.0
032 */
033@DoNotMock("Use Interners.new*Interner")
034@GwtIncompatible
035@ElementTypesAreNonnullByDefault
036public interface Interner<E> {
037  /**
038   * Chooses and returns the representative instance for any of a collection of instances that are
039   * equal to each other. If two {@linkplain Object#equals equal} inputs are given to this method,
040   * both calls will return the same instance. That is, {@code intern(a).equals(a)} always holds,
041   * and {@code intern(a) == intern(b)} if and only if {@code a.equals(b)}. Note that {@code
042   * intern(a)} is permitted to return one instance now and a different instance later if the
043   * original interned instance was garbage-collected.
044   *
045   * <p><b>Warning:</b> do not use with mutable objects.
046   *
047   * @throws NullPointerException if {@code sample} is null
048   */
049  E intern(E sample);
050}