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    
017    package com.google.common.collect;
018    
019    import com.google.common.annotations.Beta;
020    
021    /**
022     * Provides equivalent behavior to {@link String#intern} for other immutable
023     * types.
024     *
025     * @author Kevin Bourrillion
026     * @since 3.0
027     */
028    @Beta
029    public interface Interner<E> {
030      /**
031       * Chooses and returns the representative instance for any of a collection of
032       * instances that are equal to each other. If two {@linkplain Object#equals
033       * equal} inputs are given to this method, both calls will return the same
034       * instance.  That is, {@code intern(a).equals(a)} always holds, and {@code
035       * intern(a) == intern(b)} if and only if {@code a.equals(b)}. Note that
036       * {@code intern(a)} is permitted to return one instance now and a different
037       * instance later if the original interned instance was garbage-collected.
038       *
039       * <p><b>Warning:</b> do not use with mutable objects.
040       *
041       * @throws NullPointerException if {@code sample} is null
042       */
043      E intern(E sample);
044    }