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Guava Collections is a collections extension library with expanded collection types, implementations, and utility methods for dealing with aggregate data.

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/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import com.google.common.annotations.Beta;

/**
 * Provides equivalent behavior to {@link String#intern} for other immutable
 * types.
 *
 * @author Kevin Bourrillion
 * @since 3
 */
@Beta
public interface Interner {
  /**
   * Chooses and returns the representative instance for any of a collection of
   * instances that are equal to each other. If two {@link Object#equals equal}
   * inputs are given to this method, both calls will return the same instance.
   * That is, {@code intern(a).equals(a)} always holds, and {@code intern(a) ==
   * intern(b)} if and only if {@code a.equals(b)}. Note that {@code intern(a)}
   * is permitted to return one instance now and a different instance later if
   * the original interned instance was garbage-collected.
   *
   * Warning: do not use with mutable objects.
   *
   * @throws NullPointerException if {@code sample} is null
   */
  E intern(E sample);
}