https://github.com/google/guava/guava: Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. 
/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotMock;


Provides similar behavior to String.intern for any immutable type. Common implementations are available from the Interners class. 
Note that String.intern() has some well-known performance limitations, and should generally be avoided. Prefer Interners.newWeakInterner or another Interner implementation even for String interning. 
Author:Kevin Bourrillion
Since:3.0
/**
 * Provides similar behavior to {@link String#intern} for any immutable type. Common implementations
 * are available from the {@link Interners} class.
 *
 * <p>Note that {@code String.intern()} has some well-known performance limitations, and should
 * generally be avoided. Prefer {@link Interners#newWeakInterner} or another {@code Interner}
 * implementation even for {@code String} interning.
 *
 * @author Kevin Bourrillion
 * @since 3.0
 */
@Beta
@DoNotMock("Use Interners.new*Interner")
@GwtIncompatible
public interface Interner<E> {
  
Chooses and returns the representative instance for any of a collection of instances that are equal to each other. If two equal inputs are given to this method, both calls will return the same instance. That is, intern(a).equals(a) always holds, and intern(a) == intern(b) if and only if a.equals(b). Note that 
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:
/**
   * Chooses and returns the representative instance for any of a collection of instances that are
   * equal to each other. If two {@linkplain 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.
   *
   * <p><b>Warning:</b> do not use with mutable objects.
   *
   * @throws NullPointerException if {@code sample} is null
   */
  @CanIgnoreReturnValue // TODO(cpovirk): Consider removing this?
  E intern(E sample);
}