1 /*
   2  * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang;
  27 
  28 
  29 /**
  30 * A class implements the {@code Hashable32} interface to indicate that it
  31 * provides a method for generating 32-bit hash values for instances.
  32 *
  33 * @since 1.8
  34 */
  35 public interface Hashable32 {
  36 
  37     /**
  38     * Return a 32-bit hash code value for this object.
  39     * <p>
  40     * The general contract of {@code hash32} is:
  41     * <ul>
  42     * <li>Whenever it is invoked on the same object more than once during
  43     *     an execution of a Java application, the {@code hash32} method
  44     *     must consistently return the same integer, provided no information
  45     *     used in {@code equals} comparisons on the object is modified.
  46     *     This integer need not remain consistent from one execution of an
  47     *     application to another execution of the same application.
  48     * <li>If two objects are equal according to the {@code equals(Object)}
  49     *     method, then calling the {@code hash32} method on each of
  50     *     the two objects must produce the same integer result.
  51     * <li>It is <em>not</em> required that if two objects are unequal
  52     *     according to the {@link java.lang.Object#equals(java.lang.Object)}
  53     *     method, then calling the {@code hash32} method on each of the
  54     *     two objects must produce distinct integer results.  However, the
  55     *     programmer should be aware that producing distinct integer results
  56     *     for unequal objects may improve the performance of hash tables.
  57     * </ul>
  58     *
  59     * @return  a hash code value for this object.
  60     * @see     java.lang.Object#equals(java.lang.Object)
  61      */
  62     int hash32(); // default { hashCode() }
  63 }