Java tutorial
/* * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements. See the NOTICE file distributed with * this work for additional information regarding copyright ownership. * The ASF licenses this file to You 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 org.apache.commons.lang.builder; import java.util.HashSet; import java.util.Set; /** * <p> * Assists in implementing {@link Object#hashCode()} methods. * </p> * <p/> * <p> * This class enables a good <code>hashCode</code> method to be built for any class. It follows the rules laid out in * the book <a href="http://java.sun.com/docs/books/effective/index.html">Effective Java</a> by Joshua Bloch. Writing a * good <code>hashCode</code> method is actually quite difficult. This class aims to simplify the process. * </p> * <p/> * <p> * All relevant fields from the object should be included in the <code>hashCode</code> method. Derived fields may be * excluded. In general, any field used in the <code>equals</code> method must be used in the <code>hashCode</code> * method. * </p> * <p/> * <p> * To use this class write code as follows: * </p> * <p/> * <pre> * public class Person { * String name; * int age; * boolean smoker; * ... * * public int hashCode() { * // you pick a hard-coded, randomly chosen, non-zero, odd number * // ideally different for each class * return new HashCodeBuilder(17, 37). * append(name). * append(age). * append(smoker). * toHashCode(); * } * } * </pre> * <p/> * <p> * If required, the superclass <code>hashCode()</code> can be added using {@link #appendSuper}. * </p> * <p/> * <p> * Alternatively, there is a method that uses reflection to determine the fields to test. Because these fields are * usually private, the method, <code>reflectionHashCode</code>, uses <code>AccessibleObject.setAccessible</code> * to change the visibility of the fields. This will fail under a security manager, unless the appropriate permissions * are set up correctly. It is also slower than testing explicitly. * </p> * <p/> * <p> * A typical invocation for this method would look like: * </p> * <p/> * <pre> * public int hashCode() { * return HashCodeBuilder.reflectionHashCode(this); * } * </pre> * * @author Stephen Colebourne * @author Gary Gregory * @author Pete Gieser * @version $Id: HashCodeBuilder.java 447989 2006-09-19 21:58:11Z ggregory $ * @since 1.0 */ public class HashCodeBuilder { /** * <p> * A registry of objects used by reflection methods to detect cyclical object references and avoid infinite loops. * </p> * * @since 2.3 */ private static final ThreadLocal registry = new ThreadLocal() { protected synchronized Object initialValue() { // The HashSet implementation is not synchronized, // which is just what we need here. return new HashSet(); } }; /** * <p> * Returns the registry of objects being traversed by the reflection methods in the current thread. * </p> * * @return Set the registry of objects being traversed * @since 2.3 */ static Set getRegistry() { return (Set) registry.get(); } /** * <p> * Returns <code>true</code> if the registry contains the given object. Used by the reflection methods to avoid * infinite loops. * </p> * * @param value The object to lookup in the registry. * @return boolean <code>true</code> if the registry contains the given object. * @since 2.3 */ static boolean isRegistered(Object value) { return getRegistry().contains(toIdentityHashCodeInteger(value)); } // ------------------------------------------------------------------------- /** * <p> * Registers the given object. Used by the reflection methods to avoid infinite loops. * </p> * * @param value The object to register. */ static void register(Object value) { getRegistry().add(toIdentityHashCodeInteger(value)); } /** * Returns an Integer for the given object's default hash code. * * @param value object for which the hashCode is to be calculated * @return Default int hash code * @see System#identityHashCode(Object) */ private static Integer toIdentityHashCodeInteger(Object value) { return System.identityHashCode(value); } /** * <p> * Unregisters the given object. * </p> * <p/> * <p/> * Used by the reflection methods to avoid infinite loops. * * @param value The object to unregister. * @since 2.3 */ static void unregister(Object value) { getRegistry().remove(toIdentityHashCodeInteger(value)); } /** * Constant to use in building the hashCode. */ private final int iConstant; /** * Running total of the hashCode. */ private int iTotal = 0; /** * <p> * Uses two hard coded choices for the constants needed to build a <code>hashCode</code>. * </p> */ public HashCodeBuilder() { iConstant = 37; iTotal = 17; } /** * <p> * Two randomly chosen, non-zero, odd numbers must be passed in. Ideally these should be different for each class, * however this is not vital. * </p> * <p/> * <p> * Prime numbers are preferred, especially for the multiplier. * </p> * * @param initialNonZeroOddNumber a non-zero, odd number used as the initial value * @param multiplierNonZeroOddNumber a non-zero, odd number used as the multiplier * @throws IllegalArgumentException if the number is zero or even */ public HashCodeBuilder(int initialNonZeroOddNumber, int multiplierNonZeroOddNumber) { if (initialNonZeroOddNumber == 0) { throw new IllegalArgumentException("HashCodeBuilder requires a non zero initial value"); } if (initialNonZeroOddNumber % 2 == 0) { throw new IllegalArgumentException("HashCodeBuilder requires an odd initial value"); } if (multiplierNonZeroOddNumber == 0) { throw new IllegalArgumentException("HashCodeBuilder requires a non zero multiplier"); } if (multiplierNonZeroOddNumber % 2 == 0) { throw new IllegalArgumentException("HashCodeBuilder requires an odd multiplier"); } iConstant = multiplierNonZeroOddNumber; iTotal = initialNonZeroOddNumber; } /** * <p> * Append a <code>hashCode</code> for a <code>boolean</code>. * </p> * <p> * This adds <code>iConstant * 1</code> to the <code>hashCode</code> and not a <code>1231</code> or * <code>1237</code> as done in java.lang.Boolean. This is in accordance with the <quote>Effective Java</quote> * design. * </p> * * @param value the boolean to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(boolean value) { iTotal = iTotal * iConstant + (value ? 0 : 1); return this; } /** * <p> * Append a <code>hashCode</code> for a <code>boolean</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(boolean[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (boolean anArray : array) { append(anArray); } } return this; } // ------------------------------------------------------------------------- /** * <p> * Append a <code>hashCode</code> for a <code>byte</code>. * </p> * * @param value the byte to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(byte value) { iTotal = iTotal * iConstant + value; return this; } // ------------------------------------------------------------------------- /** * <p> * Append a <code>hashCode</code> for a <code>byte</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(byte[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (byte anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for a <code>char</code>. * </p> * * @param value the char to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(char value) { iTotal = iTotal * iConstant + value; return this; } /** * <p> * Append a <code>hashCode</code> for a <code>char</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(char[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (char anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for a <code>double</code>. * </p> * * @param value the double to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(double value) { return append(Double.doubleToLongBits(value)); } /** * <p> * Append a <code>hashCode</code> for a <code>double</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(double[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (double anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for a <code>float</code>. * </p> * * @param value the float to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(float value) { iTotal = iTotal * iConstant + Float.floatToIntBits(value); return this; } /** * <p> * Append a <code>hashCode</code> for a <code>float</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(float[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (float anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for an <code>int</code>. * </p> * * @param value the int to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(int value) { iTotal = iTotal * iConstant + value; return this; } /** * <p> * Append a <code>hashCode</code> for an <code>int</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(int[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (int anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for a <code>long</code>. * </p> * * @param value the long to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(long value) { iTotal = iTotal * iConstant + ((int) (value ^ (value >> 32))); return this; } /** * <p> * Append a <code>hashCode</code> for a <code>long</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(long[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (long anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for an <code>Object</code>. * </p> * * @param object the Object to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(Object object) { if (object == null) { iTotal = iTotal * iConstant; } else { if (object.getClass().isArray() == false) { // the simple case, not an array, just the element iTotal = iTotal * iConstant + object.hashCode(); } else { // 'Switch' on type of array, to dispatch to the correct handler // This handles multi dimensional arrays if (object instanceof long[]) { append((long[]) object); } else if (object instanceof int[]) { append((int[]) object); } else if (object instanceof short[]) { append((short[]) object); } else if (object instanceof char[]) { append((char[]) object); } else if (object instanceof byte[]) { append((byte[]) object); } else if (object instanceof double[]) { append((double[]) object); } else if (object instanceof float[]) { append((float[]) object); } else if (object instanceof boolean[]) { append((boolean[]) object); } else { // Not an array of primitives append((Object[]) object); } } } return this; } /** * <p> * Append a <code>hashCode</code> for an <code>Object</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(Object[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (Object anArray : array) { append(anArray); } } return this; } /** * <p> * Append a <code>hashCode</code> for a <code>short</code>. * </p> * * @param value the short to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(short value) { iTotal = iTotal * iConstant + value; return this; } /** * <p> * Append a <code>hashCode</code> for a <code>short</code> array. * </p> * * @param array the array to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(short[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (short anArray : array) { append(anArray); } } return this; } /** * <p> * Adds the result of super.hashCode() to this builder. * </p> * * @param superHashCode the result of calling <code>super.hashCode()</code> * @return this HashCodeBuilder, used to chain calls. * @since 2.0 */ public HashCodeBuilder appendSuper(int superHashCode) { iTotal = iTotal * iConstant + superHashCode; return this; } /** * <p> * Return the computed <code>hashCode</code>. * </p> * * @return <code>hashCode</code> based on the fields appended */ public int toHashCode() { return iTotal; } }