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.lang3.builder; import java.lang.reflect.AccessibleObject; import java.lang.reflect.Field; import java.lang.reflect.Modifier; import java.util.Collection; import java.util.HashSet; import java.util.Set; import org.apache.commons.lang3.ArrayUtils; import org.apache.commons.lang3.Validate; /** * <p> * Assists in implementing {@link Object#hashCode()} methods. * </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://www.oracle.com/technetwork/java/effectivejava-136174.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> * The following is the approach taken. When appending a data field, the current total is multiplied by the * multiplier then a relevant value * for that data type is added. For example, if the current hashCode is 17, and the multiplier is 37, then * appending the integer 45 will create a hashcode of 674, namely 17 * 37 + 45. * </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> * To use this class write code as follows: * </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> * If required, the superclass <code>hashCode()</code> can be added using {@link #appendSuper}. * </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> * A typical invocation for this method would look like: * </p> * * <pre> * public int hashCode() { * return HashCodeBuilder.reflectionHashCode(this); * } * </pre> * * @since 1.0 * @version $Id$ */ public class HashCodeBuilder implements Builder<Integer> { /** * The default initial value to use in reflection hash code building. */ private static final int DEFAULT_INITIAL_VALUE = 17; /** * The default multipler value to use in reflection hash code building. */ private static final int DEFAULT_MULTIPLIER_VALUE = 37; /** * <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<Set<IDKey>> REGISTRY = new ThreadLocal<Set<IDKey>>(); /* * NOTE: we cannot store the actual objects in a HashSet, as that would use the very hashCode() * we are in the process of calculating. * * So we generate a one-to-one mapping from the original object to a new object. * * Now HashSet uses equals() to determine if two elements with the same hashcode really * are equal, so we also need to ensure that the replacement objects are only equal * if the original objects are identical. * * The original implementation (2.4 and before) used the System.indentityHashCode() * method - however this is not guaranteed to generate unique ids (e.g. LANG-459) * * We now use the IDKey helper class (adapted from org.apache.axis.utils.IDKey) * to disambiguate the duplicate ids. */ /** * <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<IDKey> getRegistry() { return 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(final Object value) { final Set<IDKey> registry = getRegistry(); return registry != null && registry.contains(new IDKey(value)); } /** * <p> * Appends the fields and values defined by the given object of the given <code>Class</code>. * </p> * * @param object * the object to append details of * @param clazz * the class to append details of * @param builder * the builder to append to * @param useTransients * whether to use transient fields * @param excludeFields * Collection of String field names to exclude from use in calculation of hash code */ private static void reflectionAppend(final Object object, final Class<?> clazz, final HashCodeBuilder builder, final boolean useTransients, final String[] excludeFields) { if (isRegistered(object)) { return; } try { register(object); final Field[] fields = clazz.getDeclaredFields(); AccessibleObject.setAccessible(fields, true); for (final Field field : fields) { if (!ArrayUtils.contains(excludeFields, field.getName()) && !field.getName().contains("$") && (useTransients || !Modifier.isTransient(field.getModifiers())) && (!Modifier.isStatic(field.getModifiers())) && (!field.isAnnotationPresent(HashCodeExclude.class))) { try { final Object fieldValue = field.get(object); builder.append(fieldValue); } catch (final IllegalAccessException e) { // this can't happen. Would get a Security exception instead // throw a runtime exception in case the impossible happens. throw new InternalError("Unexpected IllegalAccessException"); } } } } finally { unregister(object); } } /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <p> * Transient members will be not be used, as they are likely derived fields, and not part of the value of the * <code>Object</code>. * </p> * * <p> * Static fields will not be tested. Superclass fields will be included. * </p> * * <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. Prime numbers are preferred, especially for the multiplier. * </p> * * @param initialNonZeroOddNumber * a non-zero, odd number used as the initial value. This will be the returned * value if no fields are found to include in the hash code * @param multiplierNonZeroOddNumber * a non-zero, odd number used as the multiplier * @param object * the Object to create a <code>hashCode</code> for * @return int hash code * @throws IllegalArgumentException * if the Object is <code>null</code> * @throws IllegalArgumentException * if the number is zero or even */ public static int reflectionHashCode(final int initialNonZeroOddNumber, final int multiplierNonZeroOddNumber, final Object object) { return reflectionHashCode(initialNonZeroOddNumber, multiplierNonZeroOddNumber, object, false, null); } /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <p> * If the TestTransients parameter is set to <code>true</code>, transient members will be tested, otherwise they * are ignored, as they are likely derived fields, and not part of the value of the <code>Object</code>. * </p> * * <p> * Static fields will not be tested. Superclass fields will be included. * </p> * * <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. Prime numbers are preferred, especially for the multiplier. * </p> * * @param initialNonZeroOddNumber * a non-zero, odd number used as the initial value. This will be the returned * value if no fields are found to include in the hash code * @param multiplierNonZeroOddNumber * a non-zero, odd number used as the multiplier * @param object * the Object to create a <code>hashCode</code> for * @param testTransients * whether to include transient fields * @return int hash code * @throws IllegalArgumentException * if the Object is <code>null</code> * @throws IllegalArgumentException * if the number is zero or even */ public static int reflectionHashCode(final int initialNonZeroOddNumber, final int multiplierNonZeroOddNumber, final Object object, final boolean testTransients) { return reflectionHashCode(initialNonZeroOddNumber, multiplierNonZeroOddNumber, object, testTransients, null); } /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <p> * If the TestTransients parameter is set to <code>true</code>, transient members will be tested, otherwise they * are ignored, as they are likely derived fields, and not part of the value of the <code>Object</code>. * </p> * * <p> * Static fields will not be included. Superclass fields will be included up to and including the specified * superclass. A null superclass is treated as java.lang.Object. * </p> * * <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. Prime numbers are preferred, especially for the multiplier. * </p> * * @param <T> * the type of the object involved * @param initialNonZeroOddNumber * a non-zero, odd number used as the initial value. This will be the returned * value if no fields are found to include in the hash code * @param multiplierNonZeroOddNumber * a non-zero, odd number used as the multiplier * @param object * the Object to create a <code>hashCode</code> for * @param testTransients * whether to include transient fields * @param reflectUpToClass * the superclass to reflect up to (inclusive), may be <code>null</code> * @param excludeFields * array of field names to exclude from use in calculation of hash code * @return int hash code * @throws IllegalArgumentException * if the Object is <code>null</code> * @throws IllegalArgumentException * if the number is zero or even * @since 2.0 */ public static <T> int reflectionHashCode(final int initialNonZeroOddNumber, final int multiplierNonZeroOddNumber, final T object, final boolean testTransients, final Class<? super T> reflectUpToClass, final String... excludeFields) { if (object == null) { throw new IllegalArgumentException("The object to build a hash code for must not be null"); } final HashCodeBuilder builder = new HashCodeBuilder(initialNonZeroOddNumber, multiplierNonZeroOddNumber); Class<?> clazz = object.getClass(); reflectionAppend(object, clazz, builder, testTransients, excludeFields); while (clazz.getSuperclass() != null && clazz != reflectUpToClass) { clazz = clazz.getSuperclass(); reflectionAppend(object, clazz, builder, testTransients, excludeFields); } return builder.toHashCode(); } /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * This constructor uses two hard coded choices for the constants needed to build a hash code. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <P> * If the TestTransients parameter is set to <code>true</code>, transient members will be tested, otherwise they * are ignored, as they are likely derived fields, and not part of the value of the <code>Object</code>. * </p> * * <p> * Static fields will not be tested. Superclass fields will be included. If no fields are found to include * in the hash code, the result of this method will be constant. * </p> * * @param object * the Object to create a <code>hashCode</code> for * @param testTransients * whether to include transient fields * @return int hash code * @throws IllegalArgumentException * if the object is <code>null</code> */ public static int reflectionHashCode(final Object object, final boolean testTransients) { return reflectionHashCode(DEFAULT_INITIAL_VALUE, DEFAULT_MULTIPLIER_VALUE, object, testTransients, null); } /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * This constructor uses two hard coded choices for the constants needed to build a hash code. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <p> * Transient members will be not be used, as they are likely derived fields, and not part of the value of the * <code>Object</code>. * </p> * * <p> * Static fields will not be tested. Superclass fields will be included. If no fields are found to include * in the hash code, the result of this method will be constant. * </p> * * @param object * the Object to create a <code>hashCode</code> for * @param excludeFields * Collection of String field names to exclude from use in calculation of hash code * @return int hash code * @throws IllegalArgumentException * if the object is <code>null</code> */ public static int reflectionHashCode(final Object object, final Collection<String> excludeFields) { return reflectionHashCode(object, ReflectionToStringBuilder.toNoNullStringArray(excludeFields)); } // ------------------------------------------------------------------------- /** * <p> * Uses reflection to build a valid hash code from the fields of {@code object}. * </p> * * <p> * This constructor uses two hard coded choices for the constants needed to build a hash code. * </p> * * <p> * It uses <code>AccessibleObject.setAccessible</code> to gain access to private fields. This means that it will * throw a security exception if run under a security manager, if the permissions are not set up correctly. It is * also not as efficient as testing explicitly. * </p> * * <p> * Transient members will be not be used, as they are likely derived fields, and not part of the value of the * <code>Object</code>. * </p> * * <p> * Static fields will not be tested. Superclass fields will be included. If no fields are found to include * in the hash code, the result of this method will be constant. * </p> * * @param object * the Object to create a <code>hashCode</code> for * @param excludeFields * array of field names to exclude from use in calculation of hash code * @return int hash code * @throws IllegalArgumentException * if the object is <code>null</code> */ public static int reflectionHashCode(final Object object, final String... excludeFields) { return reflectionHashCode(DEFAULT_INITIAL_VALUE, DEFAULT_MULTIPLIER_VALUE, object, false, null, excludeFields); } /** * <p> * Registers the given object. Used by the reflection methods to avoid infinite loops. * </p> * * @param value * The object to register. */ static void register(final Object value) { synchronized (HashCodeBuilder.class) { if (getRegistry() == null) { REGISTRY.set(new HashSet<IDKey>()); } } getRegistry().add(new IDKey(value)); } /** * <p> * Unregisters the given object. * </p> * * <p> * Used by the reflection methods to avoid infinite loops. * * @param value * The object to unregister. * @since 2.3 */ static void unregister(final Object value) { Set<IDKey> registry = getRegistry(); if (registry != null) { registry.remove(new IDKey(value)); synchronized (HashCodeBuilder.class) { //read again registry = getRegistry(); if (registry != null && registry.isEmpty()) { REGISTRY.remove(); } } } } /** * 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, odd numbers must be passed in. Ideally these should be different for each class, * however this is not vital. * </p> * * <p> * Prime numbers are preferred, especially for the multiplier. * </p> * * @param initialOddNumber * an odd number used as the initial value * @param multiplierOddNumber * an odd number used as the multiplier * @throws IllegalArgumentException * if the number is even */ public HashCodeBuilder(final int initialOddNumber, final int multiplierOddNumber) { Validate.isTrue(initialOddNumber % 2 != 0, "HashCodeBuilder requires an odd initial value"); Validate.isTrue(multiplierOddNumber % 2 != 0, "HashCodeBuilder requires an odd multiplier"); iConstant = multiplierOddNumber; iTotal = initialOddNumber; } /** * <p> * Append a <code>hashCode</code> for a <code>boolean</code>. * </p> * <p> * This adds <code>1</code> when true, and <code>0</code> when false to the <code>hashCode</code>. * </p> * <p> * This is in contrast to the standard <code>java.lang.Boolean.hashCode</code> handling, which computes * a <code>hashCode</code> value of <code>1231</code> for <code>java.lang.Boolean</code> instances * that represent <code>true</code> or <code>1237</code> for <code>java.lang.Boolean</code> instances * that represent <code>false</code>. * </p> * <p> * This is in accordance with the <i>Effective Java</i> design. * </p> * * @param value * the boolean to add to the <code>hashCode</code> * @return this */ public HashCodeBuilder append(final 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(final boolean[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final boolean element : array) { append(element); } } 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(final 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(final byte[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final byte element : array) { append(element); } } 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(final 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(final char[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final char element : array) { append(element); } } 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(final 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(final double[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final double element : array) { append(element); } } 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(final 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(final float[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final float element : array) { append(element); } } 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(final 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(final int[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final int element : array) { append(element); } } 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 */ // NOTE: This method uses >> and not >>> as Effective Java and // Long.hashCode do. Ideally we should switch to >>> at // some stage. There are backwards compat issues, so // that will have to wait for the time being. cf LANG-342. public HashCodeBuilder append(final 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(final long[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final long element : array) { append(element); } } 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(final Object object) { if (object == null) { iTotal = iTotal * iConstant; } else { if (object.getClass().isArray()) { // '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); } } else { if (object instanceof Long) { append(((Long) object).longValue()); } else if (object instanceof Integer) { append(((Integer) object).intValue()); } else if (object instanceof Short) { append(((Short) object).shortValue()); } else if (object instanceof Character) { append(((Character) object).charValue()); } else if (object instanceof Byte) { append(((Byte) object).byteValue()); } else if (object instanceof Double) { append(((Double) object).doubleValue()); } else if (object instanceof Float) { append(((Float) object).floatValue()); } else if (object instanceof Boolean) { append(((Boolean) object).booleanValue()); } else if (object instanceof String) { iTotal = iTotal * iConstant + object.hashCode(); } else { if (isRegistered(object)) { return this; } try { register(object); iTotal = iTotal * iConstant + object.hashCode(); } finally { unregister(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(final Object[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final Object element : array) { append(element); } } 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(final 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(final short[] array) { if (array == null) { iTotal = iTotal * iConstant; } else { for (final short element : array) { append(element); } } 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(final 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; } /** * Returns the computed <code>hashCode</code>. * * @return <code>hashCode</code> based on the fields appended * * @since 3.0 */ @Override public Integer build() { return Integer.valueOf(toHashCode()); } /** * <p> * The computed <code>hashCode</code> from toHashCode() is returned due to the likelihood * of bugs in mis-calling toHashCode() and the unlikeliness of it mattering what the hashCode for * HashCodeBuilder itself is.</p> * * @return <code>hashCode</code> based on the fields appended * @since 2.5 */ @Override public int hashCode() { return toHashCode(); } }