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. */ import java.io.Serializable; /** * <p>Operations on <code>Object</code>.</p> * * <p>This class tries to handle <code>null</code> input gracefully. * An exception will generally not be thrown for a <code>null</code> input. * Each method documents its behaviour in more detail.</p> * * @author <a href="mailto:nissim@nksystems.com">Nissim Karpenstein</a> * @author <a href="mailto:janekdb@yahoo.co.uk">Janek Bogucki</a> * @author Daniel L. Rall * @author Stephen Colebourne * @author Gary Gregory * @author Mario Winterer * @author <a href="mailto:david@davidkarlsen.com">David J. M. Karlsen</a> * @since 1.0 * @version $Id: ObjectUtils.java 594336 2007-11-12 22:54:02Z bayard $ */ public class ObjectUtils { /** * <p>Singleton used as a <code>null</code> placeholder where * <code>null</code> has another meaning.</p> * * <p>For example, in a <code>HashMap</code> the * {@link java.util.HashMap#get(java.lang.Object)} method returns * <code>null</code> if the <code>Map</code> contains * <code>null</code> or if there is no matching key. The * <code>Null</code> placeholder can be used to distinguish between * these two cases.</p> * * <p>Another example is <code>Hashtable</code>, where <code>null</code> * cannot be stored.</p> * * <p>This instance is Serializable.</p> */ public static final Null NULL = new Null(); /** * <p><code>ObjectUtils</code> instances should NOT be constructed in * standard programming. Instead, the class should be used as * <code>ObjectUtils.defaultIfNull("a","b");</code>.</p> * * <p>This constructor is public to permit tools that require a JavaBean instance * to operate.</p> */ public ObjectUtils() { super(); } // Defaulting //----------------------------------------------------------------------- /** * <p>Returns a default value if the object passed is * <code>null</code>.</p> * * <pre> * ObjectUtils.defaultIfNull(null, null) = null * ObjectUtils.defaultIfNull(null, "") = "" * ObjectUtils.defaultIfNull(null, "zz") = "zz" * ObjectUtils.defaultIfNull("abc", *) = "abc" * ObjectUtils.defaultIfNull(Boolean.TRUE, *) = Boolean.TRUE * </pre> * * @param object the <code>Object</code> to test, may be <code>null</code> * @param defaultValue the default value to return, may be <code>null</code> * @return <code>object</code> if it is not <code>null</code>, defaultValue otherwise */ public static Object defaultIfNull(Object object, Object defaultValue) { return object != null ? object : defaultValue; } /** * <p>Compares two objects for equality, where either one or both * objects may be <code>null</code>.</p> * * <pre> * ObjectUtils.equals(null, null) = true * ObjectUtils.equals(null, "") = false * ObjectUtils.equals("", null) = false * ObjectUtils.equals("", "") = true * ObjectUtils.equals(Boolean.TRUE, null) = false * ObjectUtils.equals(Boolean.TRUE, "true") = false * ObjectUtils.equals(Boolean.TRUE, Boolean.TRUE) = true * ObjectUtils.equals(Boolean.TRUE, Boolean.FALSE) = false * </pre> * * @param object1 the first object, may be <code>null</code> * @param object2 the second object, may be <code>null</code> * @return <code>true</code> if the values of both objects are the same */ public static boolean equals(Object object1, Object object2) { if (object1 == object2) { return true; } if ((object1 == null) || (object2 == null)) { return false; } return object1.equals(object2); } /** * <p>Gets the hash code of an object returning zero when the * object is <code>null</code>.</p> * * <pre> * ObjectUtils.hashCode(null) = 0 * ObjectUtils.hashCode(obj) = obj.hashCode() * </pre> * * @param obj the object to obtain the hash code of, may be <code>null</code> * @return the hash code of the object, or zero if null * @since 2.1 */ public static int hashCode(Object obj) { return (obj == null) ? 0 : obj.hashCode(); } // Identity ToString //----------------------------------------------------------------------- /** * <p>Gets the toString that would be produced by <code>Object</code> * if a class did not override toString itself. <code>null</code> * will return <code>null</code>.</p> * * <pre> * ObjectUtils.identityToString(null) = null * ObjectUtils.identityToString("") = "java.lang.String@1e23" * ObjectUtils.identityToString(Boolean.TRUE) = "java.lang.Boolean@7fa" * </pre> * * @param object the object to create a toString for, may be * <code>null</code> * @return the default toString text, or <code>null</code> if * <code>null</code> passed in */ public static String identityToString(Object object) { if (object == null) { return null; } StringBuffer buffer = new StringBuffer(); identityToString(buffer, object); return buffer.toString(); } /** * <p>Appends the toString that would be produced by <code>Object</code> * if a class did not override toString itself. <code>null</code> * will throw a NullPointerException for either of the two parameters. </p> * * <pre> * ObjectUtils.identityToString(buf, "") = buf.append("java.lang.String@1e23" * ObjectUtils.identityToString(buf, Boolean.TRUE) = buf.append("java.lang.Boolean@7fa" * ObjectUtils.identityToString(buf, Boolean.TRUE) = buf.append("java.lang.Boolean@7fa") * </pre> * * @param buffer the buffer to append to * @param object the object to create a toString for * @since 2.4 */ public static void identityToString(StringBuffer buffer, Object object) { if (object == null) { throw new NullPointerException("Cannot get the toString of a null identity"); } buffer.append(object.getClass().getName()).append('@') .append(Integer.toHexString(System.identityHashCode(object))); } // ToString //----------------------------------------------------------------------- /** * <p>Gets the <code>toString</code> of an <code>Object</code> returning * an empty string ("") if <code>null</code> input.</p> * * <pre> * ObjectUtils.toString(null) = "" * ObjectUtils.toString("") = "" * ObjectUtils.toString("bat") = "bat" * ObjectUtils.toString(Boolean.TRUE) = "true" * </pre> * * @see StringUtils#defaultString(String) * @see String#valueOf(Object) * @param obj the Object to <code>toString</code>, may be null * @return the passed in Object's toString, or nullStr if <code>null</code> input * @since 2.0 */ public static String toString(Object obj) { return obj == null ? "" : obj.toString(); } /** * <p>Gets the <code>toString</code> of an <code>Object</code> returning * a specified text if <code>null</code> input.</p> * * <pre> * ObjectUtils.toString(null, null) = null * ObjectUtils.toString(null, "null") = "null" * ObjectUtils.toString("", "null") = "" * ObjectUtils.toString("bat", "null") = "bat" * ObjectUtils.toString(Boolean.TRUE, "null") = "true" * </pre> * * @see StringUtils#defaultString(String,String) * @see String#valueOf(Object) * @param obj the Object to <code>toString</code>, may be null * @param nullStr the String to return if <code>null</code> input, may be null * @return the passed in Object's toString, or nullStr if <code>null</code> input * @since 2.0 */ public static String toString(Object obj, String nullStr) { return obj == null ? nullStr : obj.toString(); } // Null //----------------------------------------------------------------------- /** * <p>Class used as a null placeholder where <code>null</code> * has another meaning.</p> * * <p>For example, in a <code>HashMap</code> the * {@link java.util.HashMap#get(java.lang.Object)} method returns * <code>null</code> if the <code>Map</code> contains * <code>null</code> or if there is no matching key. The * <code>Null</code> placeholder can be used to distinguish between * these two cases.</p> * * <p>Another example is <code>Hashtable</code>, where <code>null</code> * cannot be stored.</p> */ public static class Null implements Serializable { /** * Required for serialization support. Declare serialization compatibility with Commons Lang 1.0 * * @see java.io.Serializable */ private static final long serialVersionUID = 7092611880189329093L; /** * Restricted constructor - singleton. */ Null() { super(); } /** * <p>Ensure singleton.</p> * * @return the singleton value */ private Object readResolve() { return ObjectUtils.NULL; } } }