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; 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> * * <p>#ThreadSafe#</p> * @author Apache Software Foundation * @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 Gary Gregory * @author Mario Winterer * @author <a href="mailto:david@davidkarlsen.com">David J. M. Karlsen</a> * @since 1.0 * @version $Id$ */ //@Immutable 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 <T> T defaultIfNull(T object, T defaultValue) { return object != null ? object : defaultValue; } /** * <p>Returns the first value in the array which is not <code>null</code>. * If all the values are <code>null</code> or the array is <code>null</code> * or empty then <code>null</code> is returned.</p> * * <pre> * ObjectUtils.firstNonNull(null, null) = null * ObjectUtils.firstNonNull(null, "") = "" * ObjectUtils.firstNonNull(null, null, "") = "" * ObjectUtils.firstNonNull(null, "zz") = "zz" * ObjectUtils.firstNonNull("abc", *) = "abc" * ObjectUtils.firstNonNull(null, "xyz", *) = "xyz" * ObjectUtils.firstNonNull(Boolean.TRUE, *) = Boolean.TRUE * ObjectUtils.firstNonNull() = null * </pre> * * @param values the values to test, may be <code>null</code> or empty * @return the first value from <code>values</code> which is not <code>null</code>, * or <code>null</code> if there are no non-null values */ public static <T> T firstNonNull(T... values) { if (values != null) { for (T val : values) { if (val != null) { return val; } } } return null; } // Null-safe equals/hashCode //----------------------------------------------------------------------- /** * <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>Compares two objects for inequality, where either one or both * objects may be <code>null</code>.</p> * * <pre> * ObjectUtils.notEqual(null, null) = false * ObjectUtils.notEqual(null, "") = true * ObjectUtils.notEqual("", null) = true * ObjectUtils.notEqual("", "") = false * ObjectUtils.notEqual(Boolean.TRUE, null) = true * ObjectUtils.notEqual(Boolean.TRUE, "true") = true * ObjectUtils.notEqual(Boolean.TRUE, Boolean.TRUE) = false * ObjectUtils.notEqual(Boolean.TRUE, Boolean.FALSE) = true * </pre> * * @param object1 the first object, may be <code>null</code> * @param object2 the second object, may be <code>null</code> * @return <code>false</code> if the values of both objects are the same */ public static boolean notEqual(Object object1, Object object2) { return ObjectUtils.equals(object1, object2) == false; } /** * <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(); } // Min/Max //----------------------------------------------------------------------- /** * Null safe comparison of Comparables. * * @param values the set of comparable values, may be null * @return * <ul> * <li>If any objects are non-null and unequal, the lesser object. * <li>If all objects are non-null and equal, the first. * <li>If any of the comparables are null, the lesser of the non-null object. * <li>If all the comparables are null, null is returned. * </ul> */ public static <T extends Comparable<? super T>> T min(T... values) { T result = null; if (values != null) { for (T value : values) { if (compare(value, result, true) < 0) { result = value; } } } return result; } /** * Null safe comparison of Comparables. * * @param values the set of comparable values, may be null * @return * <ul> * <li>If any objects are non-null and unequal, the greater object. * <li>If all objects are non-null and equal, the first. * <li>If any of the comparables are null, the greater of the non-null object. * <li>If all the comparables are null, null is returned. * </ul> */ public static <T extends Comparable<? super T>> T max(T... values) { T result = null; if (values != null) { for (T value : values) { if (compare(value, result, false) > 0) { result = value; } } } return result; } /** * Null safe comparison of Comparables. * {@code null} is assumed to be less than a non-{@code null} value. * * @param c1 the first comparable, may be null * @param c2 the second comparable, may be null * @return a negative value if c1 < c2, zero if c1 = c2 * and a positive value if c1 > c2 */ public static <T extends Comparable<? super T>> int compare(T c1, T c2) { return compare(c1, c2, false); } /** * Null safe comparison of Comparables. * * @param c1 the first comparable, may be null * @param c2 the second comparable, may be null * @param nullGreater if true <code>null</code> is considered greater * than a Non-<code>null</code> value or if false <code>null</code> is * considered less than a Non-<code>null</code> value * @return a negative value if c1 < c2, zero if c1 = c2 * and a positive value if c1 > c2 * @see java.util.Comparator#compare(Object, Object) */ public static <T extends Comparable<? super T>> int compare(T c1, T c2, boolean nullGreater) { if (c1 == c2) { return 0; } else if (c1 == null) { return (nullGreater ? 1 : -1); } else if (c2 == null) { return (nullGreater ? -1 : 1); } return c1.compareTo(c2); } // /** // * Clone an object. // * // * @param <T> the type of the object // * @param o the object to clone // * @return the clone if the object implements {@link Cloneable} otherwise <code>null</code> // * @throws CloneFailedException if the object is cloneable and the clone operation fails // * @since 3.0 // */ // public static <T> T clone(final T o) { // if (o instanceof Cloneable) { // final Object result; // if (o.getClass().isArray()) { // final Class<?> componentType = o.getClass().getComponentType(); // if (!componentType.isPrimitive()) { // result = ((Object[]) o).clone(); // } else { // int length = Array.getLength(o); // result = Array.newInstance(componentType, length); // while (length-- > 0) { // Array.set(result, length, Array.get(o, length)); // } // } // } else { // try { // final Method clone = o.getClass().getMethod("clone"); // result = clone.invoke(o); // } catch (final NoSuchMethodException e) { // throw new CloneFailedException("Cloneable type " // + o.getClass().getName() // + " has no clone method", e); // } catch (final IllegalAccessException e) { // throw new CloneFailedException("Cannot clone Cloneable type " // + o.getClass().getName(), e); // } catch (final InvocationTargetException e) { // throw new CloneFailedException("Exception cloning Cloneable type " // + o.getClass().getName(), e.getCause()); // } // } // @SuppressWarnings("unchecked") // final T checked = (T) result; // return checked; // } // // return null; // } /** * Clone an object if possible. This method is similar to {@link #clone(Object)}, but will * return the provided instance as the return value instead of <code>null</code> if the instance * is not cloneable. This is more convenient if the caller uses different * implementations (e.g. of a service) and some of the implementations do not allow concurrent * processing or have state. In such cases the implementation can simply provide a proper * clone implementation and the caller's code does not have to change. * * @param <T> the type of the object * @param o the object to clone * @return the clone if the object implements {@link Cloneable} otherwise the object itself * @throws CloneFailedException if the object is cloneable and the clone operation fails * @since 3.0 */ // public static <T> T cloneIfPossible(final T o) { // final T clone = clone(o); // return clone == null ? o : clone; // } // 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; } } }