code: /*
   * 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>
   *
   * @author <a href="mailto:nissim@nksystems.com">Nissim Karpenstein</a>
   * @author <a href="mailto:janekdb@yahoo.co.uk">Janek Bogucki</a>
   * @author <a href="mailto:dlr@finemaltcoding.com">Daniel Rall</a>
   * @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 491050 2006-12-29 17:04:00Z scolebourne $
   */
  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;
          }
          return appendIdentityToString(null, object).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 return <code>null</code>.</p>
       *
       * <pre>
       * ObjectUtils.appendIdentityToString(*, null)            = null
       * ObjectUtils.appendIdentityToString(null, "")           = "java.lang.String@1e23"
       * ObjectUtils.appendIdentityToString(null, Boolean.TRUE) = "java.lang.Boolean@7fa"
       * ObjectUtils.appendIdentityToString(buf, Boolean.TRUE)  = buf.append("java.lang.Boolean@7fa")
       * </pre>
       *
       * @param buffer  the buffer to append to, may be <code>null</code>
       * @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
       * @since 2.0
       */
      public static StringBuffer appendIdentityToString(StringBuffer buffer, Object object) {
          if (object == null) {
              return null;
          }
          if (buffer == null) {
              buffer = new StringBuffer();
          }
          return 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 c1  the first comparable, may be null
       * @param c2  the second comparable, may be null
       * @return
       *  <ul>
       *   <li>If both objects are non-null and unequal, the lesser object.
       *   <li>If both objects are non-null and equal, c1.
       *   <li>If one of the comparables is null, the non-null object.
       *   <li>If both the comparables are null, null is returned.
       *  </ul>
       */
      public static Object min(Comparable c1, Comparable c2) {
          if (c1 != null && c2 != null) {
              return c1.compareTo(c2) < 1 ? c1 : c2;
          } else {
              return c1 != null ? c1 : c2;
          }                              
      }
  
      /**
       * Null safe comparison of Comparables.
       * 
       * @param c1  the first comparable, may be null
       * @param c2  the second comparable, may be null
       * @return
       *  <ul>
       *   <li>If both objects are non-null and unequal, the greater object.
       *   <li>If both objects are non-null and equal, c1.
       *   <li>If one of the comparables is null, the non-null object.
       *   <li>If both the comparables are null, null is returned.
       *  </ul>
       */
      public static Object max(Comparable c1, Comparable c2) {
          if (c1 != null && c2 != null) {
              return c1.compareTo(c2) >= 0 ? c1 : c2;
          } else {
              return c1 != null ? c1 : c2;
          }
      }
  
      // 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;
          }
      }
  
  }
  