001    /*
002     $Id: NullObject.java 4240 2006-11-16 10:47:28Z blackdrag $
003    
004     Copyright 2003 (C) James Strachan and Bob Mcwhirter. All Rights Reserved.
005    
006     Redistribution and use of this software and associated documentation
007     ("Software"), with or without modification, are permitted provided
008     that the following conditions are met:
009    
010     1. Redistributions of source code must retain copyright
011        statements and notices.  Redistributions must also contain a
012        copy of this document.
013    
014     2. Redistributions in binary form must reproduce the
015        above copyright notice, this list of conditions and the
016        following disclaimer in the documentation and/or other
017        materials provided with the distribution.
018    
019     3. The name "groovy" must not be used to endorse or promote
020        products derived from this Software without prior written
021        permission of The Codehaus.  For written permission,
022        please contact info@codehaus.org.
023    
024     4. Products derived from this Software may not be called "groovy"
025        nor may "groovy" appear in their names without prior written
026        permission of The Codehaus. "groovy" is a registered
027        trademark of The Codehaus.
028    
029     5. Due credit should be given to The Codehaus -
030        http://groovy.codehaus.org/
031    
032     THIS SOFTWARE IS PROVIDED BY THE CODEHAUS AND CONTRIBUTORS
033     ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
034     NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
035     FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
036     THE CODEHAUS OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
037     INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
038     (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
039     SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
040     HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
041     STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
042     ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
043     OF THE POSSIBILITY OF SUCH DAMAGE.
044    
045     */
046    package org.codehaus.groovy.runtime;
047    
048    import java.util.Collections;
049    import java.util.Iterator;
050    
051    import groovy.lang.GroovyObjectSupport;
052    
053    public class NullObject extends GroovyObjectSupport {
054      private static NullObject ref;
055    
056      /**
057       * private constructor
058       **/
059      private NullObject() {
060      }
061    
062      /**
063       * get the NullObject reference
064       **/
065      public synchronized static NullObject getNullObject() {
066        if (ref == null)
067          ref = new NullObject();
068        return ref;
069      }
070    
071      /**
072       * Since this is implemented as a singleton, we should avoid the 
073       * use of the clone method
074       **/
075      public Object clone() throws CloneNotSupportedException {
076        throw new CloneNotSupportedException();
077      }
078    
079      /**
080       * @param property - the property to get
081       * @returns a NPE
082       **/
083      public Object getProperty(String property) {
084        throw new NullPointerException("Cannot get property " + property + "() on null object");
085      }
086    
087      /**
088       * @param property - the property to set
089       * @returns a NPE
090       **/
091      public Object setProperty(String property) {
092        throw new NullPointerException("Cannot set property " + property + "() on null object");
093      }
094    
095      /**
096       * @param name the name of the method to invoke
097       * @param args - arguments to the method
098       * @returns a NPE
099       * 
100       **/
101      public Object invokeMethod(String name, Object args) {
102        throw new NullPointerException("Cannot invoke method " + name + "() on null object");
103      }
104    
105      /**
106       * @param to - the reference object with which to compare
107       * @returns - true if this object is the same as the to argument
108       **/
109      public boolean equals(Object to) {
110        return to == null;
111      }
112    
113      /**
114       * iterator() method to be able to iterate on null. 
115       * Note: this part is from Invoker
116       * @return an iterator for an empty list
117       */
118      public Iterator iterator() {
119          return Collections.EMPTY_LIST.iterator();
120      }
121      
122      public Object plus(String s){
123          return "null"+s;
124      }
125      
126    }