groovy-core/src/main/org/codehaus/groovy/runtime/NullObject.java

/*
 $Id$

 Copyright 2003 (C) James Strachan and Bob Mcwhirter. All Rights Reserved.

 Redistribution and use of this software and associated documentation
 ("Software"), with or without modification, are permitted provided
 that the following conditions are met:

 1. Redistributions of source code must retain copyright
    statements and notices.  Redistributions must also contain a
    copy of this document.

 2. Redistributions in binary form must reproduce the
    above copyright notice, this list of conditions and the
    following disclaimer in the documentation and/or other
    materials provided with the distribution.

 3. The name "groovy" must not be used to endorse or promote
    products derived from this Software without prior written
    permission of The Codehaus.  For written permission,
    please contact info@codehaus.org.

 4. Products derived from this Software may not be called "groovy"
    nor may "groovy" appear in their names without prior written
    permission of The Codehaus. "groovy" is a registered
    trademark of The Codehaus.

 5. Due credit should be given to The Codehaus -
    http://groovy.codehaus.org/

 THIS SOFTWARE IS PROVIDED BY THE CODEHAUS AND CONTRIBUTORS
 ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
 NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 THE CODEHAUS OR ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
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 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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 */
package org.codehaus.groovy.runtime;

import java.util.Collections;
import java.util.Iterator;

import groovy.lang.GroovyObjectSupport;

public class NullObject extends GroovyObjectSupport {
  private static NullObject ref;

  /**
   * private constructor
   **/
  private NullObject() {
  }

  /**
   * get the NullObject reference
   **/
  public synchronized static NullObject getNullObject() {
    if (ref == null)
      ref = new NullObject();
    return ref;
  }

  /**
   * Since this is implemented as a singleton, we should avoid the 
   * use of the clone method
   **/
  public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException();
  }

  /**
   * @param property - the property to get
   * @returns a NPE
   **/
  public Object getProperty(String property) {
    throw new NullPointerException("Cannot get property " + property + "() on null object");
  }

  /**
   * @param property - the property to set
   * @returns a NPE
   **/
  public Object setProperty(String property) {
    throw new NullPointerException("Cannot set property " + property + "() on null object");
  }

  /**
   * @param name the name of the method to invoke
   * @param args - arguments to the method
   * @returns a NPE
   * 
   **/
  public Object invokeMethod(String name, Object args) {
    throw new NullPointerException("Cannot invoke method " + name + "() on null object");
  }

  /**
   * @param to - the reference object with which to compare
   * @returns - true if this object is the same as the to argument
   **/
  public boolean equals(Object to) {
    return to == null;
  }

  /**
   * iterator() method to be able to iterate on null. 
   * Note: this part is from Invoker
   * @return an iterator for an empty list
   */
  public Iterator iterator() {
      return Collections.EMPTY_LIST.iterator();
  }
  
  public Object plus(String s){
      return "null"+s;
  }
  
}