不管是测试驱动开发或者是其它的开发模式

				/*  Copyright (c) 2000-2004 jMock.org
				 */
				package org.jmock.core.constraint;
				
				import org.jmock.core.Constraint;
				
				
				/**
				 * Is the value null?
				 */
				public class IsNull implements Constraint
				{
				    public boolean eval( Object o ) {
				        return o == null;
				    }
				
				    public StringBuffer describeTo( StringBuffer buffer ) {
				        return buffer.append("null");
				    }
				}