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				/*  Copyright (c) 2000-2004 jMock.org
				 */
				package org.jmock.expectation;
				
				/**
				 * A class that represents the null value.
				 * The {@link org.jmock.expectation.Null Null} class is used when an
				 * {@link org.jmock.expectation.Expectation Expectation} is set to expect nothing.
				 * 
				 * Example usage:
				 * 
				 * public class MockX {
				 *    private Expectation... anExpectation = new Expectation...(...);
				 * 
				 *    public MockX() {
				 *       anExpectation.setExpectNothing();
				 *    }
				 * 
				 *    public void setAnExpectation(Object value) {
				 *       anExpectation.setExpected(value);
				 *    }
				 * 
				 *    public void setActual(Object value) {
				 *       anExpectation.setActual(value);
				 *    }
				 * }
				 * 
				 * The act of calling {@link org.jmock.expectation.Expectation#setExpectNothing() Expectation.setExpectNothing()}
				 * tells the expectation that it should expect no values to change.  Since
				 * all {@link org.jmock.expectation.Null Null} objects are equal to themselves,
				 * most expectations set their expected value to an instance of
				 * {@link org.jmock.expectation.Null Null}, and at the same time, set their actual
				 * value to another instance of {@link org.jmock.expectation.Null Null}.
				 * This way, when {@link org.jmock.expectation.Verifiable#verify() verify()} checks
				 * expectations, they will compare two {@link org.jmock.expectation.Null Null}
				 * objects together, which is guaranteed to succeed.
				 * 
				 * @author Francois Beausoleil (fbos@users.sourceforge.net)
				 * @version $Id: Null.java,v 1.4 2004/05/05 08:52:48 npryce Exp $
				 */
				public class Null
				{
				    /**
				     * The default description for all {@link org.jmock.expectation.Null Null}
				     * objects.
				     * This String is equal to "Null".
				     */
				    public static final String DEFAULT_DESCRIPTION = "Null";
				
				    /**
				     * A default {@link org.jmock.expectation.Null Null} object.
				     * Instead of always instantiating new {@link org.jmock.expectation.Null Null}
				     * objects, consider using a reference to this object instead. This way,
				     * the virtual machine will not be taking the time required to instantiate
				     * an object everytime it is required.
				     */
				    public static final Null NULL = new Null();
				
				    /**
				     * The description of this {@link org.jmock.expectation.Null Null} object.
				     */
				    final private String myDescription;
				
				    /**
				     * Instantiates a new {@link org.jmock.expectation.Null Null} object with
				     * the default description.
				     *
				     * @see org.jmock.expectation.Null#DEFAULT_DESCRIPTION
				     */
				    public Null() {
				        this(DEFAULT_DESCRIPTION);
				    }
				
				    /**
				     * Instantiates a new {@link org.jmock.expectation.Null Null} object and
				     * sets it's description.
				     *
				     * @param description
				     */
				    public Null( String description ) {
				        super();
				        myDescription = description;
				    }
				
				    /**
				     * Determines equality between two objects.
				     * {@link org.jmock.expectation.Null Null} objects are only equal to
				     * another instance of themselves.
				     *
				     * @param other
				     */
				    public boolean equals( Object other ) {
				        return other instanceof Null;
				    }
				
				    /**
				     * Returns this {@link org.jmock.expectation.Null Null} object's hashCode.
				     * All  {@link org.jmock.expectation.Null Null} return the same
				     * hashCode value.
				     */
				    public int hashCode() {
				        return 0;
				    }
				
				    /**
				     * Returns a string representation of this {@link org.jmock.expectation.Null Null}
				     * object.
				     * This merely returns the string passed to the constructor initially.
				     */
				    public String toString() {
				        return myDescription;
				    }
				}