001/* java.beans.Expression
002   Copyright (C) 2004, 2005 Free Software Foundation, Inc.
003
004This file is part of GNU Classpath.
005
006GNU Classpath is free software; you can redistribute it and/or modify
007it under the terms of the GNU General Public License as published by
008the Free Software Foundation; either version 2, or (at your option)
009any later version.
010 
011GNU Classpath is distributed in the hope that it will be useful, but
012WITHOUT ANY WARRANTY; without even the implied warranty of
013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014General Public License for more details.
015
016You should have received a copy of the GNU General Public License
017along with GNU Classpath; see the file COPYING.  If not, write to the
018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
01902110-1301 USA.
020
021Linking this library statically or dynamically with other modules is
022making a combined work based on this library.  Thus, the terms and
023conditions of the GNU General Public License cover the whole
024combination.
025
026As a special exception, the copyright holders of this library give you
027permission to link this library with independent modules to produce an
028executable, regardless of the license terms of these independent
029modules, and to copy and distribute the resulting executable under
030terms of your choice, provided that you also meet, for each linked
031independent module, the terms and conditions of the license of that
032module.  An independent module is a module which is not derived from
033or based on this library.  If you modify this library, you may extend
034this exception to your version of the library, but you are not
035obligated to do so.  If you do not wish to do so, delete this
036exception statement from your version. */
037
038package java.beans;
039
040/**
041 * <p>An Expression captures the execution of an object method
042 * that returns a value.</p>
043 * 
044 * <p>It stores an object, the method to call, and the arguments to pass to
045 * the method.</p>
046 * 
047 * <p>While this class can generally be used to describe method calls it is
048 * part of the XML serialization API.</p> 
049 * 
050 * @author Robert Schuster (robertschuster@fsfe.org)
051 * @since 1.4
052 */
053public class Expression extends Statement
054{
055  // This is a placeholder to indicate that value hasn't been set
056  // yet;
057  private static final Object UNSET = new Object();
058
059  // The value to return. This is equal to unset until getValue is called.
060  private Object value;
061
062  /**
063   * Constructor Constructs an Expression representing the invocation of
064   * object.methodName(arg[0], arg[1], ...); However, it will never be executed.
065   * Instead, value will always be returned.
066   * 
067   * @param value
068   *          The value to return.
069   * @param target
070   *          The object to invoke the method on.
071   * @param methodName
072   *          The object method to invoke.
073   * @param arguments
074   *          An array of arguments to pass to the method.
075   */
076  public Expression(Object value, Object target, String methodName,
077                    Object[] arguments)
078  {
079    super(target, methodName, arguments);
080    this.value = value;
081  }
082
083  /**
084   * Constructor Constructs an Expression representing the invocation of
085   * object.methodName(arg[0], arg[1], ...);
086   * 
087   * @param target
088   *          The object to invoke the method on.
089   * @param methodName
090   *          The object method to invoke.
091   * @param arguments
092   *          An array of arguments to pass to the method.
093   */
094  public Expression(Object target, String methodName, Object[] arguments)
095  {
096    super(target, methodName, arguments);
097    this.value = UNSET;
098  }
099
100  /**
101   * Return the result of executing the method. If the cached value has not yet
102   * been set, the method is executed in the same way as Statement.execute(),
103   * except that the value is cached, and then returned. If the value has been
104   * set, it is returned without executing the method again.
105   * 
106   * @return the result of executing the method.
107   * @exception Exception
108   *              if an error occurs
109   */
110  public Object getValue() throws Exception
111  {
112    if (value == UNSET)
113      value = doExecute();
114    return value;
115  }
116
117  /**
118   * Set the cached value to be returned by getValue()
119   * 
120   * @param value
121   *          the value to cache and return.
122   */
123  public void setValue(Object value)
124  {
125    this.value = value;
126  }
127
128  /**
129   * Return a string representation of this expression.
130   */
131  public String toString()
132  {
133    String result = super.toString();
134    if (value != UNSET)
135      return value.getClass().getName() + "=" + result;
136    return result;
137  }
138}