001    /* java.beans.Expression
002       Copyright (C) 2004, 2005 Free Software Foundation, Inc.
003    
004    This file is part of GNU Classpath.
005    
006    GNU Classpath is free software; you can redistribute it and/or modify
007    it under the terms of the GNU General Public License as published by
008    the Free Software Foundation; either version 2, or (at your option)
009    any later version.
010    
011    GNU Classpath is distributed in the hope that it will be useful, but
012    WITHOUT ANY WARRANTY; without even the implied warranty of
013    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
014    General Public License for more details.
015    
016    You should have received a copy of the GNU General Public License
017    along with GNU Classpath; see the file COPYING.  If not, write to the
018    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
019    02110-1301 USA.
020    
021    Linking this library statically or dynamically with other modules is
022    making a combined work based on this library.  Thus, the terms and
023    conditions of the GNU General Public License cover the whole
024    combination.
025    
026    As a special exception, the copyright holders of this library give you
027    permission to link this library with independent modules to produce an
028    executable, regardless of the license terms of these independent
029    modules, and to copy and distribute the resulting executable under
030    terms of your choice, provided that you also meet, for each linked
031    independent module, the terms and conditions of the license of that
032    module.  An independent module is a module which is not derived from
033    or based on this library.  If you modify this library, you may extend
034    this exception to your version of the library, but you are not
035    obligated to do so.  If you do not wish to do so, delete this
036    exception statement from your version. */
037    
038    package 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     */
053    public 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    }