libjava/classpath/java/beans/Expression.java

   1 /* java.beans.Expression
   2    Copyright (C) 2004, 2005 Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  19 02110-1301 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38 package java.beans;
  39
  40 /**
  41  * <p>An Expression captures the execution of an object method
  42  * that returns a value.</p>
  43  *
  44  * <p>It stores an object, the method to call, and the arguments to pass to
  45  * the method.</p>
  46  *
  47  * <p>While this class can generally be used to describe method calls it is
  48  * part of the XML serialization API.</p>
  49  *
  50  * @author Robert Schuster (robertschuster@fsfe.org)
  51  * @since 1.4
  52  */
  53 public class Expression extends Statement
  54 {
  55   // This is a placeholder to indicate that value hasn't been set
  56   // yet;
  57   private static final Object UNSET = new Object();
  58
  59   // The value to return. This is equal to unset until getValue is called.
  60   private Object value;
  61
  62   /**
  63    * Constructor Constructs an Expression representing the invocation of
  64    * object.methodName(arg[0], arg[1], ...); However, it will never be executed.
  65    * Instead, value will always be returned.
  66    *
  67    * @param value
  68    *          The value to return.
  69    * @param target
  70    *          The object to invoke the method on.
  71    * @param methodName
  72    *          The object method to invoke.
  73    * @param arguments
  74    *          An array of arguments to pass to the method.
  75    */
  76   public Expression(Object value, Object target, String methodName,
  77                     Object[] arguments)
  78   {
  79     super(target, methodName, arguments);
  80     this.value = value;
  81   }
  82
  83   /**
  84    * Constructor Constructs an Expression representing the invocation of
  85    * object.methodName(arg[0], arg[1], ...);
  86    *
  87    * @param target
  88    *          The object to invoke the method on.
  89    * @param methodName
  90    *          The object method to invoke.
  91    * @param arguments
  92    *          An array of arguments to pass to the method.
  93    */
  94   public Expression(Object target, String methodName, Object[] arguments)
  95   {
  96     super(target, methodName, arguments);
  97     this.value = UNSET;
  98   }
  99
 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 }