Source for java.beans.Expression

   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: }