Source for java.beans.Expression

   1: /* java.beans.Expression
   2:    Copyright (C) 2004 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: 
  39: package java.beans;
  40: 
  41: /**
  42:  * class Expression
  43:  *
  44:  * An Expression captures the execution of an object method that
  45:  * returns a value.  It stores an object, the method to call, and the
  46:  * arguments to pass to the method.
  47:  *
  48:  * @since 1.4
  49:  */
  50: public class Expression extends Statement
  51: {
  52:   // This is a placeholder to indicate that value hasn't been set
  53:   // yet;
  54:   private static final Object unset = new Object();
  55: 
  56:   // The value to return.  This is equal to unset until getValue is called.
  57:   private Object value;
  58:    
  59: 
  60:   /**
  61:    * Constructor
  62:    *
  63:    * Constructs an Expression representing the invocation of
  64:    * object.methodName(arg[0], arg[1], ...);  However, it will never
  65:    * be executed.  Instead, value will always be returned.
  66:    *
  67:    * @param value The value to return.
  68:    * @param target The object to invoke the method on.
  69:    * @param methodName The object method to invoke.
  70:    * @param arguments An array of arguments to pass to the method.
  71:    */
  72:   public Expression(Object value, Object target, String methodName,
  73:             Object[] arguments)
  74:   {
  75:     super(target, methodName, arguments);
  76:     this.value = value;
  77:   }
  78: 
  79:   /**
  80:    * Constructor
  81:    *
  82:    * Constructs an Expression representing the invocation of
  83:    * object.methodName(arg[0], arg[1], ...);
  84:    *
  85:    * @param target The object to invoke the method on.
  86:    * @param methodName The object method to invoke.
  87:    * @param arguments An array of arguments to pass to the method.
  88:    */
  89:   public Expression(Object target, String methodName, Object[] arguments)
  90:   {
  91:     super(target, methodName, arguments);
  92:     this.value = unset;
  93:   }
  94: 
  95:   /**
  96:    * Return the result of executing the method.
  97:    *
  98:    * If the cached value has not yet been set, the method is
  99:    * executed in the same way as Statement.execute(), except that
 100:    * the value is cached, and then returned.  If the value has been
 101:    * set, it is returned without executing the method again.
 102:    *
 103:    * @return the result of executing the method.
 104:    * @exception Exception if an error occurs
 105:    */
 106:   public Object getValue() throws Exception
 107:   {
 108:     if (value == unset)
 109:       value = doExecute();
 110:     return value;
 111:   }
 112: 
 113:   /**
 114:    * Set the cached value to be returned by getValue()
 115:    *
 116:    * @param value the value to cache and return.
 117:    */
 118:   public void setValue(Object value)
 119:   {
 120:     this.value = value;
 121:   }
 122:     
 123:   /**
 124:    * Return a string representation of this expression.
 125:    */
 126:   public String toString()
 127:   {
 128:     String result = super.toString();
 129:     if (value != unset)
 130:       return value.getClass().getName() + " " + result;
 131:     return result;
 132:   }
 133: }