/* java.beans.Expression
   Copyright (C) 2004 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
 
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02111-1307 USA.

Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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exception statement from your version. */


package java.beans;

/**
 * class Expression
 *
 * An Expression captures the execution of an object method that
 * returns a value.  It stores an object, the method to call, and the
 * arguments to pass to the method.
 *
 * @since 1.4
 */
public class Expression extends Statement
{
  // This is a placeholder to indicate that value hasn't been set
  // yet;
  private static final Object unset = new Object();

  // The value to return.  This is equal to unset until getValue is called.
  private Object value;
   

  /**
   * Constructor
   *
   * Constructs an Expression representing the invocation of
   * object.methodName(arg[0], arg[1], ...);  However, it will never
   * be executed.  Instead, value will always be returned.
   *
   * @param value The value to return.
   * @param target The object to invoke the method on.
   * @param methodName The object method to invoke.
   * @param arguments An array of arguments to pass to the method.
   */
  public Expression(Object value, Object target, String methodName,
		    Object[] arguments)
  {
    super(target, methodName, arguments);
    this.value = value;
  }

  /**
   * Constructor
   *
   * Constructs an Expression representing the invocation of
   * object.methodName(arg[0], arg[1], ...);
   *
   * @param target The object to invoke the method on.
   * @param methodName The object method to invoke.
   * @param arguments An array of arguments to pass to the method.
   */
  public Expression(Object target, String methodName, Object[] arguments)
  {
    super(target, methodName, arguments);
    this.value = unset;
  }

  /**
   * Return the result of executing the method.
   *
   * If the cached value has not yet been set, the method is
   * executed in the same way as Statement.execute(), except that
   * the value is cached, and then returned.  If the value has been
   * set, it is returned without executing the method again.
   *
   * @return the result of executing the method.
   * @exception Exception if an error occurs
   */
  public Object getValue() throws Exception
  {
    if (value == unset)
      value = doExecute();
    return value;
  }

  /**
   * Set the cached value to be returned by getValue()
   *
   * @param value the value to cache and return.
   */
  public void setValue(Object value)
  {
    this.value = value;
  }
    
  /**
   * Return a string representation of this expression.
   */
  public String toString()
  {
    String result = super.toString();
    if (value != unset)
      return value.getClass().getName() + " " + result;
    return result;
  }
}
