/* java.beans.Expression
   Copyright (C) 2004, 2005 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
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any later version.
 
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WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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02110-1301 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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permission to link this library with independent modules to produce an
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package java.beans;

/**
 * <p>An Expression captures the execution of an object method
 * that returns a value.</p>
 * 
 * <p>It stores an object, the method to call, and the arguments to pass to
 * the method.</p>
 * 
 * <p>While this class can generally be used to describe method calls it is
 * part of the XML serialization API.</p> 
 * 
 * @author Robert Schuster (robertschuster@fsfe.org)
 * @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;
  }
}