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/* * @(#)Expression.java 1.15 06/04/07 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.beans; /** * An <code>Expression</code> object represents a primitive expression * in which a single method is applied to a target and a set of * arguments to return a result - as in <code>"a.getFoo()"</code>. * <p> * In addition to the properties of the super class, the * <code>Expression</code> object provides a <em>value</em> which * is the object returned when this expression is evaluated. * The return value is typically not provided by the caller and * is instead computed by dynamically finding the method and invoking * it when the first call to <code>getValue</code> is made. * * @see #getValue * @see #setValue * * @since 1.4 * * @version 1.3 11/15/00 * @author Philip Milne */ public class Expression extends Statement { private static Object unbound = new Object(); private Object value = unbound; /** * Creates a new <code>Statement</code> object with a <code>target</code>, * <code>methodName</code> and <code>arguments</code> as per the parameters. * * @param target The target of this expression. * @param methodName The methodName of this expression. * @param arguments The arguments of this expression. If <code>null</code> then an empty array will be used. * * @see #getValue */ public Expression(Object target, String methodName, Object[] arguments) { super(target, methodName, arguments); } /** * Creates a new <code>Expression</code> object for a method * that returns a result. The result will never be calculated * however, since this constructor uses the <code>value</code> * parameter to set the value property by calling the * <code>setValue</code> method. * * @param value The value of this expression. * @param target The target of this expression. * @param methodName The methodName of this expression. * @param arguments The arguments of this expression. If <code>null</code> then an empty array will be used. * * @see #setValue */ public Expression(Object value, Object target, String methodName, Object[] arguments) { this(target, methodName, arguments); setValue(value); } /** * If the value property of this instance is not already set, * this method dynamically finds the method with the specified * methodName on this target with these arguments and calls it. * The result of the method invocation is first copied * into the value property of this expression and then returned * as the result of <code>getValue</code>. If the value property * was already set, either by a call to <code>setValue</code> * or a previous call to <code>getValue</code> then the value * property is returned without either looking up or calling the method. * <p> * The value property of an <code>Expression</code> is set to * a unique private (non-<code>null</code>) value by default and * this value is used as an internal indication that the method * has not yet been called. A return value of <code>null</code> * replaces this default value in the same way that any other value * would, ensuring that expressions are never evaluated more than once. * <p> * See the <code>excecute</code> method for details on how * methods are chosen using the dynamic types of the target * and arguments. * * @see Statement#execute * @see #setValue * * @return The result of applying this method to these arguments. */ public Object getValue() throws Exception { if (value == unbound) { setValue(invoke()); } return value; } /** * Sets the value of this expression to <code>value</code>. * This value will be returned by the getValue method * without calling the method associated with this * expression. * * @param value The value of this expression. * * @see #getValue */ public void setValue(Object value) { this.value = value; } /*pp*/ String instanceName(Object instance) { return instance == unbound ? "<unbound>" : super.instanceName(instance); } /** * Prints the value of this expression using a Java-style syntax. */ public String toString() { return instanceName(value) + "=" + super.toString(); } }