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 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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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
 *
 * @author Philip Milne
 */
public class Expression extends Statement {

 private static Object unbound = new Object();

 private Object value = unbound;

 /**
 * Creates a new {@link Expression} object
 * for the specified target object to invoke the method
 * specified by the name and by the array of arguments.
 * <p>
 * The {@code target} and the {@code methodName} values should not be {@code null}.
 * Otherwise an attempt to execute this {@code Expression}
 * will result in a {@code NullPointerException}.
 * If the {@code arguments} value is {@code null},
 * an empty array is used as the value of the {@code arguments} property.
 *
 * @param target the target object of this expression
 * @param methodName the name of the method to invoke on the specified target
 * @param arguments the array of arguments to invoke the specified method
 *
 * @see #getValue
 */
 @ConstructorProperties({"target", "methodName", "arguments"})
 public Expression(Object target, String methodName, Object[] arguments) {
 super(target, methodName, arguments);
 }

 /**
 * Creates a new {@link Expression} object with the specified value
 * for the specified target object to invoke the method
 * specified by the name and by the array of arguments.
 * The {@code value} value is used as the value of the {@code value} property,
 * so the {@link #getValue} method will return it
 * without executing this {@code Expression}.
 * <p>
 * The {@code target} and the {@code methodName} values should not be {@code null}.
 * Otherwise an attempt to execute this {@code Expression}
 * will result in a {@code NullPointerException}.
 * If the {@code arguments} value is {@code null},
 * an empty array is used as the value of the {@code arguments} property.
 *
 * @param value the value of this expression
 * @param target the target object of this expression
 * @param methodName the name of the method to invoke on the specified target
 * @param arguments the array of arguments to invoke the specified method
 *
 * @see #setValue
 */
 public Expression(Object value, Object target, String methodName, Object[] arguments) {
 this(target, methodName, arguments);
 setValue(value);
 }

 /**
 * {@inheritDoc}
 * <p>
 * If the invoked method completes normally,
 * the value it returns is copied in the {@code value} property.
 * Note that the {@code value} property is set to {@code null},
 * if the return type of the underlying method is {@code void}.
 *
 * @throws NullPointerException if the value of the {@code target} or
 * {@code methodName} property is {@code null}
 * @throws NoSuchMethodException if a matching method is not found
 * @throws SecurityException if a security manager exists and
 * it denies the method invocation
 * @throws Exception that is thrown by the invoked method
 *
 * @see java.lang.reflect.Method
 * @since 1.7
 */
 @Override
 public void execute() throws Exception {
 setValue(invoke());
 }

 /**
 * 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>execute</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.
 * @throws Exception if the method with the specified methodName
 * throws an exception
 */
 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();
 }
}