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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
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* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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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 <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();
}
}