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package java.beans;
An Expression 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 "a.getFoo()".
In addition to the properties of the super class, the
Expression object provides a value 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 getValue is made.
Author: Philip Milne See Also: - getValue
- setValue
Since: 1.4
/**
* 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 Statement object with a target,
methodName and arguments as per the parameters.
Params: - target – The target of this expression.
- methodName – The methodName of this expression.
- arguments – The arguments of this expression. If
null then an empty array will be used.
See Also:
/**
* 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 Expression object for a method
that returns a result. The result will never be calculated
however, since this constructor uses the value
parameter to set the value property by calling the
setValue method.
Params: - value – The value of this expression.
- target – The target of this expression.
- methodName – The methodName of this expression.
- arguments – The arguments of this expression. If
null then an empty array will be used.
See Also:
/**
* 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 getValue. If the value property
was already set, either by a call to setValue
or a previous call to getValue then the value
property is returned without either looking up or calling the method.
The value property of an Expression is set to
a unique private (non-null) value by default and
this value is used as an internal indication that the method
has not yet been called. A return value of null
replaces this default value in the same way that any other value
would, ensuring that expressions are never evaluated more than once.
See the excecute method for details on how
methods are chosen using the dynamic types of the target
and arguments.
See Also: - execute.execute
- setValue
Returns: The result of applying this method to these arguments.
/**
* 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 value.
This value will be returned by the getValue method
without calling the method associated with this
expression.
Params: - value – The value of this expression.
See Also:
/**
* 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.
/**
* Prints the value of this expression using a Java-style syntax.
*/
public String toString() {
return instanceName(value) + "=" + super.toString();
}
}