Expressionpublic class Expression extends Statement 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. |
| Fields Summary |
|---|
| private static Object | unbound | | private Object | value |
| Constructors Summary |
|---|
public Expression(Object target, String methodName, Object[] arguments)Creates a new Statement object with a target,
methodName and arguments as per the parameters.
super(target, methodName, arguments);
| public Expression(Object value, Object target, String methodName, Object[] 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.
this(target, methodName, arguments);
setValue(value);
|
| Methods Summary |
|---|
| public java.lang.Object | getValue()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.
if (value == unbound) {
setValue(invoke());
}
return value;
| | java.lang.String | instanceName(java.lang.Object instance)
return instance == unbound ? "<unbound>" : super.instanceName(instance);
| | public void | setValue(java.lang.Object 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.
this.value = value;
| | public java.lang.String | toString()Prints the value of this expression using a Java-style syntax.
return instanceName(value) + "=" + super.toString();
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