FileDocCategorySizeDatePackage
Expression.javaAPI DocJava SE 6 API4695Tue Jun 10 00:25:30 BST 2008java.beans

Expression

public 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.

see
#getValue
see
#setValue
since
1.4
version
1.3 11/15/00
author
Philip Milne

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.

param
target The target of this expression.
param
methodName The methodName of this expression.
param
arguments The arguments of this expression. If null then an empty array will be used.
see
#getValue

 
    
                                                                  
            
        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.

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 null then an empty array will be used.
see
#setValue

 
        this(target, methodName, arguments); 
        setValue(value); 
    
Methods Summary
public java.lang.ObjectgetValue()
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
Statement#execute
see
#setValue
return
The result of applying this method to these arguments.

 
        if (value == unbound) { 
            setValue(invoke());
        }
        return value;
    
java.lang.StringinstanceName(java.lang.Object instance)

 
        return instance == unbound ? "<unbound>" : super.instanceName(instance); 
    
public voidsetValue(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.

param
value The value of this expression.
see
#getValue

 
        this.value = value;
    
public java.lang.StringtoString()
Prints the value of this expression using a Java-style syntax.

 
        return instanceName(value) + "=" + super.toString();