org.openmdx.dalvik.uses.java.beans.Expression Maven / Gradle / Ivy
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package org.openmdx.dalvik.uses.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.
*
* @see #getValue
* @see #setValue
*
*
* openMDX/Dalvik Notice (January 2013):
* THIS CODE HAS BEEN MODIFIED AND ITS NAMESPACE HAS BEEN PREFIXED WITH
* org.openmdx.dalvik.uses.
*
* @since openMDX 2.12
* @author openMDX Team
*
* @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.
*
* @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
*/
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.
*
* @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
*/
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 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.
*
* @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 ? "" : super.instanceName(instance);
}
/**
* Prints the value of this expression using a Java-style syntax.
*/
public String toString() {
return instanceName(value) + "=" + super.toString();
}
}