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package language;
import org.aspectj.testing.Tester;
/** @author Wes Isberg */
public class DoubleDispatch {
public static void main(String[] a) {
Worker worker = new Worker();
worker.run((SuperType) new SubTypeOne());
worker.run((SuperType) new SubTypeTwo());
worker.run(new SuperType());
Tester.checkAllEvents();
}
static aspect A {
static int counter;
static {
Tester.expectEvent("language.SubTypeOne-1");
Tester.expectEvent("language.SubTypeTwo-2");
Tester.expectEvent("language.SuperType-3");
}
before(Object o) : execution(void Worker.run(..)) && args(o) {
Tester.event(o.getClass().getName() + "-" + ++counter);
}
}
}
// START-SAMPLE language-doubleDispatch Implementing double-dispatch
/**
* By hypothesis, there is a working class with
* methods taking a supertype and subtypes.
* The goal of double-dispatch is to execute the
* subtype method rather than the supertype
* method selected when the compile-time
* reference is of the super's type.
*/
class Worker {
void run(SuperType t) {}
void run(SubTypeOne t) {}
void run(SubTypeTwo t) {}
}
class SuperType {}
class SubTypeOne extends SuperType {}
class SubTypeTwo extends SuperType {}
/** Implement double-dispatch for Worker.run(..) */
aspect DoubleDispatchWorker {
/**
* Replace a call to the Worker.run(SuperType)
* by delegating to a target method.
* Each target subtype in this method dispatches back
* to the subtype-specific Worker.run(SubType..) method,
* to implement double-dispatch.
*/
void around (Worker worker, SuperType targ):
!withincode(void SuperType.doWorkerRun(Worker)) &&
target (worker) && call (void run(SuperType)) &&
args (targ) {
targ.doWorkerRun(worker);
}
void SuperType.doWorkerRun(Worker worker) {
worker.run(this);
}
// these could be in separate aspects
void SubTypeOne.doWorkerRun(Worker worker) {
worker.run(this);
}
void SubTypeTwo.doWorkerRun(Worker worker) {
worker.run(this);
}
}
// END-SAMPLE language-doubleDispatch
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