double-dispatch example

docs/sandbox/common/language/DoubleDispatch.java

@@ -0,0 +1,81 @@
+
+
+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 
+ 

docs/sandbox/sandbox-test.xml

@@ -62,6 +62,11 @@
 		<run class="language.ControlFlow"/>
 		</ajc-test>
 
+	<ajc-test dir="common" title="language-doubleDispatch">
+		<compile files="language/DoubleDispatch.java"/>
+		<run class="language.DoubleDispatch"/>
+	</ajc-test>
+
 
 	<ajc-test dir="common" title="tracing-simpleLogging">
 		<compile