DOCUMENTATION FROM APACHE AVALON DELEGATE CLASS

Delegates are a typesafe pointer to another method. Since Java does not have language support for such a construct, this utility will construct a proxy that forwards method calls to any method with the same signature. This utility is inspired in part by the C# delegate mechanism. We implemented it in a Java-centric manner.

Delegate

Any interface with one method can become the interface for a delegate. Consider the example below:

  public interface MainDelegate {
      int main(String[] args);
  }

The interface above is an example of an interface that can become a delegate. It has only one method, and the interface is public. In order to create a delegate for that method, all we have to do is call MethodDelegate.create(this, "alternateMain", MainDelegate.class). The following program will show how to use it:

  public class Main {
      public static int main( String[] args ) {
          Main newMain = new Main();
          MainDelegate start = (MainDelegate)
              MethodDelegate.create(newMain, "alternateMain", MainDelegate.class);
          return start.main( args );
      }

      public int alternateMain( String[] args ) {
          for (int i = 0; i < args.length; i++) {
              System.out.println( args[i] );
          }
          return args.length;
      }
  }

By themselves, delegates don't do much. Their true power lies in the fact that they can be treated like objects, and passed to other methods. In fact that is one of the key building blocks of building Intelligent Agents which in tern are the foundation of artificial intelligence. In the above program, we could have easily created the delegate to match the static main method by substituting the delegate creation call with this: MethodDelegate.createStatic(getClass(), "main", MainDelegate.class).

Another key use for Delegates is to register event listeners. It is much easier to have all the code for your events separated out into methods instead of individual classes. One of the ways Java gets around that is to create anonymous classes. They are particularly troublesome because many Debuggers do not know what to do with them. Anonymous classes tend to duplicate alot of code as well. We can use any interface with one declared method to forward events to any method that matches the signature (although the method name can be different).

Equality

The criteria that we use to test if two delegates are equal are:

• They both refer to the same instance. That is, the instance parameter passed to the newDelegate method was the same for both. The instances are compared with the identity equality operator, ==.
• They refer to the same method as resolved by Method.equals.