Function pointers/delegates in Java?

Question

For my Java game server I send the Action ID of the packet which basically tells the server what the packet is for. I want to map each Action ID (an integer) to a function.

Answer 1

Have you ever used Swing/AWT? Their Event hierarchy solves a similar problem. The way Java passes functions around is with an interface, for example

public interface ActionHandler {
    public void actionPerformed(ActionArgs e);
}

Then, if you want to map integers onto these objects, you could use something like a java.util.HashMap to manage that. The actual implementations can either go in anonymous classes (Java's best approximation of "lambda") or in proper classes somewhere. Here's the anonymous class way:

HashMap handlers;
handlers.put(ACTION_FROB, new ActionHandler() {
    public void actionPerformed(ActionArgs e) {
        // Do stuff
        // Note that any outer variables you intend to close over must be final.
    }
});
handlers.get(ACTION_FROB).actionPerformed(foo);

(edit) If you want to be even more abusive, you can initialize the HashMap like so:

HashMap m = new HashMap() {{
    put(0,"hello");
    put(1,"world");
}};