In the code example that you linked the KillMonsterEventListener
public interface KillMonsterEventListener {
void onKillMonster ();
}
provides a way for users of your API to tell you something like this:
Here is a piece of code. When a monster is killed, call it back. I will decide what to do.
This is a way for me to plug in my code at a specific point in your execution stream (specifically, at the point when a monster is killed). I can do something like this:
yourClass.addKillMonsterEventListener(
new KillMonsterEventListener() {
public onKillMonster() {
System.out.println("A good monster is a dead monster!");
}
}
);
Somewhere else I could add another listener:
yourClass.addKillMonsterEventListener(
new KillMonsterEventListener() {
public onKillMonster() {
monsterCount--;
}
}
);
When your code goes through the list of listeners on killing a monster, i.e.
for (KillMonsterEventListener listener : listeners) {
listener.onKillMonster()
}
both my code snippets (i.e. the monsterCount--
and the printout) get executed. The nice thing about it is that your code is completely decoupled from mine: it has no idea what I am printing, what variable I am decrementing, and so on.