How to Implement callback routines in Java ?

Java's support of interfaces provides a mechanism by which we can get the equivalent of callbacks. The trick is to define a simple interface that declares the method we wish to be invoked.
For example, suppose we want to be notified when an event happens. We can define an interface:

public interface InterestingEvent {
// This is just a regular method so it can return something or
// take arguments if you like.
public void interestingEvent();
}

This gives us a grip on any objects of classes that implement the interface. So, we need not concern ourselves with any other extraneous type information. The class that will signal the event needs to expect objects that implement the InterestingEvent interface and then invoke theinterestingEvent() method as appropriate.

public class EventNotifier {
private InterestingEvent ie;
private boolean somethingHappened;


public EventNotifier(InterestingEvent event) {
// Save the event object for later use.
ie = event;
// Nothing to report yet.
somethingHappened = false;
}


// ...
public void doWork() {
// Check the predicate, which is set elsewhere.
if (somethingHappened) {
// Signal the even by invoking the interface's method.
ie.interestingEvent();
}
// ...
}
// ...
}

In that example, I used the somethingHappened predicate to track whether or not the event should be triggered. In many instances, the very fact that the method was called is enough to warrant signaling the interestingEvent().
The code that wishes to receive the event notification must implement the InterestingEvent interface and just pass a reference to itself to the event notifier.

public class CallMe implements InterestingEvent {
private EventNotifier en;


public CallMe() {
// Create the event notifier and pass ourself to it.
en = new EventNotifier(this);
}


// Define the actual handler for the event.
public void interestingEvent() {
// Wow! Something really interesting must have occurred!
// Do something...
}
// ...
}