A callback method in java is a method that gets called when an event (call it E) occurs. Usually you can implement that by passing an implementation of a certain interface to the system that is responsible for triggering the event E (see example 1).
Also in bigger and more complex systems you simply can annotate a method and the system will identify all annotated methods and will call them when the event occurs (see example 2). Of course the system defines what parameters the method should receive and other constraints.
Example 1:
public interface Callback {
//parameters can be of any types, depending on the event defined
void callbackMethod(String aParameter);
}
public class CallbackImpl implements Callback {
void callbackMethod(String aParameter) {
//here you do your logic with the received paratemers
//System.out.println("Parameter received: " + aParameter);
}
}
//.... and then somewhere you have to tell the system to add the callback method
//e.g. systemInstance.addCallback(new CallbackImpl());
Example 2:
//by annotating a method with this annotation, the system will know which method it should call.
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface CallbackAnnotation {}
public class AClass {
@CallbackAnnotation
void callbackMethod(String aParameter) {
//here you do your logic with the received paratemers
//System.out.println("Parameter received: " + aParameter);
}
}
//.... and then somewhere you have to tell the system to add the callback class
//and the system will create an instance of the callback class
//e.g. systemInstance.addCallbackClass(AClass.class);
