I'm coming to Java from C#, and I'm really just trying to figure out how to do things in Java world. I'm running Java 8 in IntelliJ IDEA. I found this explaining events in Java as basically being hand-made through manual registration and an interface method call. The code example has enough problems that I assume it was never compiled. After cleaning that up a bit I have this:
Interface MetronomeEvent:
public interface MetronomeEvent {
void Tick(Date tickDate);
}
Class EventFiringSource:
public class EventFiringSource {
// Our collection of classes that are subscribed as listeners of our
protected List<MetronomeEvent> _listeners=new ArrayList();
// Method for listener classes to register themselves
public void addMetronomeEventListener(MetronomeEvent listener)
{
_listeners.add(listener);
}
// "fires" the event
protected void fireMetronomeEvent()
{
if (_listeners != null && !_listeners.isEmpty())
{
for (MetronomeEvent e:_listeners)
{
e.Tick(new Date());
}
}
}
public void Start()
{
fireMetronomeEvent();
}
}
Main console application:
public class MainApp implements MetronomeEvent {
public static void main(String[] args) {
EventFiringSource source = new EventFiringSource();
source.addMetronomeEventListener(this); // Adds itself as a listener for the event
source.Start();
}
public void Tick(Date tickDate)
{
System.out.println(tickDate.toString());
}
}
The one remaining error is source.addMetronomeEventListener(this);
where the compiler complains that it cannot reference MyApp.this
from a static context. That makes sense, but I don't see any way then that I could, after implementing the MetronomeEvent interface on the main program class, actually pass it to source.addMetronomeEventListener()
for registration. Is it impossible to directly register the main program class for events? Am I supposed to create and register a Listener class that implements MetronomeEvent and will act on behalf of the main application? Like this?
public class Listener implements MetronomeEvent {
public void Tick(Date tickDate){
System.out.println(tickDate.toString());
}
}
And then:
public static void main(String[] args) {
EventFiringSource source = new EventFiringSource();
Listener l=new Listener();
source.addMetronomeEventListener(l); // Adds another object to listen on behalf of main()
source.Start();
}
Based on Vince Emigh's comment/answer I was led to this Oracle doc on lamda expressions and to this one on method references . I've found 3 ways to do this so far.
1) Anonymous class:
source.addMetronomeEventListener(
new MetronomeEvent() {
@Override
public void Tick(Date tickDate) {
System.out.println("anonymous class:");
System.out.println(tickDate.toString());
}
}
); // Adds itself as a listener for the event
2) Lambda expression:
source.addMetronomeEventListener(d -> System.out.println("lambda:\n"+d.toString()));
3) Method reference, which is the closest to what I am accustomed to. A method is defined in the main class:
public static void processTick(Date tickDate){
System.out.println("method reference:");
System.out.println(tickDate.toString());
}
...and then in the body of main() it is added as an event handler like this:
source.addMetronomeEventListener(MainApp::processTick);
This is not about events, it's about main()
and static methods in general.
I would suggest writing your main() as
public static void main(String[] args) {
new MainApp(args).execute();
}
This way you're immediately jumping from static function world into object-oriented world.
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