We Keep Coding

How or why do listener interfaces work? And do interfaces have any other use rather than being a listener?

Whenever we want to create a listener, we implement a listener interface. For example, lets implement SensorEventListener.
Now we have to override the methods of this listener interface.
public void onSensorChanged(SensorEvent event);
and
public void onAccuracyChanged(Sensor sensor, int accuracy);
What I don't understand is:
Why and how these methods work when I automatically use them?
Why does onAccuracyChanged method gets called when the accuracy changes?
After all, onAccuracyChanged is just an empty method that we override because our formula (or the interface we implement) requires us to do so. If it is something magical caused by the lower levels
When and why would someone actually use an interface in his/her
self-project regardless of android?

Here is a suitable answer. Allow me to give you an example about listeners.
Listeners:
Suppose there is a class that fetches data in the background, the Worker, and another class that is interested in that data, the InterestedClass.
public class Worker extends Thread{
interface DataFetchedListener{
void onDataFetched(String data);
}
private DataFetchedListener listener;
#Override
public void run(){
String data = fetchData();
// Data fetched inform your listener so he can take action
listener.onDataFetched(data);
}
public void setDataFetchedListener(DataFetchedListener listener){
this.listener = listener;
}
private String fetchData(){
// returns the fetched data after some operations
return "Data";
}
}
public class InterestedClass implements Worker.DatafetchedListener{
#Override
public void onDataFetched(String data){
doSomethingWith(data);
}
private doSomethingWith(String data){
// just print it in the console
System.out.println("Data fetched is -> " + data);
}
}
The Worker does not care which class will manipulate its data, as long as that class follows the contract of DataFetchedListener.
Equally this means that any class is able to do something with the data (InterestedClass just prints it in the console) but Worker does not need to know which class is that, just that it implements its interface.
The main could go like this...
public class Application{
public static void main(String[] args){
InterestedClass interested = new InterestedClass();
Worker worker = new Worker();
worker.setDataFetchedListener(intereseted);
worker.start(); // Starts Worker's thread
}
}
When the Worker will fetch the data then it will notify its listener (currently the interested object) and the listener will act accordingly (interested will print the data to the console).

In computing, an interface is a shared boundary across which two or more separate components of a computer system exchange information.(Wikipedia)
You may wish to respond to some events either system events or user events. But for that you need to know when the event you wish to capture occurs and also what must be done at that time.
And for that you open a confidential EAR to listen to events. But that will not be sufficient since you need to be notified too so that you can reply according to the event. You set callbacks that will notify when an event occur. Those empty body methods we create inside an interface.
A Listener is that interface that hears and notify back through callbacks.
So how can all that be used? And how all these do interact?
First create an interface with empty bodies methods that you intend to call when an event occurs:
public interface MyListener{
void actionOneHappens(Object o);
void actionTwo();
void actionThree();
}
Create a class that handles something, for example counts:
public class MyCounter{
//create a member of type MyListener if you intend to exchange infos
private MyListener myListener;
//let's create a setter for our listener
public void setMyListener(MyListener listener)
{
this.myListener=listener;
}
MyCounter(){
}
//this method will help us count
public void startCounting()
{
new CountDownTimer(10000,1000)
{
#Override
public void onTick(long millisUntilFinished) {
//I want to notify at third second after counter launched
if(millisUntilFinished/1000==3)
{
// I notify if true :
//as someone can forget to set the listener let's test if it's not //null
if(myListener!=null){
myListener.actionThree();
}
}
}
#Override
public void onFinish() {
}
}.start();
}
}
You can then create an object of type MyCounter and know when it's at three:
MyCounter myCounter=new MyCounter();
myCounter.setMyListener(new MyListener()
{
//then override methods here
#override
void actionOneHappens(Object o){
}
#override
void actionTwo()
{}
#override
void actionThree()
{
//Add you code here
Toast.makeText(getApplicationContext(),"I'm at 3",Toast.LENGTH_LONG).show()
}
});
//start your counter
myCounter.startCounting();
And it's done!! That's how we proceed.

Interfaces have no implementation and for using them we have two options:
A class that implement them
An anonymous class
And consider this code:
interface TestInterface {
void doSomething();
}
class TestClass{
private TestInterface ti;
public TestClass(TestInterface ti){
this.ti = ti;
}
public void testActionMethod(){
ti.doSomething();
//some other codes
}
}
class OurOwnLauncherApp{
public static void main(String[] args) {
TestClass tc = new TestClass(new TestInterface() {
#Override
public void doSomething() {
System.out.println("Hi!");
}
});
tc.testActionMethod();
TestClass tc2 = new TestClass(new TestInterface() {
#Override
public void doSomething() {
System.out.println("Bye!");
}
});
tc2.testActionMethod();
}
}
In here we have:
An Interface (Just like what you asked)
A function class the uses that interface
An application somewhere that we don't know (Maybe your phone app, maybe your friends phone app, etc)
What this code does, it gives an anonymous class (which implements TestInterface) to the testActionMethod and with calling doSomething method inside testActionMethod, we invert the calling back to our own method. that's why you will see this result:
Hi!
Bye!
This is exactly a simplified version of listener interfaces and how they work

There is no magic thing. Generally, the event-listener mechanism is as follow:
For some entities, there is the possibility to listen to some events on that entity (let name this entity as event generator). So some way should exist for other entities to listen to these changes (let name these entities as listeners). Now a listener registers itself as a listener of event generator. When an event occurs on the event generator, it calls the related method of registered listeners.
As a simple example assume a button. The button may generate an event for some actions such as click. Now if a listener wants to aware when the button is clicked, it should register itself as a listener of that button. On the other hand, the button should provide a unified way of registering the listeners. This unified way is the interface. Each entity which implements the interface could register itself as a listener for click on that button:
1- Listener implements the interface
2- Listener registers itself as a listener of button (Event Generator)
3- Event Generator calls the appropriate method of all registered listeners (this method is a method of the interface).
For your case, android provides a manager which you could register a listener on some sensors by it: android.hardware.SensorManager.registerListener(). All things occurs here (which is not magic!). When you register an entity (which implemented the related interface, SensorEventListener) as a sensor listener, changes in that sensor will cause to call methods of the listener).

Android Redirection (delegates pointers)

I would like to call different code (callbacks) from within a background thread loop and use that background thread to perform the work. It would be similar to delegates in C#.
public class test {
private boolean keepRunning;
private boolean messageReady;
private MyClass myClass;
void LongBackgroundWork(){
Thread thread = new Thread(new Runnable() {
#Override
public void run() {
while (keepRunning) {
if (messageReady){
myClass.callback(); // call different methods here
// to be decided at runtime and run on this worker thread
}
}
}
});
thread.start();
}
}
I want to use the background thread not the UI thread. I want to set a callback from within myClass to choose what code is called. It's easy in C# how to do it Java.
I don't have much experience in Java and don't understand what mechanism to use. Should I be using a handler? Can a handler run code on a background thread?

I'd wager you want to have a pattern where an event or some occurence happens and you need to initiate a code block somewhere.
A pattern that could help you is perhaps an Observer Wiki and firing off to the event. You can also check out this SO question here if you'd like: Delegate vs Callback in Java
In your case, I think you'd want to have someone handle the responsibility of what you have to do when a message is ready. So what you're looking for is someone to perform the action, once the event is read (message ready).
Take for example Class Foo is your container of listeners, or also called an Observer that will be notified of any events. You can have a list of callbacks here to some object that is responsible for your logic to do what you need to do next.
Then you would have an Observable object or a class that would implement some logic when notified. You could then have various class objects perform the necessary logic by implementing the callback function required.
Example:
// Observer
public class Foo {
// List of objects that implement Callbacks interface
private List<Callbacks> mList;
public Foo() {
// Initialize here
}
public void addListener(Callbacks cb) {
mList.add(cb);
}
public void notifyListeners() {
for ( Callback cb : mList) {
cb.onCallback();
}
}
// Public interface to be implemented by users
public interface Callback {
void onCallback();
}
}
Then just have a class implement this object and you can pass it along if you'd like.
// Bar implements Foo.Callback interface
public class Bar implements Foo.Callback {
public class Bar() {}
#Override
public void onCallback() {
// Perform logic here
}
}
Finally in your code, you'd just create the Foo object, add a listener, and notify them when it's time to fire your event.

if i understood you properly,you cant do this on UI thread, basically when android see Thread like this it will expect that it's a long operation however you can call it by AsyncTask or Handler
you can make something like this
private class MyAsyncTask extends AsyncTask<Void,Void,Void>{
protected Void doInBackground() {
MyClass myClass=new MyClass();
myClass.LongBackgroundWork();
}
return totalSize;
}
}
this is how yo can call your thread otherwise you have to use Handler instead
Handler handler=new Handler();
handler.post(new Runnable(){
MyClass myClass=new MyClass();
myClass.LongBackgroundWork();
})

difference between addDomHandler and implements handler

I'm creating a composite panel which have ability to drag and drop files from system.
It looks like bellow
public abstract class Upload extends Composite implements DragEnterHandler, DragLeaveHandler, DropHandler, DragOverHandler {
...
#Override
public void onDragOver(DragOverEvent event) {
// TODO Auto-generated method stub
}
and it doesn't work at all. But "almost" the same code
initWidget(uiBinder.createAndBindUi(this));
...
addDomHandler(new DragOverHandler() {
#Override
public void onDragOver(DragOverEvent event) {
// TODO Auto-generated method stub
}
}, DragOverEvent.getType());
works pretty well.
The question:
Where is a difference? Here I read that there should be no difference.
Is there still possibility to use it in "interface implements" way?

Just because you implement the interface, doesn't mean that your implementation is used.
Let me give you an example:
public class FancyButton extends Button implements SomeFancyButtonListener
{
public FancyButton()
{
// This line is necessary, otherwise the implemented code isn't used.
this.addFancyButtonListener(this);
}
#Override
public void fancyButtonClicked(FancyClickEvent e)
{
// Do something
}
}
is equivalent to:
public class FancyButton extends Button
{
public FancyButton()
{
this.addFancyButtonListener(new SomeFancyButtonListener()
{
#Override
public void fancyButtonClicked(FancyClickEvent e)
{
// Do something
}
});
}
}
From what I can see in your first code snippet, you aren't adding this as the Handler.

Handlers don't work until an unless it is added on the component as simple as it is.
In first case you have overridden the handler's method but you haven't added this handler on component itself but in second case you have added it on component along with its implementations that's why its working.
Handlers are just like listeners that works in same manner as Observer Pattern works.
If a component wants to listen for a specific event then it has to register for it. Whenever that event is triggered in the system it will be notified to all the registered components.

Is it possible to create my own event listener list in Java containing multiple listener types?

I'm implementing a client-server system where the client is in a continuous blocking read loop listening for messages from the server. When a message is received I'd like to raise an "event" based on the type of the message, which other GUI classes may add listeners to. I'm more familiar with C# events so I am still getting used to the Java way of doing things.
There will be many message types so I will need an interface for each, call it MessageTypeAListener, MessageTypeBListener, etc., each of which will contain one handle method, which my GUI classes will implement. However, there will be be many types and instead of maintaining a list of listeners per type and having several "fire" methods I wanted to have one big listener list and a typed fire method. Then the fire method could say "only fire listeners whose type is what I specify."
So for example (pseudocode):
ListenerList.Add(MessageTypeAListener);
ListenerList.Add(MessageTypeBListener);
<T> fire(message) {
ListenerList.Where(type is T).handle(message)
}
...
fire<MessageTypeAListener>(message);
However, type erasure seems to be making this difficult. I could try casting and catching exceptions but that seems wrong. Is there a clean way of implementing this or is it just wiser to keep a separate list of listeners for every type, even though there will be tons of types?

I implemented something like this, cause I have a visceral dislike of Java's EventListenerList. First, you implement a generic Listener. I defined the listener based upon the Event it was receiving, with basically one method
interface GenericListener<T extends Event> {
public void handle(T t);
}
This saves you having to define ListenerA, ListernerB etc... Though you could do it your way with ListenerA, ListenerB, etc, all extending some base like MyListener. Both ways have plusses and minuses.
I then used a CopyOnWriteArraySet to hold all these listeners. A set is something to consider cause all too often listeners get added twice by sloppy coders. YMMV. But, effectively you have a Collection<GenericListener<T extends Event>> or a Collection<MyListener>
Now, as you've discovered, with type erasure, the Collection can only hold one type of listener. That is often a problem. Solution: Use a Map.
Since I'm basing everything upon the event, I used
Map<Class<T extends Event>, Collection<GenericListener<T extends Event>>>
based upon the class of the event, get the list of listeners who want to get that event.
Your alternative is to base it upon the class of the listener
Map<Class<T extends MyListener>, Collection<MyListener>>
There's probably some typos above...

Old-fashioned pattern approach, using Visitor pattern:
class EventA {
void accept(Visitor visitor) {
System.out.println("EventA");
}
}
class EventB {
void accept(Visitor visitor) {
System.out.println("EventB");
}
}
interface Visitor {
void visit(EventA e);
void visit(EventB e);
}
class VisitorImpl implements Visitor {
public void visit(EventA e) {
e.accept(this);
}
public void visit(EventB e) {
e.accept(this);
}
}
public class Main {
public static void main(String[] args) {
Visitor visitor = new VisitorImpl();
visitor.visit(new EventA());
}
}
More modern approach is just to have Map between classes of events, which should not derive each other, and respective handlers of these events. This way you avoid disadvantages of Visitor pattern (which is, you'll need to change all your visitor classes, at least, base of them, every time you add new Event).
And another way is to use Composite pattern:
interface Listener {
void handleEventA();
void handleEventB();
}
class ListenerOne implements Listener {
public void handleEventA() {
System.out.println("eventA");
}
public void handleEventB() {
// do nothing
}
}
class CompositeListener implements Listener {
private final CopyOnWriteArrayList<Listener> listeners = new CopyOnWriteArrayList<Listener>();
void addListener(Listener l) {
if (this != l)
listeners.add(l);
}
public void handleEventA() {
for (Listener l : listeners)
l.handleEventA();
}
public void handleEventB() {
for (Listener l : listeners)
l.handleEventB();
}
}

After going through iterations of just about everyone's suggestions here, I ended up going a very slightly modified route of the standard Listener interfaces and listener lists. I started with Swing's EventListenerList, only to be disappointed with the amount of add/remove methods for dozens of message types. I realized this could not be condensed while still maintaining a single EventListenerList, so I started thinking about a separate list for each type. This makes it similar to .NET events where each event holds its own list of delegates to fire when raised. I wanted to avoid tons of add/remove methods, so I made a quick Event class that just looks like this:
public class Event<T extends EventListener> {
private List<T> listeners = new ArrayList<T>();
public void addListener(T listener) {
listeners.add(listener);
}
public void removeListener(T listener) {
listeners.remove(listener);
}
public List<T> getListeners() {
return listeners;
}
}
Then I keep several instances of this class around, each typed according to a listener, so Event<MessageTypeAListener>, etc. My classes can then call the add method to add themselves to that particular event. I would've like to be able to call a generic Raise method on the Event instance to then fire all the handlers, but I did not want them to all have to have the same "handle" method, so this was not possible. Instead, when I'm ready to fire the listeners, I just do
for (MessageTypeAListener listener : messageTypeAEvent.getListeners())
listener.onMessageTypeA(value);
I'm sure this is not a new idea and has probably been done before and in better/more robust ways, but it's working great for me and I'm happy with it. Best of all, it's simple.
Thanks for all the help.

If you only have simple events, i.e. events without data or where all events have the same data types, enum could be a way forward:
public enum Event {
A,
B,
C
}
public interface EventListener {
void handle(Event event);
}
public class EventListenerImpl implements EventListener {
#Override
public void handle(Event event) {
switch(event) {
case A:
// ...
break;
}
}
}
public class EventRegistry {
private final Map<Event, Set<EventListener>> listenerMap;
public EventRegistry() {
listenerMap = new HashMap<Event, Set<EventListener>>();
for (Event event : Event.values()) {
listenerMap.put(event, new HashSet<EventListener>());
}
}
public void registerEventListener(EventListener listener, Event event) {
Set<EventListener> listeners = listenerMap.get(event);
listeners.add(listener);
}
public void fire(Event event) {
Set<EventListener> listeners = listenerMap.get(event);
for (EventListener listener : listeners) {
listener.handle(event);
}
}
}
Comments:
The switch statement in the EventListnerImpl may be omitted if it is only registered to a single event, or if it should always act in the same way regardless of which Event it receives.
The EventRegister has stored the EventListener(s) in a map, meaning that each listener will only get the kind of Event(s) that it has subscribed to. Additionally, the EventRegister uses Sets, meaning that an EventListener will only receive the event at most once (to prevent that the listener will receive two events if someone accidentally registers the listener twice).

Java Listener Design Pattern for Subscribing

I am trying to design a Java system that is simliar to the concept of c# delegates.
Here is the basic functionality i wish to achieve:
public class mainform
{
public delegate onProcessCompleted
//......
processInformation()
{
onProcessCompleted(this);
}
//......
}
//PLUGIN
public class PluginA
{
public PluginA()
{
//somehow subscribe to mainforms onProcessingCompleted with callback myCallback()
}
public void myCallback(object sender)
{
}
}
I have read through this site: http://www.javaworld.com/javaqa/2000-08/01-qa-0804-events.html?page=1
They make reference to implementing the whole 'subscription list' manually. But the code is not a complete example, and I'm so used to c# that I'm having trouble grasping how I could do it in java.
Does anyone have a working examle of this that I could see?
thanks
Stephanie

In Java you don't have function delegates (effectively method references); you have to pass an entire class implementing a certain interface. E.g.
class Producer {
// allow a third party to plug in a listener
ProducerEventListener my_listener;
public void setEventListener(ProducerEventListener a_listener) {
my_listener = a_listener;
}
public void foo() {
...
// an event happened; notify the listener
if (my_listener != null) my_listener.onFooHappened(new FooEvent(...));
...
}
}
// Define events that listener should be able to react to
public interface ProducerEventListener {
void onFooHappened(FooEvent e);
void onBarOccured(BarEvent e);
// .. as many as logically needed; often only one
}
// Some silly listener reacting to events
class Consumer implements ProducerEventListener {
public void onFooHappened(FooEvent e) {
log.info("Got " + e.getAmount() + " of foo");
}
...
}
...
someProducer.setEventListener(new Consumer()); // attach an instance of listener
Often you have trivial listeners that you create via an anonymous classes in place:
someProducer.setEventListener(new ProducerEventListener(){
public void onFooHappened(FooEvent e) {
log.info("Got " + e.getAmount() + " of foo");
}
public void onBarOccured(BarEvent e) {} // ignore
});
If you want to allow many listeners per event (as e.g. GUI components do), you manage a list which you usually want to be synchronized, and have addWhateverListener and removeWhateverListener to manage it.
Yes, this is insanely cumbersome. Your eyes don't lie to you.