I am coming from JavaScript, in which callbacks are pretty easy. I am trying to implement them into JAVA, without success.
I have a Parent class:
import java.net.Socket;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Server {
ExecutorService workers = Executors.newFixedThreadPool(10);
private ServerConnections serverConnectionHandler;
public Server(int _address) {
System.out.println("Starting Server...");
serverConnectionHandler = new ServerConnections(_address);
serverConnectionHandler.newConnection = function(Socket _socket) {
System.out.println("A function of my child class was called.");
};
workers.execute(serverConnectionHandler);
System.out.println("Do something else...");
}
}
Then I have a child class, that is called from the parent:
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Level;
import java.util.logging.Logger;
public class ServerConnections implements Runnable {
private int serverPort;
private ServerSocket mainSocket;
public ServerConnections(int _serverPort) {
serverPort = _serverPort;
}
@Override
public void run() {
System.out.println("Starting Server Thread...");
try {
mainSocket = new ServerSocket(serverPort);
while (true) {
newConnection(mainSocket.accept());
}
} catch (IOException ex) {
Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
}
}
public void newConnection(Socket _socket) {
}
}
What is the right way of implementing the
serverConnectionHandler.newConnection = function(Socket _socket) {
System.out.println("A function of my child class was called.");
};
part, in the Parent class, which is clearly not correct?
IMO, you should have a look at the Observer Pattern, and this is how most of the listeners work
In this particular case, the following should work:
serverConnectionHandler = new ServerConnections(_address) {
public void newConnection(Socket _socket) {
System.out.println("A function of my child class was called.");
}
};
It's an anonymous subclass.
I don't know if this is what you are looking for, but you can achieve this by passing a callback to the child class.
first define a generic callback:
public interface ITypedCallback<T> {
void execute(T type);
}
create a new ITypedCallback instance on ServerConnections instantiation:
public Server(int _address) {
serverConnectionHandler = new ServerConnections(new ITypedCallback<Socket>() {
@Override
public void execute(Socket socket) {
// do something with your socket here
}
});
}
call the execute methode on the callback object.
public class ServerConnections implements Runnable {
private ITypedCallback<Socket> callback;
public ServerConnections(ITypedCallback<Socket> _callback) {
callback = _callback;
}
@Override
public void run() {
try {
mainSocket = new ServerSocket(serverPort);
while (true) {
callback.execute(mainSocket.accept());
}
} catch (IOException ex) {
Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
btw: I didn't check if it's 100% correct, directly coded it here.
Use the observer pattern. It works like this:
interface MyListener{
void somethingHappened();
}
public class MyForm implements MyListener{
MyClass myClass;
public MyForm(){
this.myClass = new MyClass();
myClass.addListener(this);
}
public void somethingHappened(){
System.out.println("Called me!");
}
}
public class MyClass{
private List<MyListener> listeners = new ArrayList<MyListener>();
public void addListener(MyListener listener) {
listeners.add(listener);
}
void notifySomethingHappened(){
for(MyListener listener : listeners){
listener.somethingHappened();
}
}
}
You create an interface which has one or more methods to be called when some event happens. Then, any class which needs to be notified when events occur implements this interface.
This allows more flexibility, as the producer is only aware of the listener interface, not a particular implementation of the listener interface.
In my example:
MyClass
is the producer here as its notifying a list of listeners.
MyListener
is the interface.
MyForm
is interested in when somethingHappened
, so it is implementing MyListener
and registering itself with MyClass
. Now MyClass
can inform MyForm
about events without directly referencing MyForm
. This is the strength of the observer pattern, it reduces dependency and increases reusability.
Source: Stackoverflow.com