Observer Pattern - Example

Example

Below is an example that takes keyboard input and treats each input line as an event. The example is built upon the library classes java.util.Observer and java.util.Observable. When a string is supplied from System.in, the method notifyObservers is then called, in order to notify all observers of the event's occurrence, in the form of an invocation of their 'update' methods - in our example, ResponseHandler.update(...).

The file MyApp.java contains a main method that might be used in order to run the code.

/* File Name : EventSource.java */ package org.wikipedia.obs; import java.util.Observable; //Observable is here import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; public class EventSource extends Observable implements Runnable { public void run { try { final InputStreamReader isr = new InputStreamReader(System.in); final BufferedReader br = new BufferedReader(isr); while (true) { String response = br.readLine; setChanged; notifyObservers(response); } } catch (IOException e) { e.printStackTrace; } } } /* File Name: ResponseHandler.java */ package org.wikipedia.obs; import java.util.Observable; import java.util.Observer; /* this is Event Handler */ public class ResponseHandler implements Observer { private String resp; public void update(Observable obj, Object arg) { if (arg instanceof String) { resp = (String) arg; System.out.println("\nReceived Response: " + resp ); } } } /* Filename : MyApp.java */ /* This is the main program */ package org.wikipedia.obs; public class MyApp { public static void main(String args) { System.out.println("Enter Text >"); // create an event source - reads from stdin final EventSource eventSource = new EventSource; // create an observer final ResponseHandler responseHandler = new ResponseHandler; // subscribe the observer to the event source eventSource.addObserver(responseHandler); // starts the event thread Thread thread = new Thread(eventSource); thread.start; } }