Observer and Observable

An introduction to the Observer interface and Observable class using the Model/View/Controller architecture as a guide

By Todd Sundsted, JavaWorld.com, 10/01/96

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Observer and Observable functions

The following are code listings for observer and observable functions:

Observer

public void update(Observable obs, Object obj)

Called when a change has occurred in the state of the observable.

Observable

public void addObserver(Observer obs)

Adds an observer to the internal list of observers.

public void deleteObserver(Observer obs)

Deletes an observer from the internal list of observers.

public void deleteObservers()

Deletes all observers from the internal list of observers.

public int countObservers()

Returns the number of observers in the internal list of observers.

protected void setChanged()

Sets the internal flag that indicates this observable has changed

state.

protected void clearChanged()

Clears the internal flag that indicates this observable has

changed state.

public boolean hasChanged()

Returns the boolean value true if this observable has changed

state.

public void notifyObservers()

Checks the internal flag to see if the observable has changed

state and notifies all observers.

public void notifyObservers(Object obj)

Checks the internal flag to see if the observable has changed

state and notifies all observers. Passes the object specified in the

parameter list to the notify() method of the observer.

How to use interface Observer and class Observable

The following section describes in detail how to create a new observable class and a new observer class, and how to tie the two together.

Extend an observable

A new class of observable objects is created by extending class Observable. Because class Observable already implements all of the methods necessary to provide the observer/observable behavior, the derived class need only provide some mechanism for adjusting and accessing the internal state of the observable object.

In the class ObservableValue listing below, the internal state of the model is captured by the integer n. This value is accessed (and, more importantly, modified) only through public accessors. If the value is changed, the observable object invokes its own setChanged() method to indicate that the state of the model has changed. It then invokes its own notifyObservers() method in order to update all of the registered observers.

import java.util.Observable;
public class ObservableValue extends Observable
{
   private int n = 0;
   public ObservableValue(int n)
   {
      this.n = n;
   }
   public void setValue(int n)
   {
      this.n = n;
      setChanged();
      notifyObservers();
   }
   public int getValue()
   {
      return n;
   }
}

Implement an observer

A new class of objects that observe the changes in state of another object is created by implementing the Observer interface. The Observer interface requires that an update() method be provided in the new class. The update() method is called whenever the observable changes state and announces this fact by calling its notifyObservers() method. The observer should then interrogate the observable object to determine its new state, and, in the case of the Model/View/Controller architecture, adjust its view appropriately.