Advanced Uses of the Observer Pattern in Java

Advanced Uses of the Observer Pattern in Java
Multicasting, Property Changes, and Other Issues

Computer Science Department

bernstdh@jmu.edu

A Common Use of the Oberver Pattern in Java GUIs

Subjects:
- GUI components (e.g., JButton , JList )
Observers:
- Application-specific classes implement one or more interfaces (e.g., ActionListener , ItemListener )

Creating Subjects of AWT Events

The Situation:
- You need to create a Component that generates AWT events
One Easy Aspect of the Problem:
- There are many classes that encapsulate AWT events (e.g., ActionEvent , ItemEvent )
Complicating Factors:
- Using the event dispatch thread
- Maintaining the order of the queue
- Thread safety

Creating Subjects of AWT Events (cont.)

A Useful Class:
- AWTEventMulticaster provides efficient and thread-safe message passing to multiple observers
Usage:
- Is a little unusual but straightforward once you understand

Creating Subjects of AWT Events (cont.)

javaexamples/multicaster/CombinationField.java

Creating Subjects of Changes in Object State

The Situation:
- You have an object that needs to inform observers of changes in its state
One Approach:
- Maintain a collection of observers and send each an appropriate message when state changes
Complicating Factors:
- Maintaining a one-to-one correspondence between state changes and notifications
- Maintaining the order of the queue
- Thread safety

Creating Subjects of Changes in Object State (cont.)

A Useful Class:
- PropertyChangeSupport provides message passing about arbitrary property changes to multiple observers
Usage:
- Straightforward delegation

Creating Subjects of Changes in Object State (cont.)

A Subject javaexamples/propertychange/Rectangle.java

Creating Subjects of Changes in Object State (cont.)

An Observer javaexamples/propertychange/PropertyLabel.java

Creating Subjects of Changes in Object State (cont.)

Usage javaexamples/propertychange/RectanglePanel.java

What About Observer and Observable?

What Were They?
- Observable was a class that could be extended to create subjects of various kinds
- Observer was an interface that could be implemented by observers
Why Were They Deprecated in v9?
- Limited event model
- Unspecified order of notifications
- State changes not one-to-one with notifications

There's Always More to Learn