The Observer Pattern

The Observer Pattern
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

The Problem:
- Objects often need to communicate with each other (i.e., send messages to each other/invoke methods on each other)
- Communication increases coupling
One Solution:
- Change your perspective

A Trading System Example

A Bad Design

The Problem with this Design

An Observation:
- Every TickReader must have an associated TickWriter and TickerTape
The Problem:
- This makes the TickReader much less re-usable

A Trading System Example (cont.)

A Better Design

Advantage of this Design

An Observation:
- The TickReader plays a more passive role
The Advantage:
- A TickReader need only have a list of (zero or more) TickListener objects that it will inform

Operationalizing this Observation

What's Needed?
- A TickListener interface
An Important Question:
- Can this specific example be generalized into a pattern?

The Observer Pattern

Intent:
- Define a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified
Participants:
- A subject
- Some observers

The Observer Pattern (cont.)

Issues Related to the Choice of Collection

Frequency of notifications vs. modifications (think about HashMap , ArrayList , and LinkedList )
Need for concurrent notifications and modifications (think about CopyOnWriteArrayList )

Other Terminology

Listener
Publish-Subscribe Pattern

A Complete Example

A Silly Text Processor:
- Counts the number of words that start with an uppercase letter
- Save the lines to a file
- Shows the progress (e.g., the number of lines processed)
Some Observations:
- This is not going to make us any money
- We can use it to explore different designs
- Java has an Observer interface and an Observable class that are not used in the following example so that you can see the details

A Complete Example (cont.)

An Implementation of the Worst Possible Design javaexamples/observer/notcohesive/SillyTextProcessor.java

A Complete Example (cont.)

A Bad Design

A Complete Example (cont.)

A Better Design

A Complete Example (cont.)

This design is better
It is, however, too tightly coupled

A Complete Example (cont.)

Using the Observer Pattern

A Complete Example (cont.)

Implementing the Good Design

javaexamples/observer/LineObserver.java

A Complete Example (cont.)

javaexamples/observer/LineSubject.java

A Complete Example (cont.)

javaexamples/observer/LineReader.java

A Complete Example (cont.)

javaexamples/observer/SillyTextProcessor.java

A Complete Example (cont.)

javaexamples/observer/ProgressWindow.java

A Complete Example (cont.)

javaexamples/observer/UCWordCounter.java

A Complete Example (cont.)

javaexamples/observer/LineArchiver.java

There's Always More to Learn