Head First Design Patterns

Creators of the Head First Series
Table of Contents (the real thing)
Intro: How to use this Book
Who is this book for?
Who should probably back away from this book?
We know what you’re thinking.
And we know what your brain is thinking.
Metacognition: thinking about thinking
Here’s what WE did:
Here’s what YOU can do to bend your brain into submission
Read Me
We use simple UML-like diagrams.
We don’t cover every single Design Pattern ever created.
The activities are NOT optional.
We use the word “composition” in the general OO sense, which is more flexible than the strict UML use of “composition.”
The redundancy is intentional and important.
The code examples are as lean as possible.
The Brain Power exercises don’t have answers.
Tech Reviewers
Tech Reviewers, 2nd Edition
Acknowledgments
From the first edition
Acknowledgments
From the second edition
Very Special Thanks

1. Welcome to Design Patterns: Intro to Design Patterns
It started with a simple SimUDuck app
But now we need the ducks to FLY
But something went horribly wrong...
Joe thinks about inheritance...
How about an interface?
What would you do if you were Joe?
The one constant in software development
Zeroing in on the problem...
Separating what changes from what stays the same
Designing the Duck Behaviors
Implementing the Duck Behaviors
there are no Dumb Questions
Integrating the Duck Behaviors
More integration...
Testing the Duck code
Setting behavior dynamically
The Big Picture on encapsulated behaviors
HAS-A can be better than IS-A
Speaking of Design Patterns...
Design Puzzle
Overheard at the local diner...
Overheard in the next cubicle...
The power of a shared pattern vocabulary
How do I use Design Patterns?
there are no Dumb Questions
Tools for your Design Toolbox
Design Patterns Crossword
Design Puzzle Solution
Design Patterns Crossword Solution

2. Keeping your Objects in the Know: The Observer Pattern
The Weather Monitoring application overview
Unpacking the WeatherData class
Our Goal
Stretch Goal
Taking a first, misguided implementation of the Weather Station
What’s wrong with our implementation anyway?
Meet the Observer Pattern
Publishers + Subscribers = Observer Pattern
A day in the life of the Observer Pattern
Five-minute drama: a subject for observation
Two weeks later...
The Observer Pattern defined
The Observer Pattern: the Class Diagram
there are no Dumb Questions
The Power of Loose Coupling
Cubicle conversation
Designing the Weather Station
Implementing the Weather Station
Implementing the Subject interface in WeatherData
Now, let’s build those display elements
there are no Dumb Questions
Power up the Weather Station
Looking for the Observer Pattern in the Wild
The Swing library
A little life-changing application
Coding the life-changing application
there are no Dumb Questions
Meanwhile, back at Weather-O-Rama
For the Subject to send notifications...
For an Observer to receive notifications...
Code Magnets
Test Drive the new code
Tools for your Design Toolbox
Design Principle Challenge
Design Patterns Crossword
Design Principle Challenge Solution
Code Magnets Solution
Design Patterns Crossword Solution

3. Decorating Objects: The Decorator Pattern
Welcome to Starbuzz Coffee
The Open-Closed Principle
there are no Dumb Questions
Meet the Decorator Pattern
Constructing a drink order with Decorators
Okay, here’s what we know about Decorators, so far...
The Decorator Pattern defined
Decorating our Beverages
Cubicle Conversation
New barista training
Writing the Starbuzz code
Coding beverages
Coding condiments
Serving some coffees
there are no Dumb Questions
Real-World Decorators: Java I/O
Decorating the java.io classes
Writing your own Java I/O Decorator
Test out your new Java I/O Decorator
Tools for your Design Toolbox

4. Baking with OO Goodness: The Factory Pattern
Identifying the aspects that vary
But the pressure is on to add more pizza types
Encapsulating object creation
Building a simple pizza factory
there are no Dumb Questions
Reworking the PizzaStore class
The Simple Factory defined
Franchising the pizza store
We’ve seen one approach...
But you’d like a little more quality control...
A framework for the pizza store
Allowing the subclasses to decide
Let’s make a Pizza Store
Declaring a factory method
Let’s see how it works: ordering pizzas with the pizza factory method
So how do they order?
Let’s check out how these pizzas are really made to order...
We’re just missing one thing: Pizzas!
Our Pizza Store isn’t going to be very popular without some pizzas, so let’s implement them
Now we just need some concrete subclasses...how about defining New York and Chicago-style cheese pizzas?
You’ve waited long enough. Time for some pizzas!
It’s finally time to meet the Factory Method Pattern
The Creator classes
The Product classes
View Creators and Products in Parallel
Design Puzzle
Factory Method Pattern defined
there are no Dumb Questions
Looking at object dependencies
The Dependency Inversion Principle
Applying the Principle
Inverting your thinking...
A few guidelines to help you follow the Principle...
Meanwhile, back at the Pizza Store...
Ensuring consistency in your ingredients
Families of ingredients...
Building the ingredient factories
Building the New York ingredient factory
Reworking the pizzas...
Reworking the pizzas, continued...
Revisiting our pizza stores
What have we done?
More pizza for Ethan and Joel...
Ethan and Joel can’t get enough Objectville Pizza! What they don’t know is that now their orders are making use of the new ingredient factories. So now when they order...
From here things change, because we are using an ingredient factory
Abstract Factory Pattern defined
Factory Method and Abstract Factory compared
Tools for your Design Toolbox
Design Patterns Crossword
Design Puzzle Solution
Design Patterns Crossword Solution

5. One-of-a-Kind Objects: The Singleton Pattern
The Little Singleton
A small Socratic exercise in the style of The Little Lisper
Dissecting the classic Singleton Pattern implementation
The Chocolate Factory
Singleton Pattern defined
Hershey, PA, we have a problem...
BE the JVM
Dealing with multithreading
Can we improve multithreading?
1. Do nothing if the performance of getInstance() isn’t critical to your application.
2. Move to an eagerly created instance rather than a lazily created one.
3. Use “double-checked locking” to reduce the use of synchronization in getInstance().
Meanwhile, back at the Chocolate Factory...
Congratulations!
there are no Dumb Questions
Tools for your Design Toolbox
Design Patterns Crossword
Design Patterns Crossword Solution

6. Encapsulating Invocation: The Command Pattern
Home Automation or Bust, Inc.
Free hardware! Let’s check out the Remote Control...
Taking a look at the vendor classes
Cubicle Conversation
Meanwhile, back at the Diner..., or, A brief introduction to the Command Pattern
Let’s study the interaction in a little more detail...
The Objectville Diner roles and responsibilities
From the Diner to the Command Pattern
Our first command object
Using the command object
Creating a simple test to use the Remote Control
The Command Pattern defined
The Command Pattern defined: the class diagram
Assigning Commands to slots
Implementing the Remote Control
Implementing the Commands
Putting the Remote Control through its paces
Now, let’s check out the execution of our remote control test...
Time to write that documentation...
What are we doing?
Time to QA that Undo button!
Using state to implement Undo
Adding Undo to the Ceiling Fan commands
Get ready to test the ceiling fan
Testing the ceiling fan...
Every remote needs a Party Mode!
Using a macro command
there are no Dumb Questions
More uses of the Command Pattern: queuing requests
More uses of the Command Pattern: logging requests
Command Pattern in the Real World
Tools for your Design Toolbox
Design Patterns Crossword

7. Being Adaptive: The Adapter and Facade Patterns
Adapters all around us
Object-oriented adapters
If it walks like a duck and quacks like a duck, then it must might be a duck turkey wrapped with a duck adapter...
Test drive the adapter
The Adapter Pattern explained
Here’s how the Client uses the Adapter
there are no Dumb Questions
Adapter Pattern defined
Object and class adapters
Duck Magnets
Duck Magnets Answer
Real-world adapters
Enumerators
Iterators
Using Enumerators with code that expects Iterators
Adapting an Enumeration to an Iterator
Designing the Adapter
Dealing with the remove() method
Writing the EnumerationIterator adapter
And now for something different...
Home Sweet Home Theater
Watching a movie (the hard way)
Lights, Camera, Facade!
there are no Dumb Questions
Constructing your home theater facade
Implementing the simplified interface
Time to watch a movie (the easy way)
Facade Pattern defined
The Principle of Least Knowledge
How NOT to Win Friends and Influence Objects
Keeping your method calls in bounds...
there are no Dumb Questions
The Facade Pattern and the Principle of Least Knowledge
Tools for your Design Toolbox
Design Patterns Crossword
Design Patterns Crossword Solution

8. Encapsulating Algorithms: The Template Method Pattern
It’s time for some more caffeine
Whipping up some coffee and tea classes (in Java)
And now the Tea...
Let’s abstract that Coffee and Tea
Taking the design further...
Abstracting prepareRecipe()
What have we done?
Meet the Template Method
Let’s make some tea...
What did the Template Method get us?
Template Method Pattern defined
Hooked on Template Method...
Using the hook
Let’s run the Test Drive
And let’s give it a run...
there are no Dumb Questions
The Hollywood Principle
The Hollywood Principle and Template Method
there are no Dumb Questions
Template Methods in the Wild
Sorting with Template Method
We’ve got some ducks to sort...
What is compareTo()?
Comparing Ducks and Ducks
Let’s sort some Ducks
Let the sorting commence!
The making of the sorting duck machine
there are no Dumb Questions
Swingin’ with Frames
Custom Lists with AbstractList
Design Patterns Crossword
Tools for your Design Toolbox
Design Patterns Crossword Solution

9. Well-Managed Collections: The Iterator and Composite Patterns
Breaking News: Objectville Diner and Objectville Pancake House Merge
Check out the Menu Items
Lou and Mel’s Menu implementations
What’s the problem with having two different menu representations?
The Java-Enabled Waitress Specification
Implementing the spec: our first attempt
What now?
Can we encapsulate the iteration?
Meet the Iterator Pattern
Adding an Iterator to DinerMenu
Reworking the DinerMenu with Iterator
Fixing up the Waitress code
Testing our code
Here’s the test run...
What have we done so far?
Reviewing our current design...
Making some improvements...
there are no Dumb Questions
Cleaning things up with java.util.Iterator
We are almost there...
What does this get us?
Iterator Pattern defined
The Iterator Pattern Structure
The Single Responsibility Principle
there are no Dumb Questions
Meet Java’s Iterable interface
Java’s enhanced for loop
Not so fast; Arrays are not Iterables
Taking a look at the Café Menu
Reworking the Café Menu code
Adding the Cafe Menu to the Waitress
Breakfast, lunch, AND dinner
Here’s the test run; check out the new dinner menu from the Café!
What did we do?
We decoupled the Waitress....
...and we made the Waitress more extensible
But there’s more!
Iterators and Collections
Code Magnets
Is the Waitress ready for prime time?
Just when we thought it was safe...
What do we need?
The Composite Pattern defined
there are no Dumb Questions
Designing Menus with Composite
Implementing MenuComponent
Implementing the MenuItem
Implementing the Composite Menu
Fixing the print() method
Getting ready for a test drive...
Now for the test drive...
Getting ready for a test drive...
Design Patterns Crossword
Tools for your Design Toolbox
Code Magnets Solution
Design Patterns Crossword Solution

10. The State of Things: The State Pattern
Java Breakers
Cubicle Conversation
State machines 101
Writing the code
In-house testing
You knew it was coming...a change request!
Design Puzzle
The messy STATE of things...
The new design
Defining the State interfaces and classes
Implementing our State classes
Reworking the Gumball Machine
Now, let’s look at the complete GumballMachine class...
Implementing more states
Let’s take a look at what we’ve done so far...
The State Pattern defined
there are no Dumb Questions
We still need to finish the Gumball 1 in 10 game
Finishing the game
Demo for the CEO of Mighty Gumball, Inc.
there are no Dumb Questions
Sanity check...
We almost forgot!
Tools for your Design Toolbox
Design Puzzle Solution

11. Controlling Object Access: The Proxy Pattern
Coding the Monitor
Testing the Monitor
The role of the ‘remote proxy’
Adding a remote proxy to the Gumball Machine monitoring code
Remote methods 101
Walking through the design
How the method call happens
Java RMI, the Big Picture
Making the Remote service
Step one: make a Remote interface
Step two: make a Remote implementation
Step four: start the service
Step four: start the service
there are no Dumb Questions
Complete code for the server side
Complete code for the client side
Back to our GumballMachine remote proxy
Getting the GumballMachine ready to be a remote service
Registering with the RMI registry...
Now for the GumballMonitor client...
Writing the Monitor test drive
Another demo for the CEO of Mighty Gumball...
And now let’s put the monitor in the hands of the CEO. Hopefully, this time he’ll love it:
The Proxy Pattern defined
Get ready for the Virtual Proxy
Remote Proxy
Virtual Proxy
Displaying Album covers
Designing the Album Cover Virtual Proxy
How ImageProxy is going to work:
Writing the Image Proxy
Testing the Album Cover Viewer
Things to try...
What did we do?
there are no Dumb Questions
Using the Java API’s Proxy to create a protection proxy
Geeky Matchmaking in Objectville
The Person implementation
Five-minute drama: protecting subjects
Big Picture: creating a Dynamic Proxy for the Person
Step one: creating Invocation Handlers
Creating Invocation Handlers, continued...
Step two: creating the Proxy class and instantiating the Proxy object
Testing the matchmaking service
Running the code...
there are no Dumb Questions
The Proxy Zoo
Design Patterns Crossword
Tools for your Design Toolbox
Design Patterns Crossword Solution
The code for the Album Cover Viewer

12. Patterns of Patterns: Compound Patterns
Working together
Duck reunion
there are no Dumb Questions
What did we do?
A duck’s-eye view: the class diagram
The King of Compound Patterns
If Elvis were a compound pattern, his name would be Model-View-Controller, and he’d be singing a little song like this...
Meet Model-View-Controller
A closer look...
there are no Dumb Questions
Understanding MVC as a set of Patterns
Observer
Strategy
Composite
Using MVC to control the beat...
Meet the Java DJ View
The controller is in the middle...
Let’s not forget about the model underneath it all...
Putting the pieces together
Building the pieces
Let’s check out the BeatModelInterface before looking at the implementation:
Now let’s have a look at the concrete BeatModel class
The View
Implementing the View
Now for the Controller
And here’s the implementation of the controller:
Putting it all together...
And now for a test run...
Things to try
Exploring Strategy
Adapting the Model
Now we’re ready for a HeartController
And that’s it! Now it’s time for some test code...
And now for a test run...
Things to try
there are no Dumb Questions
Tools for your Design Toolbox
Exercise Solutions
The Beat Model
The View
The Controller
The Heart Model
The Heart Adapter
The Controller

13. Patterns in the Real World: Better Living with Patterns
Design Pattern defined
Looking more closely at the Design Pattern definition
there are no Dumb Questions
So you wanna be a Design Patterns writer
Organizing Design Patterns
Pattern Categories
Thinking in Patterns
Keep it simple (KISS)
Design Patterns aren’t a magic bullet; in fact, they’re not even a bullet!
You know you need a pattern when...
Refactoring time is Patterns time!
Take out what you don’t really need. Don’t be afraid to remove a Design Pattern from your design.
If you don’t need it now, don’t do it now.
Your Mind on Patterns
Don’t forget the power of the shared vocabulary
Cruisin’ Objectville with the Gang of Four
Your journey has just begun...
The definitive Design Patterns text
The definitive Patterns texts
Other Design Patterns resources
The Patterns Zoo
Annihilating evil with Anti-Patterns
Tools for your Design Toolbox
Leaving Objectville...
Boy, it’s been great having you in Objectville.

A. Leftover Patterns
Bridge
Your dilemma
Why use the Bridge Pattern?
Builder
You need a flexible design
Why use the Builder Pattern?
Chain of Responsibility
A scenario
Your task
How to use the Chain of Responsibility Pattern
Flyweight
A scenario
Your big client’s dilemma
Why use the Flyweight Pattern?
Interpreter
A scenario
Now what?
How to implement an interpreter
Mediator
A scenario
HouseOfTheFuture’s dilemma
Mediator in action...
Memento
A scenario
The Memento at work
Prototype
A scenario
Prototype to the rescue
Visitor
A scenario
The Visitor drops by

Index