1. Chapter 10
Design Patterns

2. Motivation for Design Patterns
Most software systems contain certain common aspects that are frequently reinvented for each system
Solutions to these common problems may vary in quality from system to system
Design patterns seeks to communicate these classic solutions in an easy to understand manner

3. What are Design Patterns?
Design Patterns communicate solutions to common programming problems
The seminal book on design patterns, Design Patterns, Elements of Reusable Object-Oriented Software by Gamma et al, identifies three categories of design patterns
Creational
Structural
Behavioral

4. Creational Patterns Abstract factory Builder Factory method
Facilitates the creation of related objects without concrete class reference
Builder
If a class supports a number of different data representations, the builder encapsulates the details of the necessary conversions
Factory method
Defers determination of specific class to create to a subclass

5. More Creational Patterns
Prototype
Embodies information comprising prototypical instances
Singleton
Ensures that the class has at most one instance and that other classes have global access to it

6. Structural Patterns Adapter Bridge Composite Decorator
Translates between interacting objects for which the method signatures are incompatible
Bridge
Creates a more flexible mechanism for multiple implementations of an abstract class than inheritance
Composite
Facilitates a generic hierarchical grouping of objects where the class and number of objects may change
Decorator
Allows dynamic addition of new behavior to objects by acting as an outer wrapper containing the added features

7. More Structural Patterns
Facade
Provides a front end for a series of classes so that users of these classes only need to know of and use the standard set of methods implemented by the facade
Flyweight
Describes a strategy for using an otherwise prohibitive number of very small-scale objects with minimal overhead
Proxy
A means by which the instantiation of certain classes may be delayed until the proxied object is needed

8. Behavioral Patterns Chain of responsibility Command Interpreter
A strategy that sets up a series of objects that may respond to a message depending on the context of the request
Command
Allows methods to be encapsulated in objects and invoked by a standard means as determined by the interface of the common class
Interpreter
If a set of problems are conducive to being represented as a simple grammar, a class may be defined for each rule

9. More Behavioral Patterns
Iterator
Separates the class elements necessary for access and traversal from the aggregating class and encapsulates them in the iterator
Mediator
Helps to avoid highly coupled class definitions by moving explicit object references to an intermediary object
Memento
A means to store the otherwise encapsulated internal state of an object for the purpose of creating a check-point or undoing transactions

10. More Behavioral Patterns
Observer
Allows multiple dependent instances of a class to be notified of relevant changes to the observed class
State
A class hierarchy is defined around a set of states for an object, allowing the behavior of an object to vary depending on a state by substituting the state-specific subclass in its composition
Strategy
Selects a set of related algorithms and encapsulates them in such a way that they may be interchangeable for solving certain problems

11. More Behavioral Patterns
Template method
Structures a method in such a way that elements of the algorithm are deferred to subclasses
Visitor
Allows the introduction of new operations on the objects that comprise the visited object