Observations

• There are repeated design structures and roles of classes used in most software
• Not possible to capture these structures and roles into specific classes, i.e., they are part of a class but do not make up the entire class
• Language features may support these if organized correctly
• Need a way to communicate about them

Issues

Patterns

• Must be widely applicable
• Solution must be safe
• Solution should be efficient

Origins

Each pattern describes a problem which occurs over and over again in our environment and then describes the core of the solution to that problem, in such a way that you can use the solution a million times over, without ever doing it in the same way twice

Christopher Alexander, "A Pattern Language", 1977

Software Design Patterns

• Design Patterns: Elements of Reusable Object-Oriented Software Gamma, Helm, Johnson, Vlissides
• AKA, "Gang of 4 Book" (GOF'95)

Elements of Design Patterns

• Name
• Problem
• Solution
• Consequences

Pattern Categories

• Creational Patterns E.g., Factory Method
• Structural Patterns E.g., Proxy
• Behavioral Patterns E.g., Template Method

Creational Patterns

How the program creates objects

• E.g., Factory Method
• Are more important when we choose object composition over class inheritance
• Class inheritance provides a fixed set of behaviors, while object composition provides a greater variety of ways to combine functionality

Creational Pattern List

• Factory Method - Instantiation is deferred to subclasses
• Builder - Separate object construction from representation
• Abstract Factory - Create families of objects using only abstract classes
• Prototype - Create new objects by copying an existing object
• Singleton - Only allow one object of a class to exist

Structural Patterns

How classes and objects are composed to form larger objects

• structural class patterns
• Use inheritance to compose an interface or implementation
• Static, Compile-time only
• structural object patterns
• Compose objects for new objects
• Static and dynamic, compile-time and runtime

Structural Patterns List

• Proxy - A surrogate for an object
• Adapter - Convert the interface of an object into one that clients expect
• Bridge - Decouple an abstraction from implementation so both can vary
• Composite - Allows clients to treat individual objects and the composition of objects uniformly
• Decorator - Attach features to an object dynamically instead of subclassing
• Facade - Unified interface to a set of interfaces, making a subsystem more straightforward to use
• Flyweight - Efficiently handles large numbers of fine-grained objects

Behavioral Patterns

Algorithms and the assignment of responsibilities between objects

• class behavioral patterns Use inheritance
• object behavioral patterns Use composition

Behavioral Patterns

• Template Method - A class pattern that defines the skeleton of an algorithm as an abstract class, allowing its subclasses to provide concrete behavior
• Interpreter - Implements a specialized language
• Mediator - Allows loose coupling between classes by being the only class that has detailed knowledge of their methods
• Chain of Responsibility - Delegates commands to a chain of processing objects
• Observer - Is a publish/subscribe pattern that allows many observer objects to see an event
• Strategy - Allows one of a family of algorithms to be selected on the fly at runtime
• Command - Creates objects which encapsulate actions and parameters
• State - Allows an object to alter its behavior when its internal state changes
• Iterator - Accesses the elements of an object sequentially without exposing its underlying representation
• Memento - Provides the ability to restore an object to its previous state (undo)
• Visitor - Separates an algorithm from an object structure by moving the hierarchy of methods into one object