Object-Oriented Programming

Design Patterns

Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron


  • 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



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


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