CS 325: Software Engineering

February 12, 2015

Applying Responsibility-Assignment Patterns• Design Patterns• Situation-Specific Patterns• Responsibility-Assignment Patterns• The Controller Pattern• The Expert Pattern• The Creator Pattern

Design Patterns

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Design patterns are general repeatable solutions to commonly occurring problems in software design.Patterns originated

as an architectural concept by Christopher Alexander in the late 1970s.

Design Patterns

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In 1987, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming.

Design patterns gained popularity in computer science after the 1994 publication of Design Patterns: Elements of Reusable Object-Oriented Software by the “Gang of Four”: Erich Gamma, Richard Helm, Ralph Johnson, & John Vlissides.

Situation-Specific Patterns

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The classical Gang-of-Four design patterns are situation-specific, where each pattern is applied to particular type of software design problem.

Stuctural Patterns

describe how to combine objects into

larger structures.

Creational Patterns employ

methods and objects to instantiate

other objects.

Behavioral Patterns address common object

interactions.

Responsibility-Assignment Patterns

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A second type of design pattern, developed by Craig Larman in 2005, focuses on assigning appropriate responsibilities to the objects within a software system.High

Cohesion ensures that the

responsibilities of a

given software

element are strongly

related and highly

focused (e.g.,

breaking programs

into classes and

subsystems).

Loose Coupling assigns

responsibilities to support

lower dependency between the classes and higher reuse potential by

ensuring that change in

one class has little impact

on other classes.

Polymorphism

assigns the

responsibility of

defining any

variation of

behavior based

upon type to the

types for which the variation happens.

The Controller Pattern

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Having the user interface of a system handle user requests to the system forces the UI to know too much about the internal system, but having the internal system handle user requests forces the internal system to know too much about the UI.

Inserting a “controller” class in between the UI and the internal system loosens their coupling, reducing the change effect that they have on each other.

The Expert Pattern

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The Expert Pattern assigns the responsibility for handling a functional request to an object that has the information (e.g., the attributes) to fulfill the request.

In this partial card game model, which class should handle requests for the current scoreboard?

The Creator Pattern

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The Creator Pattern addresses the issue of assigning responsibility for object creation.Consider giving Class A responsibility for creating objects of Class B if:• Class A is an aggregation of Class B (i.e., A

consists of B objects)• Objects of Class A contain objects of Class B

• Objects of Class A maintain objects of Class B

• Objects of Class A frequently use objects of Class B• Objects of Class A have the information to create objects of Class B