Şabloane de Proiectare

Daniel POP, Ph.D

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The Plan

• Introducing fundamental patterns (today)

• 18 design patterns will be covered based on the case study presented in the text book (next 8 classes):– Design Patterns: Elements of

Reusable Object-Oriented Software

• Remaining patterns (5) will be presented individually (2 classes)

• Last 3 classes will be devoted to another case study: Designing a Payroll System

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Definition

• A 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 this solution a million times over, without ever doing it the same way twice. [Christopher Alexander, Sara Ishikawa, Murray Silverstein, Max Jacobson, Ingrid Fiksdahl-King, and Shlomo Angel. A Pattern Language. Oxford University Press, New York, 1977]

• 4 essential elements:– Pattern name

– Problem

– Solution

– Consequences

– Implementation

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Types of Patterns

• Architectural Patterns: MVC, Layers etc.

• Design Patterns: Singleton, Observer etc – We’ll do these ones

• GUI Design Patterns: Window per task, Disabled irrelevant things, Explorable interface etc• Database Patterns: decoupling patterns, resource patterns, cache patterns etc.• Concurrency Patterns: Double buffering, Lock object, Producer-consumer, Asynchronous processing etc.• Enterprise (J2EE) Patterns: Data Access Object, Transfer Objects etc.• GRASP (General Responsibility Assignment Patterns): Low coupling/high cohesion, Controller, Law of Demeter (don’t talk to strangers), Expert, Creator etc.• Anti-patterns = bad solutions largely observed: God class, Singletonitis, Basebean etc.

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Types of Design Patterns

• Based on the type of the problem it addresses (Purpose)– Fundamental

– Creational

– Structural

– Behavioral

• Bases on the technique it uses (Scope)– Class: uses inheritance

– Object: uses object composition

Remark: we’ll cover patterns from the text book, but many other design patterns exist as well; also classification may differ from one author to another.

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Fundamental PatternsDelegation

• Delegation = Handing of a task over another object

• Consequences:– Behavior can be changed at run-time (comparing to

inheritance that is static)

– The ‘delegate’ is hidden to delegator’s clients

– More difficult to implement comparing to inheritance

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Fundamental PatternsInterface

• Interface – decouples the service provider from its client

• Consequences:– Programming to abstraction

– Easily change the service provider

– Transparency for client

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Fundamental PatternsAbstract Superclass

• Abstract superclass – ensures consistent behavior for its subclasses

• Consequences:– Common behavior is

consistent over subclasses

– Variant behavior is abstract in superclass

– Clients are using the abstract superclass

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Fundamental PatternsInterface and Abstract Superclass

• Combines Interface and Abstract Superclass patterns

• Consequences:– Combines the

advantages of both patterns

– May provide a default behavior for the entire, or just a subset, of the ServiceIF via AbstractService

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Fundamental PatternsImmutable Object

• Immutable object – the internal state of the object doesn’t change after its creation

• Consequences:– Only constructors can change

object’s state

– All member functions are const functions (in C++)

– Any member function that need to change the state will create a new instance

– Increase design’s robustness and maintainability

• Example: – String class in JDK

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Fundamental PatternsMarker Interface

• A class implements a marker interface in order to support a semantic attribute of the system

• Consequences:– Used by utility classes that need a specific behavior from

their elements, without requesting a common base class

• Example: – Cloneable, Serializable in JDK

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Bibliography

http://www.oodesign.com

http://www.amazon.com/Design-Patterns-Elements-Reusable-Object-Oriented/dp/0201633612/ref=sr_1_1?ie=UTF8&qid=1324985583&sr=8-1

http://www.amazon.com/First-Design-Patterns-Elisabeth-Freeman/dp/0596007124/ref=sr_1_2?ie=UTF8&qid=1324985583&sr=8-2