Transcript Design Patterns: Case Study

Şabloane de Proiectare
Daniel POP, Ph.D
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
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•
•
2
Remaining patterns (5) will be
presented individually (2 classes)
Last 3 classes will be devoted to
another case study: Designing a Payroll
System
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]
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•
4 essential elements:
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Pattern name
Problem
Solution
Consequences
Implementation
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, Producerconsumer, 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
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Based on the type of the problem it addresses (Purpose)
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–
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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.
5
Fundamental Patterns
Delegation
•
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
6
Fundamental Patterns
Interface
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Interface – decouples the service provider from its client
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Consequences:
– Programming to abstraction
– Easily change the service provider
– Transparency for client
7
Fundamental Patterns
Abstract Superclass
Abstract superclass – ensures consistent behavior for its
subclasses
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Consequences:
– Common behavior is consistent
over subclasses
– Variant behavior is abstract in
superclass
– Clients are using the abstract
superclass
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Fundamental Patterns
Interface and Abstract Superclass
•
Combines Interface and Abstract Superclass patterns
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Consequences:
– Combines the
advantages of both
patterns
– May provide a default
behavior for the entire,
or just a subset, of the
ServiceIF via
AbstractService
9
Fundamental Patterns
Immutable 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
10
Fundamental Patterns
Marker Interface
A class implements a marker interface in order to support a
semantic attribute of the system
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Consequences:
•
– Used by utility classes that need a specific behavior from their
elements, without requesting a common base class
• Example:
– Cloneable, Serializable in JDK
11
Bibliography
http://www.amazon.com/D
esign-Patterns-ElementsReusable-ObjectOriented/dp/0201633612/r
ef=sr_1_1?ie=UTF8&qid=132
4985583&sr=8-1
12
http://www.amazon.com/F
irst-Design-PatternsElisabethFreeman/dp/0596007124/r
ef=sr_1_2?ie=UTF8&qid=132
4985583&sr=8-2
http://www.oodesign.com