Selected design patterns (as part of the the PTT lecture)

Selected Design Patterns
Prof. Dr. Ralf Lämmel
Universität Koblenz-Landau
Software Languages Team

(C) 2010-2013 Prof. Dr. Ralf Lämmel, Universität Koblenz-Landau (where applicable)
Reusable solutions to
recurrent problems

Selected patterns
(Pattern names with intents)
Composite -- provide uniform interface on part-whole hierarchies
Command -- encapsulate the execution of functionality; enable undo
Visitor -- represent operations on object structure as objects
Observer -- provide change notiﬁcations to objects depending on state
MVC -- decouple model, view, control in for interactive applications
Proxy -- reﬁne or replace behavior of a given object
Object Adapter -- provide a different interface for an existing object
Template Method -- capture general structure of an algorithm
...
Intent = short combo
of problem + solution

Observer -- Problem
The observer pattern (aka publish/subscribe) involves
an object, called the subject, which maintains a list of
its dependents, called observers, and notiﬁes them
automatically of any state changes.
Do you know the problem?
Do you know a solution?

Observer -- Solution
http://en.wikipedia.org/wiki/File:Observer.svg

Essential elements of a
design pattern
Name
The problem describes when to apply the pattern.
The solution describes elements of the design, their
relationships, responsibilities, and collaborations.
The consequences are the results and trade-offs of
applying the pattern.

Describing design patterns
Pattern Name
Classiﬁcation
Intent
Also Known As
Motivation
Applicability
Structure
Participants
Collaborations
Consequences
Implementation
Sample Code
Known Uses
Related Patterns
We do
not go into all
these details in this
course.

Classiﬁcation of
design patterns
Aspects
Creational patterns
Structural patterns
Behavioral patterns
Scope
Class
Object

Design patterns -- Overview
[Gamma et al., 1995, S. 10]

Design patterns
of this slide deck
Template Method
Abstract Factory
Object Adapter
Observer
Proxy
You are deﬁnitely
expected to expand on
your knowledge of
design patterns.

Abstract Factory -- Problem
Object models often give rise to variation. For instance,
there may be multiple GUI libraries subject to different
widget hierarchies. Whenever components want to
abstract from the speciﬁc choice, then substantial efforts
are required. For instance, the construction of objects
must be tunneled through a factory.

Abstract Factory -- Solution
http://sourcemaking.com/design_patterns/abstract_factory

Abstract Factory
DEMO
Two alternative object models for companies.
POJOs
Some sort of beans
Make the choice of the object model conﬁgurable.
Contribution:javaExorcism

Proxy -- Problem
Additional access behavior for objects
Access management for remote objects
Access management for expensive objects

Proxy -- Solution
http://en.wikipedia.org/wiki/File:Proxy_pattern_diagram.svg

Proxy -- Scenarios
Additional access behavior for objects
Access management for remote objects
Access management for expensive objects

Proxy
DEMO
Use security proxy to protect salary access.
Interesting issue of proxy deployment.

Observer -- Problem
The observer pattern (aka publish/subscribe) involves
an object, called the subject, which maintains a list of
its dependents, called observers, and notiﬁes them
automatically of any state changes.

Observer
DEMO
Use observer to providing logging.
Use observer to enforce data constraint.

Object Adapter -- Problem
A given object often has about the right capabilities for a
speciﬁc client but not necessarily the precise interface
expected. Accordingly, an interface adaptation can be
achieved by an extra wrapper around the given object.

Adapter -- Solution
(object adapters)

Adapter -- Solution
(class adapters)

Adapter
DEMO
Class adapter: add observability of list interface.
Object adapter: downgrade java.util.List to simple list.

Template Method -- Problem
Some algorithms or strategies in a system may be very
similar and hence it may be desirable to capture their
commonalities in a sort of template so that speciﬁcs can
be expressed through reﬁnement giving rise to a high
degree of reuse.

Template Method -- Solution
http://stephane.ducasse.free.fr/FreeBooks/SmalltalkDesignPatternCompanion/templateMethod.pdf
Intent
Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Temp
Method lets subclasses redefine certain steps of an algorithm without changing the algorith
structure.
Structure
Description
Template Method may be #1 on the Design Patterns Pop Charts. Its structure and use are at the h
of much of object-oriented programming. Template Method turns on the fundamental object-orie
templateMethod
AbstractClass
ConcreteClassA
primitiveMethod2
primitiveMethod1
primitiveMethod1
primitiveMethod2
...
self primitiveMethod1.
...
self primitiveMethod2.
...
ConcreteClassB
primitiveMethod1
primitiveMethod2
ConcreteClassC
primitiveMethod2

Template
DEMO
Two general class of algorithms can identiﬁed:
Queries such as 101feature:Total
Transformations such as 101feature:Cut
Contribution:javaTemplate

Summary
The use of design patterns is a basic element of software
development. If you wouldn’t use them, your designs
and implementations are unnecessarily unstandardized,
idiosyncratic, incomprehensible, unmaintainable, etc.
That is, if you are interested in a software development
career, or you plan to talk to software developers at an
interesting level of detail, you have to aggregate
substantial knowledge of design patterns.

Selected design patterns (as part of the the PTT lecture)

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