Design patterns. OOD Lecture 6

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1 Design patterns OOD Lecture 6

2 Next lecture Monday, Oct 1, at 1:15 pm, in 1311 Remember that the poster sessions are in two days Thursday, Sep 27 1:15 or 3:15 pm (check which with your TA) Room

3 The object design activity Select sub-system Specification Re-use Specifying types & signatures Specifying visibility Identifying missing attributes & operations Identifying components Adjusting components Specifying constraints Specifying exceptions Adjusting patterns Identifying patterns Check use cases Restructuring Optimization Revisiting inheritanc Optimizing access paths Collapsing classes Caching complex computations Realizing associations Delaying complex computations

4 What are design patterns? Software problems generally have more than one solution: How do you choose which to use? Patterns give guidance in the form of solution types based on experience Patterns are also communication tools: I m using pattern X instead of lengthy descriptions of solution details. Communication abstraction Design patterns are not designs only helpful hints

5 Pattern elements A name Description of a typical problem The elements that make up the pattern: Classes, relations, responsibilities, collaborations Consequences: Pros, cons, trade-offs An example solution

6 The concept of design patterns Nothing new in itself. Patterns are to a large degree verbalized and externalized experience, no matter what your discipline The Gang of Four book (GoF) started people thinking more in these terms for s/w design Gamma, Erich, Richard Helm, Ralph Johnson, John Vlissides (1995) Design Patterns: Elements of Reusable Object-Oriented Software. Addison- Wesley. ISBN Inspired by architectural theory: Christopher Alexander (1977) A Pattern Language: Towns, Buildings, Construction

7 Classic patterns Creational patterns Abstract factory, Builder, Factory method, Singleton, Prototype Structural patterns Adapter, Bridge, Composite, Decorator, Façade, Flyweight, Proxy Behavioural patterns Chain of Responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template method, Visitor

8 Factory method Creational pattern Problem Hard-coding class names in code makes code less flexible Damages re-use, changes might require multiple consistent updates Testing: E.g. stubbing production data Shielding user from complex creation, possibly involving data the user shouldn t have access to Solution Decouple creation point from class name Use separate method for instance creation Encapsulates the places needing changes Multiple pluggable creators with common interface

9 Factory method UML «interface» Creator +factorymethod() : Product Product ConcreteCreator +factorymethod() : Product DIP + OCP (+ ISP + LSP), decoupling Dependency injection (technique for achieving dependency inversion) Dependency defined by Creator Injector/Provider/Container = ConcreteCreator Dependent = whatever uses Product

10 Factory method: Before class Bonniers { void publish(string title, String author, String text) { String isbn = kb.lookup(title, author); Book b = new ISBNBook(title, author, isbn); public class ISBNBook { public ISBNBook(String title, String author, String isbn) {

11 Factory method: After public interface BookCreator { Book makebook(string title, String author); public class BooksWithISBN implements BookCreator Book makebook(string title, String author) { String isbn = kb.lookup(title, author); return new ISBNBook(title, author, isbn); // different package class Bonniers { BookCreator bc; void publish(string title, String author, String text) {... Book b = bc.makebook(title, author);... Product «interface» Creator +factorymethod() : Product ConcreteCreator +factorymethod() : Product

12 Factory method Other uses Also used to encapsulate object creation when it s complex or it s convenient to delay decision class ImageReaderFactory { static ImageReader getimagereader(inputstream is) { switch(determineimagetype(is)) { case ImageReaderFactory.GIF: return new GifReader(is); case ImageReaderFactory.JPEG: return new JpegReader(is); //

13 Factory method Limitations Introducing factory methods might break existing clients Reduces coupling by increasing complexity

14 Abstract factory pattern Generalization of factory method pattern «interface» AbstractFactory «uses» Client +createproducta() : AbstractProductA +createproductb() : AbstractProductB «uses» «interface» AbstractProductA ConcreteFactory1 «instantiates» ProductA1 ProductA2 +createproducta() : ProductA1 +createproductb() : ProductB1 ConcreteFactory2 «instantiates» «uses» +createproducta() : ProductA2 +createproductb() : ProductB2 «interface» AbstractProductB «instantiates» ProductB1 ProductB2 «instantiates»

15 Composite Structural pattern Description: Represent a hierarchy of variable width and depth so that leaves and composites can be uniformly accessed through a common interface Examples Syntax trees for arithmetical expressions (such as (((x+2)*y)+3)*(z+1)) Organizational chart In general, anything that can be seen as tree structured

16 Composite Example Domain object Solution object Instance diagram Add x + (y * 2) x Multiply y 2

17 Composite Solution Structure Leaf Component Composite Navigability uni- or bidirectional not part of the pattern * parent leaves 1 Participants Component Provides the common interface May provide interface to both parent and children (e.g. evaluate for arithmetic expressions) Composite Primitive with children Implements behaviour for composite objects Leaf Primitive w/out children Implements behaviour for terminal objects

18 Composite Example in code interface Expression { int evaluate(map<string, int> environment); class Add implements Expression { private Expression lhs; private Expression rhs; public Add(Expression l, Expression r) { lhs = l; rhs = r; public int evaluate(map<string, int> environment) { return lhs.evaluate(environment) + rhs.evaluate(environment); class Integer implements Expression { private int value; public Integer(int v) { value = v; public int evaluate(map<string, int> _env) { return value; C o m p o n e n t * p a r e n t L e a f C o m p o s i t e l e a v e s class Variable implements Expression { private String name; public Variable(String n) { name = n; public int evaluate(map<string, int> env) { return environment.get(name); 1

19 Composite Consequences and issues Pro Simplifies the client Easy to add new kinds of components (e.g. division) Contra Recursive composition of objects Behaviour depends on composition Issues How to provide access to children? In Component (Expression) or in Composite (Add etc) Pseudo-problem Explicit parent references? Sharing components? Child ordering?

20 Proxy Structural pattern Expose only a subset of an object interface to clients Improve the performance or the security of a system by delaying expensive computations, using memory only when needed, or checking access before loading an object into memory

21 Proxy Solution Structure Client ProxyObject attribute1 attribute2 operation1() operation2() Subject attribute1 attribute2 operation1() operation2() RealObject attribute1 attribute2 moreattributes operation1() operation2() moreoperations() Participants Subject The interface that clients see RealObject Class with expensive computations or security issues ProxyObject Acts on behalf of RealObject until the expensive computation is needed, when it delegates to the real object

22 Proxy Consequences Pro: The Client is shielded from any optimizations for handling RealObjects Contra: Adds a level of indirection between Client and RealObject (i.e. increases complexity)

23 Proxy Example in code interface AlgebraServer { int calculate(expression expression); Client Subject attribute1 attribute2 operation1() operation2() class RemoteServer implements AlgebraServer{ int calculate(expression expression) {... class CachingProxy implements AlgebraServer { HashMap<Expression, Value> cache = new HashMap<Expression, Value>(); RemoteServer server; int calculate(expression expression) { int result; ProxyObject attribute1 attribute2 operation1() operation2() RealObject attribute1 attribute2 moreattributes operation1() operation2() moreoperations() if (cache.haskey(expression)) { result = cache.get(expression); else { result = server.calculate(expression); cache.put(expression, new Value(result)); return result;

24 Strategy Behavioural pattern Decouple a policy-deciding class from a set of mechanisms so that different mechanisms can be changed transparently from a client

25 Strategy pattern Library example University library lending policies Normal book, no outstanding requests: Undergraduate borrower: 2 weeks Everyone else: 4 weeks Normal book, outstanding request: 1 week Reference book: Cannot be borrowed New book: 1 week Reserved book: 3 days

26 Strategy pattern Library example: Implementation #1 int getloanlength(book b, Patron p) { if (p.isreserved()) { return 3; if (b.isnormalbook()) { if (p.hasoutstandingrequest()) { return 7; if (p.isundergraduate()) { return 14; else { return 28; if(p.isnewbook()) { return 7; return 0; // is reference book Logic implementation in single method Inflexible, as logic is hard-coded Results in ugly code that is hard to read

27 Strategy pattern Library example: Design #2 Book -loanlength : unsigned short(idl) +getloanlength() : unsigned short(idl) NewBook -loanlength : unsigned short(idl) = 7 ReferenceBook -loanlength : unsigned short(idl) = 0 ReserveBook -loanlength : unsigned short(idl) = 3 Logic spread among special subclasses Neater, but Very inflexible Every time a book is reserved, it has to change class

28 Strategy pattern Problem Many related classes differ only in their behaviour OR need custom behaviour for individual instances OR class has many behaviours implemented through many conditional statements Solution Define interface abstracting algorithm Each algorithm variant encapsulated into separate class implementing algorithm interface Context class calls strategy object to perform algorithm

29 Strategy pattern Solution Structure Client «uses» Context «interface» Strategy 1 1..* ConcreteStrategyA ConcreteStrategyB Participants Strategy: Interface that abstracts algorithm Concrete strategy: A class for each algorithm variant that implements the interface Context: A class that calls the Strategy object to perform the algorithm

30 Strategy pattern Library example, solution #3 UML Client «uses» Book «interface» LendingPolicy 1 1..* +getloanlength() NewBookPolicy ReferencePolicy +getloanlength() +getloanlength() NormalPolicy ReservedPolicy +getloanlength() +getloanlength()

31 Strategy pattern Library example, solution #3 code class Book { LendingPolicy policy; int loanlength(patron p) { return policy.loanlength(p); interface LendingPolicy { int loanlength(patron p); class NewBookPolicy implements LendingPolicy { int loanlength(patron p) { return 7; class ReferencePolicy implements LendingPolicy { int loanlength(patron p) {return 0; // Context // Strategy // Concrete strategy // Concrete strategy class NormalPolicy implements LendingPolicy { int loanlength(patron p) { if (p.isundergraduate()) { return 14; else { return 28; // Concrete strategy Client «uses» Context «interface» Strategy 1 1..* ConcreteStrategyA ConcreteStrategyB

32 Strategy pattern However Some wouldn t agree with that definition of the strategy pattern, but would rather use this Policy Client «uses» Context «interface» Strategy 1 1..* ConcreteStrategyA ConcreteStrategyB where the Policy determines which concrete strategy that Context uses

33 Strategy pattern Consequences Pro ConcreteStrategies can be substituted transparently from Context New algorithms can be added without modifying Context or Client Contra Increased complexity (especially in the variant with a separate Policy object)

34 Summary of remaining patterns Creational Abstract factory Encapsulating platforms Builder Separate the construction of a complex object from its representation allowing the same construction process to create various representations. Fits Composites well. Incremental + director Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Singleton Ensure a class has only one instance, and provide a global point of access to it.

35 Summary of remaining patterns Structural Adapter Convert the interface of a legacy class into a different interface expected by the client, so that the client and the legacy class can work together without changes Bridge (cf Strategy p) Decouple an interface from an implementation so that implementations can be substituted, possibly at runtime. Decorator Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality. Façade Reduce coupling between a set of related classes and the rest of the system Flyweight Factor out common data into separate objects with pointer Cf database design

36 Decorator pattern

37 Decorator pattern example

38 Summary of remaining patterns Behavioural Chain of Responsibility Messages passed along extensible chain of processors Command Encapsulate requests so that they can be executed, undone, or queued independently of the request Interpreter Behavioural variant on Composite pattern Iterator Sequential access to aggregate elements Mediator Class encapsulating object interactions -> loose coupling Memento Originator memento caretaker. E.g. pseudo-rng Observer (Publish/Subscribe) Maintain consistency across the states of one Publisher and many Subscribers State Cf Strategy and Bridge. Interface attribute changeable subclass value Template method Algorithm steps overridden by subclasses Visitor Separate operations from structure. Fits Composites well

39 Visitor pattern

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