Using Design Patterns to Elaborate upon Design Models

Using Design Patterns to Elaborate upon Design Models Moving from Analysis to Design Examine Design models and initial code to: • Improve cohesion • Reduce coupling • Enhance Reusability GoF Design Patterns should be used to rewrite code to promote these goals

Concordance Example A Concordance consists of an alphabetized list of words appearing in a short document together with an ordered list of the distinct line numbers of lines in which each word appears. Design and implement a short document concordance program. Your program should be able to accept any document, generate a Concordance for the document, and display (or print) the document with its concordance. The domain model will contain the following classes: Concordance Document Word WordList Line LineNumberList

makes/displays WordList Concordance parses * * Line Document contains Number * Word LineNumberList forms Concordance Domain Model

Implementation Consider class WordList. It has the following requirements: Quick Access -- HashTable O(1), BinarySearchTree O(lg(n)), Vector O(n) Alphabetically ordered display – BinarySearchTree O(ln), Vector O(n), HashTable Choose a BinarySearchTree as the most appropriate data structure Next, consider class LineNumberList. Line numbers are entered in the order they appear, and are read from front to back. Consider using either a Vector or a Queue to list line numbers. Choose the Vector for simplicity and because it can be read non-destructively.

BinSTree Concordance root BinSTreeNode 0..2 Document children parent holds Vector Word Line number Class Diagram – First Iteration

Consequences of Implementation 1 Class Interfaces class Word { private String word; private Vector theLines; public Word (String s, int lineNum) {….} public void addLine(int num) { theLines.add(new Integer(num)); public boolean equals(Word w) { … } public String toString( ) { … } public String getWord( ) { … } public String getLineNums( ) { … } } The statements highlighted in blue indicate attributes and operations flagged by a “House Unwordy Activities Committee” as not properly belonging to class Word. In the interest of high cohesion we will redesign the class

TreeIterator BinSTree Association Vector root root key holds BinSTreeNode 0..2 children parent Word Modified Class Diagram

Builder Director makePart( ) getPart( ) ConcreteBuilder Product makePart( ) getPart( ) Additional Design Modification Who is responsible for creating Associations? Concordance? Better Solution – use a Builder to construct Association objects (Concordance) (Association) Builder Pattern (AssocBuilder)

Implementation of Builder Pattern public class Concordance { private Builder assBuilder = new AssocBuilder( ); private BinsSTree bst = new BinsSTree( ); public void readLines(Document doc) { String delims = " \t\n.,!?;:"; for (int line = 1; true; line++) { try { String text = doc.readLine( ); if (text == null) return; text = text.toLowerCase( ); Enumeration e = new StringTokenizer(text, delims); while (e.hasMoreElements( )) enterWord((String)e.nextElement( ),line); }catch(IOException e) {System.err.println(e.toString( ));} } } private void enterWord(String word, int line) { Word w = new Word(word); assBuilder.buildPart(w, line); Association ass = assBuilder.getPart( ); if (!bst.contains(ass) ) { bst.add(ass); } else { boolean flag = false: Iterator itr = bst.iterator( ); while (itr.hasNext( ) && !flag) { Association visit = (Association) itr.next( ); if (visit.equals(ass)) { flag = true; visit.addLine(Integer(line)); } } } }

Implementation of Builder Pattern class AssocBuilder implements Builder{ private Association theProduct; public void buildPart(Word w, int lineNum) { theProduct = new Association(w, new Integer(linNum) ); } public Object getPart( ) { return theProduct; } } class Association implements Comparable{ private Word word; private Vector v; private String key; public Association ( ) { word = null; v = new Vector( ); key = null; } public Association(Word w, Integer lineNum) { word = w; v = new Vector( ); key = w.getWord( ); v.add(lineNum); } public addWord(Word w) { word = w; key = w.getWord( ); } public addLine(Integer lineNum) { v.add(lineNum); } //methods equals, compareTo, toString, etc. } Note! getPart( returns a generic Object – client must cast to get the desired return type.

Implementation of Builder Pattern When there is only one part to build, as the Association in the Concordance example, it makes sense to omit the abstract builder interface and just use a concrete class. We compromised and kept the abstract interface, but wrote the buildPart method with parameters specific to the single product (Association) in the example.

Builder Pattern Intent – Separate the construction of a complex object from its representation, so that the same construction process can create different representations (in our example – Association and Document) Participants • Builder –an Abstract Interface for creating parts of a Product object. • ConcreteBuilder – • Implements the Builder Interface • Defines and keeps track of the representation it creates • provides an interface for retrieving the product • Director (Concordance) • Constructs a Product object using the Builder Interface • Product (Association) • includes classes that define the constituent parts, including interfaces for assembling the parts into the final result.

Builder Pattern Collaborations • The client creates the Director object and configures it with the desired Builder object. • The Director notifies the ConcreteBuilder whenever a part of the Product should be built. • The Builder handles requests from the Director and adds parts to the Product • The client retrieves the Product from the Builder

create( ) new Director( ) new Association Alternative Design Follow the pattern more closely and create a separate Director that directs the construction of the BinSTree and the Document (not shown) Concordance aDirector aTreeBuilder aBinSTree new TreeBuilder( ) parseText(Document ) anAssoc loop buildPart(word,line) add(anAssoc) printConcordance( ) getResult( ) iterator( )

The docBuilder constructs a String – the Document – from a text file Alternative Design Classes public class Concordance { private Builder docBuilder = new DocumentBuilder( ); private DocParser director; private Document theText; public Concordance( ) { director = new DocParser( ); } public void makeConcordance( String filename) { docBuilder.buildPart( filename); theText = docBuilder.getPart( ); director .parseText(theText); } public void printConcordance( ) {…} }

Alternative Design Classes class DocParser { private Builder treeBuilder = new TreeBuilder( ); private Document theText; public parseText(Document doc) { theText = doc; readLines(theText); } public void readLines( ) { String delims = " \t\n.,!?;:"; for (int line = 1; true; line++) { try { String text = doc.readLine( ); if (text == null) return; text = text.toLowerCase( ); Enumeration e = new StringTokenizer(text, delims); while (e.hasMoreElements( )) enterWord((String)e.nextElement( ),line); } } catch(IOException e) {System.err.println(e.toString( ));} } private void enterWord(String word, int line) { Word w = new Word(word); treeBuilder.buildPart(w, line); } } The Concordance is relieved of any responsibility for forming Associations or performing operations on a tree.

Alternative Design class TreeBuilder implements Builder{ private BinSTree bst = new BinSTree( ); private Association ass; publicvoid buildPart(Word w, int line) { ass = new Association(w, new Integer(line) ); if (!bst.contains(ass) ) { bst.add(ass); } else { boolean flag = false: Iterator itr = bst.iterator( ); while (itr.hasNext( ) && !flag) { Association visit = (Association) itr.next( ); if (visit.equals(ass)) { flag = true; visit.addLine(Integer(line)); } } } } public Iterator getResult ( ) { return bst.iterator( ); }

Additional Patterns Singleton Adapter Composite Decorator Flyweight

Singleton Pattern Intent Ensure a class only has one instance, and provide a global point of access to it. Applicability • Use Singleton pattern when • There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. • When the sole instance should be extensible by sub-classing, and clients should be able to use an extended instance without modifying their code. Consequences • Controlled access to sole instance. Singleton class encapsulates its sole instance and has strict control over how and when clients access it. • Reduced name space. It avoids polluting the name space with global variables that store sole instances. • Permits refinement of operations and representation. It can be subclassed.

Singleton Pattern – Example code Singleton class declaration Implementation of Singleton class Singleton { public: static Singleton* Instance( ); protected: Singleton( ); private: static Singleton * _instance; } Singleton * Singleton:: _instance = 0; Singleton * Singleton::Instance( ) { if (_instance == 0) _instance = new Singleton( ); return _instance; }

Target Adaptee Client request( ) specialRequest( ) Adapter specialRequest( ) request( ) Adapter Pattern Class Adapter Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.

Target Adaptee Client request( ) specialRequest( ) Adapter adaptee  specialRequest( ) request( ) Adapter Pattern adaptee Object Adapter Adapter can add additional functionality to the Adaptee object that the Adaptee object lacks but that Target requires.

Adapter (a.k.a. Wrapper) Applicability • Use the Adapter pattern when • You want to use an existing class and its interface does not match the one you need. • You want to create a reusable class that cooperates with unrelated or unforseen classes, that is, classes that don’t necessarily have compatible interfaces. • (object adapter) you need to use several existing subclasses, but it’s impractical to adapt their interface by subclassing every one. An object adapter can adapt the interface of its parent class. Participants Target -- defines the domain-specific interface that Client uses. Client -- collaborates with objects conforming to the Target interface. Adaptee -- defines an existing interface that needs adapting. Adapter -- adapts the interface of Adaptee to the Target interface

Component Client * Operation( ) Add(Component) Remove(Component) GetChild(int) Composite Leaf For all g in children g.Operation( ); Operation( ) Add(Component) Remove(Component) GetChild(int) Operation( ) Composite Pattern

Composite Pattern Applicability You want to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly. Participants • Component -- declares the interface for objects in the composition -- implements default behavior for the interface common to all classes, as appropriate -- declares an interface for accessing and managing its child components. • Leaf -- defines behavior for primitive objects in the composition. • Composite -- defines behavior for components having children. -- stores child components -- implements child-related operations in the Component interface.

Composite Pattern Collaborations Clients use the Component class interface to interact with object in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwards request to its child components, possibly performing additional operations before and/or after forwarding. Consequences • Defines class hierarchies consisting of primitive objects and composite objects. Wherever client code expects a primitive object, it can also take a composite object. • Makes the client simple. Clients can treat composite structures and individual objects uniformly. • Makes it easier to add new kinds of components. Newly defined Composite or Leaf subclasses work automatically with existing structures and client code. • Can make your design overly general. It makes it harder to restrict the components of a composite.

Equipment Client * Operation( ) Add(Component) Remove(Component) GetChild(int) CompositeEquipment FloppyDisk CDDrive For all g in children g.Operation( ); Operation( ) Operation( ) Add(Component) Remove(Component) GetChild(int) Operation( ) Example of Composite Pattern

Sample Code class Equipment { public: virtual ~Equipment( ); constchar* name( ) { return _name; } virtual Watt Power( ); virtual Currency NetPrice( ); virtual Currency DiscountPrice( ); virtual void Add(Equipment *); virtual void Remove(Equipment *); virtual Iterator<Equipment*> * CreateIterator( ); protected: Equipment (const char * ); private: constchar * _name; } class FloppyDisk : public Equipment { public: FloppyDisk(const char*); virtual ~FloppyDisk( ); virtual Watt Power( ); virtual Currency NetPrice( ); virtual Currency DiscountPrice( ); } class Chassis : public CompositeEquipment{ public: Chassis (const char* ); virtual ~Chassis( ); virtual Watt Power( ); virtual Currency NetPrice( ); virtual Currency DiscountPrice( ); }

Sample Code for Composite Example class CompositeEquipment: public Equipment { public: virtual ~CompositeEquipment( ); virtual Watt Power( ); virtual Currency NetPrice( ); virtual Currency DiscountPrice( ); virtual void Add(Equipment *); virtual void Remove(Equipment *); virtual Iterator<Equipment *> * CreateIterator( ); protected: CompositeEquipment(const char *); private: List<Equimpent *> _equipment; } An implementation of NetPrice( ) Currency CompositeEquipoment::NetPrice( ) { Iterator<Equipment *> * i = CreateIterator( ); Currency total = 0; for (i -> first( ); i -> isDone( ); i -> next( ) ) { total += i -> currentItem( ) -> NetPrice( ); } delete i; return total; }

Sample code for Composite Pattern Example Assume we have additional Equipment classes such as Bus, Cabinet, etc. We can assemple equipment into a (simple) computer (CompositeEquipment object). Cabinet * cabinet = new Cabinet(“PC Cabinet”); Chassis * chassis = new Chassis(PC Chassis”); cabinet -> Add(chassis); Bus * bus = new Bus(“MCA Bus”); bus -> Add(new Card(“100 Mbs Ethernet”) ); chassis ->Add(bus); chassis -> Add (new FloppyDisk(“3.5 in Floppy”) ); cout << “ the net price is “ << chassis -> NetPrice( ) << endl;

aBorderDecorator aScrollPaneDecorator component component aTextView Decorator Pattern Allows the programmer to add responsibilities to individual objects, not to an entire class. The decorator conforms to the interface of the component it decorates so that its presence is transparent to the component’s clients. An example of a decorator is the scroll pane in java. We may add scroll bars as needed. We may also add a border around the scroll pane. Another example of the Decorator Pattern in java is the Stream classes.

Component operation( ) ConcreteComponent Decorator component.operation(); operation( ) operation( ) ConcreteDecoratorA ConcreteDecoratorB operation( ) super.operation(); addedBehavior(); operation( ) addedBehavior( ) addedState Decorator Pattern

Decorator Pattern Participants • Component – defines the interface for objects that can be “decorated” • ConcreteComponent (JTextArea) – defines an object to which additional responsibilities can be added (can be “decorated”) • Decorator – maintains a reference to a Component object and defines an interface that conforms to Components interface. • ConcreteDecorator (JScrollPane) – adds responsibilities to the Component.

Decorator Pattern Consequences More flexible than static inheritance. Avoids feature-laden classes high up in the hierarchy. Instead of trying to support all foreseeable features in a complex, customizable class, you can define a simple class and add functionality incrementally with Decorator objects. A Decorator and its Component are not identical. A decorator acts as a transparent enclosure.

Decorator Pattern -- Example import java.awt.*; import javax.swing.*; public class JScrollDemo extends JFrame { JTextArea area = new JTextArea(10, 60); JScrollPane sp = new JScrollPane(area, JScrollPane.VERTICAL_SCROLLBAR_ASNEEDED, JScrollPane.HORIZONTAL_SCROLLBAR_ASNEEDED); public JScrollBarDemo( ) { Container cp = getContentPane( ); area.setFont(new Font(“Monospaced”, Font.PLAIN, 12)); cp.add(sp, BorderLayout.CENTER); setVisible(true); setDefaultCloseOperation(EXIT_ON_CLOSE); } }

Flyweight Pattern Intent Use sharing to support a large number of fine-grained objects efficiently. The Flyweight pattern describes how to share objects to allow their use at fine granularities without prohibitive storage cost. Applicability -- Apply the Flyweight pattern when all of the following are true. • An application uses a large number of objects. • Storage costs are high because of the sheer quantity of objects. • Most object state can be made extrinsic. (Intrinsic state is stored in the flyweight – it is context independent. Extrinsic state depends upon the flyweight’s context and can’t be shared.) • Many groups of objects may be replaced by relatively few shared objects once extrinsic state is removed. • The application does not depend upon object identity (since flyweight objects are shared,identity tests will return true for conceptually distinct objects.)

FlyweightFactory Flyweight getFlyweight(key) operation(extrinsicState) if(flyweight(key) exists) { return existing flyweight; } else { create new flyweight; add it to pool of flyweights return the new flyweight; } ConcreteFlyweight UnsharedConcreteFlyweight Client operation(extrinsicState) operation(extrinsicState) intrinsicState allState Flyweight Pattern

aClient aClient flyweight pool aConcreteFlyweight aConcreteFlyweight aFlyweightFactory intrinsicState intrinsicState flyweights Flyweight Pattern The following object diagram shows how flyweights are shared

Flyweight Pattern Participants • Flyweight (Glyph) – declares an interface through which flyweights can receive and act on extrinsic state. • ConcreteFlyweight (Character) – implements the Flyweight interface and adds storage for intrinsic state. It must be sharable and any stored state must be independent of the flyweight’s context. • UnsharedConcreteFlyweight (Row, Column) – not all Flyweight subclasses need to be shared. The Flyweight interface enables sharing; it doesn’t enforce it. It’s common for UnsharedConcreteFlyweight objects to have ConcreteFlyweight objects as children at some level of the flyweight object structure. • FlyweightFactory – creates and manages flyweight objects. When a client requests a flyweight object, the factory supplies an existing flyweight object if one exists and creates one if it doesn’t. • Client – maintains a reference to flyweights and computes or stores their extrinsic state.

Flyweight Pattern ConcreteFlyweight -- Character Intrinsic state – character code Extrinsic state – font, color, position public interface Glyph { public void draw(Window w, GlyphContext gc); public void setFont(Font f, GlyphContext gc); public Font getFont(GlyphContext gc); …….. } public class Character implements Glyph { //implement all of the Glyph methods public Character(char c) {_charcode = c;} private char _charcode; } public class Row implements Glyph { //implement all of the Glyph mehods }

Shared flyweights Unshared flyweights Flyweight Pattern public class GlyphFactory { private static final int NUMCHARS = 128; private Glyph[ ] charPool; public GlyphFactory { charPool = new Glyph[NUMCHARS]; for (int i = 0; i < NUMCHARS; i++) {charPool[i] = null; } } public Character createCharacter(char ch) { if (charPool[ch] == null) charPool[ch] = new Character(ch); return charPool[ch]; } public Row createRow( ) { returnnew Row( ); } }

Flyweight Pattern Class GlyphContext must store or compute the extrinsic state of each flyweight object in the document. If each character in the document is indexed, then we would need to locate the start index and duration of substrings using a particular font.

Using Design Patterns to Elaborate upon Design Models