Design Patterns and Graphical User Interfaces Horstmann ,

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

Design patterns For each design pattern –Prototypical example –Context, problem –Solution, decoupling/interface –UML diagram Patterns –Iterator, observer, strategy, composite, decorator

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

Example: accessing elements in an ArrayList ArrayList filmList = new ArrayList (); filmList.add(”E.T.”); filmList.add(”Gran Torino”); Printing all elements for (String film : filmList) System.out.println(film); Printing all elements using iterator Iterator iter = filmList.iterator(); while (iter.hasNext()) System.out.println(iter.next()); Printing all elements ”manually” for (int i=0; i< filmList.size(); i++) System.out.println(filmList.get(i));

public Class FilmCatalogue { private String[] films = {”Dogville”,”Chinatown”}; public String get(int i) {return films[i];} public int size() {return films.length;} } Uses encapsulation: YES Fully decoupled: NO –Only ”manual” printing of elements in a FilmCatalogue object possible –very inefficient if array representation changed to linked lists Example: Accessing elements in arbitrary container

Better: decoupling with iterator: public class FilmCatalogue implements Iterable { public Iterator iterator() { return new Iterator () { private int position = 0; public boolean hasNext() {return position<films.length;} public String next() { if (hasNext()) return films[position++]; else return null; }... }; }... }

Example: accessing elements in iterable FilmCatalogue FilmCatalogue filmList = new ArrayList (); filmList.add(”E.T.”); filmList.add(”Gran Torino”); FilmCatalogue filmList = new FilmCatalogue(); Printing all elements for (String film : filmList) System.out.println(film); Printing all elements using iterator Iterator iter = filmList.iterator(); while (iter.hasNext()) System.out.println(iter.next()); Printing all elements ”manually” for (int i=0; i< filmList.size(); i++) System.out.println(filmList.get(i));

Iterator Pattern Context An aggregate object contains element objects Clients need access to the element objects The aggregate object should not expose its internal structure Multiple clients may want independent access Solution Define an iterator that fetches one element at a time Each iterator object keeps track of the position of the next element If there are several aggregate/iterator variations, it is best if the aggregate and iterator classes realize common interface types.

Iterator Pattern

Iterable iterator() FilmCatalogue Iterator next() ! hasNext() next() anonymous EXAMPLE

QUIZ Library myLib = new Library(...); for (Book text : myLib) System.out.println(text); Iterator iter = myLib.iterator(); while (iter.hasNext()) System.out.println(iter.next()); Book[] list = myLib.getBooks(); for (int i=0; i< list.length; i++) System.out.println(list[i]); for (Book text : myLib.getBooks()) System.out.println(text); Which of the code pieces obey the iterator pattern in the UML diagram ? 1.None 2.A only 3.B only 4.C only 5.D only 6.Two of A,B,C,D 7.Three of A,B,C,D 8.All of A,B,C,D 9.I don’t know A B C D

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

Model/View/Controller Extra/changed view transparent to content

Model/View/Controller Model: data structure, no visual representation Views: visual representations Controllers: user interaction Views/controllers update model Model tells views that data has changed Views redraw themselves

Model/View/Controller

Button / Listener recall old example helloButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { textField.setText("Hello, World"); } );

Observer Pattern Model notifies views when something interesting happens Button notifies action listeners when something interesting happens Views attach themselves to model in order to be notified Action listeners attach themselves to button in order to be notified Generalize: Observers attach themselves to subject

Observer Pattern

JButtonActionListener anonymous addActionListener() actionPerformed()

QUIZ a)MVC: Frame is Model Display Components are Views Buttons are Controllers b)MVC: Model is not visible, but contains a number Display Components are Views Listeners attached to Buttons are Controllers c)Observer: A Button is a Subject A Listener attached to a button is an Observer Which interpretations are reasonable? 1.None 2.a 3.b 4.c 5.a+b 6.a+c 7.b+c 8.a+b+c 9.I dont’t know Display Components Button Components

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

GUI: components and containers 3 button components textfield component label component frame container with 5 components (textfield, 3 buttons, label)

Layout Managers User interfaces made up of components Components placed in containers Container needs to arrange components Swing doesn't use hard-coded pixel coordinates Advantages: –Can switch "look and feel" –Can internationalize strings Layout manager controls arrangement

Layout Managers

Example JPanel decDisplay = new JPanel(); final JTextField digits = new JTextField(“98",15); decDisplay.add(new JLabel("Decimal:")); decDisplay.add(digits); JPanel display = new JPanel(); display.setLayout(new BorderLayout()); display.add(decDisplay, BorderLayout.NORTH); display.add(binDisplay, BorderLayout.SOUTH); JPanel Default : FlowLayout BorderLayout

Example JPanel keyboard = new JPanel(); keyboard.setLayout(new GridLayout(2,2)); keyboard.add(new JButton("0")); keyboard.add(new JButton("inc")); keyboard.add(new JButton("1")); keyboard.add(new JButton("dec")); JFrame f = new JFrame(); f.add(display,BorderLayout.NORTH); f.add(keyboard,BorderLayout.CENTER); GridLayout JFrame Default : BorderLayout

Strategy Pattern Pluggable strategy for layout management Layout manager object responsible for executing concrete strategy Generalizes to Strategy Design Pattern Other manifestation: Comparators Comparator comp = new CountryComparatorByName(); Collections.sort(countries, comp);

Strategy Pattern Context A class can benefit from different variants for an algorithm Clients sometimes want to replace standard algorithms with custom versions Solution Define an interface type that is an abstraction for the algorithm Actual strategy classes realize this interface type. Clients can supply strategy objects Whenever the algorithm needs to be executed, the context class calls the appropriate methods of the strategy object

Strategy Pattern

Collections Compare() anonymous Comparator

Strategy Pattern LayoutManager Container BorderLayout layoutContainer()

QUIZ public class Graph { private Function f = new Logarithm(); public draw() { plot(1,f.evaluate(1)); … } } public class Graph { private Function f; public Graph(Function fun) { f=fun; } public draw() { plot(1,f.evaluate(1)); … } } Which versions of Graph class use strategy pattern? 1. None 2. A 3. B 4. A,B 5.I don’t know A B

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

Containers and Components Containers collect GUI components Sometimes, want to add a container to another container Container should be a Component Composite design pattern Composite method typically invoke component methods E.g. Container.getPreferredSize invokes getPreferredSize of components

Composite Pattern Context Primitive objects can be combined to composite objects Clients treat a composite object as a primitive object Solution Define an interface type that is an abstraction for the primitive objects Composite object collects primitive objects Composite and primitive classes implement same interface type. When implementing a method from the interface type, the composite class applies the method to its primitive objects and combines the results

Composite Pattern

Component JPanel JButton getPreferredSize()

QUIZ class Rectangle: public void paint(Graphics2D g) { g.draw(...); } class Composite: private ArrayList cubes; public void paint(Graphics2D g) { for (Rectangle r: cubes) r.paint(g); } Does UML and code for paint methods realize composite design pattern? 1.Yes, both UML and code 2.Only UML 3.Only code 4.Neither UML nor code 5.I don’t know

Design patterns Design Patterns Iterator Model-View-Controller and Observer Strategy Composite Decorator

Scroll Bars Scroll bars can be attached to components Approach #1: Component class can turn on scroll bars Approach #2: Scroll bars can surround component JScrollPane pane = new JScrollPane(component); Swing uses approach #2 JScrollPane is again a component

Decorator Pattern Context Component objects can be decorated (visually or behaviorally enhanced) The decorated object can be used in the same way as the undecorated object The component class does not want to take on the responsibility of the decoration There may be an open-ended set of possible decorations Solution Define an interface type that is an abstraction for the component Concrete component classes realize this interface type. Decorator classes also realize this interface type. A decorator object manages the component object that it decorates When implementing a method from the component interface type, the decorator class applies the method to the decorated component and combines the result with the effect of the decoration.

Decorator Pattern

JScrollPane JComponent JTextArea paintComponent()

QUIZ class Rectangle: public void paint(Graphics2D g) { g.draw(...); } class Decor: private Cubism cube; public void paint(Graphics2D g) { g.draw(new Rectangle2D(...)); g.draw(... cube...); } Does UML and code for paint methods realize decorator design pattern? 1.Yes, both UML and code 2.Only UML 3.Only code 4.Neither UML nor code 5.I don’t know