2/10/15 Today ’ s goals Design Patterns ❚ What are Design Patterns? ❚ Announcements: ❙ Thank you for your Early Informal Feedback Composite, Visitor, ❙ HW3 – Phase 2, peer reviews due this Thu at Template Method, Façade, 5pm Adapter Examples from Eclipse cs361 2 cs361 1 Defining Patterns How to Design Design Patterns – expert solutions to recurring problems in a certain domain ❚ Problem Description usually involves problem definition, driving forces, ❙ Model the problem, analysis, requirements solution, benefits, difficulties, related patterns. ❚ Patterns ❙ Remember similar solutions Pattern Language - a collection of patterns, guiding the users ❚ Patch through the decision process in building a system ❙ Test and fix, repair Patterns are related (high level-low level) cs361 3 4 Example Design Space Software Patterns “Lunch” Pattern “Eat” Pattern Language Definition ❚ Experts build up a set of techniques - we get hungry every lunch hour. - usually around 11:30am – 1:00pm. ❚ “ Best practices ” - good enough practices Wake-Up - must resolve hunger. Driving Forces ❚ Patterns - appetite, intensity of hunger, nearby restaurants Breakfast ❙ Recurring arrangements of elements Solution ❙ Documenting expertise Brunch - eat at nearest restaurant that satisfies minimum appetite requirement. Lunch Benefits ❙ An excuse to describe something that doesn ’ t fall - hunger is resolved Difficulties into standard categories (algorithm, data structure) Dinner - must not fall asleep afterwards. Related Patterns - dinner, breakfast, brunch, and snack cs361 6 5 1
2/10/15 Design Patterns: Elements of Lots of books on patterns Reusable Object-Oriented Software ❚ Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides ❚ Addison-Wesley 1995 ❚ 23 object-oriented patterns cs361 7 cs361 1-8 Composite design Fundamental OO design pattern principles ❚ Context: ❚ Open-Closed principle: ❙ Developing OO software ❙ “classes should be open for extension, but ❚ Problem: closed for modification” ❙ Complex part-whole hierarchy has lots of similar classes. ❚ Distribute the implementation ❘ Example: document, chapter, section, ❙ Don’t have one object that gets too paragraph. complicated and too smart. Each object ❘ Want to be able to seamlessly add new parts knows how to do it’s small part. cs361 1-9 cs361 10 Composite Forces ❚ Idea: make abstract "component" class. ❚ Alternative 1: every component has a ❙ power -- create new kind of part by (possibly empty) set of components. composing existing ones ❚ Problem: many components have no ❙ simplicity -- treat composition of parts like a part components. * ❙ safety -- no special cases, treat everything Component Children the same Chapter Paragraph ... cs361 11 cs361 12 2
2/10/15 Composite Two o des design ign alt alter ernat nativ ives es for or Pattern Par art-of -of Composite and : 1. Component does not know what it is a part of Component have the + Component can be in many composites. exact same interface. Component * - Component can be accessed only through its container composite. • enumerating children children: • Component has empty 2. Component knows what it is a part of iterator Leaf Composite - Component can be in only one composite. • only Composite adds + Component can be accessed directly. removes children. cs361 13 cs361 14 Part-of Ensuring Consistency ❚ Rules when component knows its single ❚ The public operations on components and composite. composites are: ❙ A is a part of B if and only if B is the composite of A. ❙ Composite can enumerate components. ❚ Duplicating information is dangerous! ❙ Component knows its container. ❙ Add/remove a component to/from the ❚ Problem: how to ensure that pointers composite. from components to composite and composite to components are consistent? cs361 15 cs361 16 addChild() in Composite public void addChild(Component child) { ❚ The operation to add a component to a ensureCapacity(); composite updates the container of the childArray[numChildren++] = child; component. There should be no other way child.container = this; to change the container of a component. } cs361 17 cs361 18 3
2/10/15 Book Example A book ❚ Book * DocumentComponent ❙ Chapter ❘ Section • Paragraph • Paragraph Paragraph Composite ❘ Section • Paragraph ❙ Chapter Book Section ❘ Section Chapter • Paragraph cs361 19 cs361 20 Composite in Eclipse UI (SWT) Composite in Eclipse Nodes ❚ Widget (abstract class) ❚ IJavaElement (interface) ❙ Control ❙ Leaf nodes IMember: ❘ Button, Label, ProgressBar, … IField, IMethod, … ❙ Composite ❙ Composite nodes: ❘ Browser, Canvas, Table, Tree, List, … ICompilationUnit, IPackage..... cs361 21 cs361 22 When to Centralize Visitor pattern Algorithm Use centralized algorithm when you need to: Visitor lets you centralize algorithm, lets • change entire algorithm at once you create a family of algorithms by • look at entire algorithm at once inheritance, and lets you define a new • work with only a few kinds of components operation without changing the classes • change algorithm often, but not add new classes of the elements on which it operates. of components - change the algorithm without changing the components (open-closed principle) Major problem is that adding a new kind of parse node requires adding a new function to each visitor. 427-21 23 427-21 24 4
2/10/15 Related Patterns ❚ Several patterns are often used with Interpreter/Visitor ❙ Composite - to make tree ❙ Iterator - to make traversal more abstract ❙ Template Method - to put reusable code in abstract class cs361 1-25 427-21 26 Who is responsible for the 1: The Components traversal algorithm ? If the component handles traversal, it looks like: The components? public Object accept(Visitor visitor) { The Visitor? visitor.visitA(this); for (Enumeration e = children(); A separate iterator in the client? e.hasMoreElements) { item = (Item) e.NextElement(); item.accept(visitor); } } Otherwise, it looks like: public Object accept(Visitor visitor) 427-21 27 427-21 28 {visitor.visitA(this);} 3: An Iterator from 2: The Visitor Client code If the visitor handles iteration, it looks like: If client calls iterator, it looks like: public Object visitA(ComponentA c) { visitor = new ConcreteVisitor. // do something with c for (Iterator i = component.iterator(); for (Iterator i = c.children(); !i.isDone(); i.Next()) { !i.isDone(); i.next()) { ((Component) i.CurrentItem()).accept(visitor); ((Component) } i.CurrentItem()).accept(visitor); } } } Otherwise Visitor visitA just interacts with Otherwise, the client looks like: componentA. ConcreteVisitor visitor; treeRoot.accept(visitor); 427-21 29 427-21 30 5
2/10/15 Visitor in Eclipse Visitor in Eclipse public public abstract class abstract class ASTVisitor { class Assignment { public public void void visit(Assignment node) { return true; public void void accept(ASTVisitor visitor){ } boolean boolean visitChildren = visitor.visit( visitChildren = visitor.visit(this this); ); if if (visitChildren) { (visitChildren) { several dozens more // visit children in normal left to right reading order acceptChild(visitor, getLeftHandSide()); } acceptChild(visitor, getRightHandSide()); } Many Eclipse tools (e.g., refactoring, code completion) walk } over the AST nodes 427-21 31 427-21 32 Visitor in Eclipse Template Method ❚ Problem: Some classes have a similar algorithm, but it ❚ Visitor works best when there are no new is a little different for each class. subclasses ❚ Solution: Define the skeleton of the algorithm as a method in a superclass, deferring some steps to ❚ Visitors work well for compilers. Why? subclasses. 427-21 33 427-22 34 In Swing’s JComponent Template Method public void paint(Graphics g) { ❚ A template method calls abstract methods. Graphics cg = getComponentGraphics(g); ❚ Subclasses provide concrete implementations Graphics co = of abstract methods. SwingGraphics.createSwingGraphics(cg); ❚ Template method in superclass calls methods paintComponent(co); in subclass paintBorder(co); ❚ Template Method separates the invariant part paintChildren(co); of an algorithm from the parts that vary with } each subclass 427-22 35 427-22 36 6
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