Outline. Design Patterns, cont. Wrappers. Adapter Pattern. Based on material by Michael Ernst, University of Washington

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1 Outline Design Patterns, cont. Based on material by Michael Ernst, University of Washington Design patterns Creational patterns, recap Subtypes: Factory method, Factory object, Prototype Sharing: Singleton and Interning Structural patterns Adapter, Composite, Decorator, Proxy Behavioral patterns Interpreter, Procedural and Visitor Spring 8 CSCI 600, K. Kuzmin, A Milanova Wrappers Adapter Pattern A wrapper uses composition/delegation A wrapper is a thin layer over an encapsulated object Modify the interface Extend behavior Restrict access to encapsulated object The encapsulated object (delegate) does most work Adapter: modifies interface, same functionality Decorator: same interface, extends functionality Proxy: same interface, same functionality Spring 8 CSCI 600, K. Kuzmin, A Milanova (based on slide by Michael Ernst) 3 Line Motivation: reuse a class with an interface different that the class interface Shape BoundingBox() CreateManipulator() BoundingBox() CreateManipulator() Text Editor BoundingBox() CreateManipulator() text TextView GetExtent() return text.getextent(); return new TextManipulator; 4

2 Adapter Example: Scaling Rectangles interface Rectangle void scale(int factor); //grow or shrink by factor void setwidth(); float getwidth(); float area(); class Client void clientmethod(rectangle r) r.scale(); class NonScalableRectangle void setwidth(); // no scale method! Spring 8 CSCI 600, K. Kuzmin, A Milanova (example due to Michael Ernst) 5 Class Adapter Class adapter adapts via subclassing class ScalableRectangle extends NonScalableRectangle implements Rectangle void scale(int factor) setwidth(factor*getwidth()); setheight(factor*getheight()); Spring 8 CSCI 600, K. Kuzmin, A Milanova 6 Object Adapter Object adapter adapts via delegation: it forwards work to delegate class ScalableRectangle implements Rectangle NonScalableRectangle r; // delegate ScalableRectangle(NonScalableRectangle r) this.r = r; void scale(int factor) setwidth(factor * getwidth()); setheight(factor * getheight()); float getwidth() return r.getwidth(); Spring 8 CSCI 600, K. Kuzmin, A Milanova 7 Adapter Pattern The purpose of the Adapter is: change an interface, without changing the functionality of the encapsulated class Allows reuse of functionality Protects client from modification Reasons Rename methods Convert units Implement a method in terms of another Spring 8 CSCI 600, K. Kuzmin, A Milanova 8

3 Exercise A Point-of-Sale system needs to support various services provided by third-party vendors: Tax calculator service from some vendor Credit authorization service from some vendor Inventory systems from some vendor Accounting systems from some vendor We anticipate to change vendors Each vendor service has its own API, which can t be changed What design pattern helps address this problem? Spring 8 CSCI 600, K. Kuzmin, A Milanova 9 POS System The Solution: Object Adapter TaxMasterAdapter gettaxes(sale) : List of TaxLineItems <<interface>> IAccountingServiceAdapter postreceivable(creditpayment) postsale(sale) <<interface>> ITaxCalculatorAdapter gettaxes(sale) : List of TaxLineItems GoodAsGoldTaxProAdapter gettaxes(sale) : List of TaxLineItems <<interface>> ICreditAuthorizationServiceAdapter requestapproval(creditpayment, TerminalID, MerchantID) Exercise Who creates the appropriate adapter object? Is it a good idea to let some domain object from Point-of-Sale system (e.g., Register, Sale) create the adapters? That would assigns responsibility beyond the object s logic. We would like to keep domain classes focused, so, this is not a good idea How to determine what type of adapter object to create? We expect adapters to change What design patterns address this problem? The Solution: Factory ServiceFactory - accountingadapter : IAccountingAdapter - inventoryadapter : IInventoryAdapter - taxcalculatoradapter : ITaxCalculatorAdapter + getaccountingadapter() : IAccountingAdapter + getinventoryadapter () : IInventoryAdapter + gettaxcalculatoradapter () : ITaxCalculatorAdapter Spring 8 CSCI 600, K. Kuzmin, A Milanova 3

4 Using the Factory public ITaxCalculatorAdapter gettaxcalculatoradapter() if (taxcalculatoradapter == null) String classname = System.getProperty( taxcalculator.classname ); taxcalculatoradapter = (ITaxCalculatorAdapter) Class.forName(className).newInstance(); return taxcalculatoradapter; What design pattern(s) do you see here? Java reflection: creates a brand new object from String classname! 3 Exercise Who creates the ServiceFactory? How is it accessed? We need a single instance of the ServiceFactory What pattern addresses these problems? Spring 8 CSCI 600, K. Kuzmin, A Milanova 4 The Solution: Singleton Special UML notation. Outline - instance: ServiceFactory ServiceFactory - accountingadapter : IAccountingAdapter - inventoryadapter : IInventoryAdapter - taxcalculatoradapter : ITaxCalculatorAdapter + getinstance() : ServiceFactory + getaccountingadapter() : IAccountingAdapter + getinventoryadapter() : IInventoryAdapter + gettaxcalculatoradapter() : ITaxCalculatorAdapter In UML, - means private, + means public. All (shown) fields in ServiceFactory are private and all methods are public. underline means static. instance and getinstance are static. Single instance of ServiceFactory ensures single instance of adapter objects. Design patterns Creational patterns, recap Subtypes: Factory method, Factory object, Prototype Sharing: Singleton and Interning Structural patterns Adapter, Composite, Decorator, Proxy Behavioral patterns Interpreter, Procedural and Visitor Spring 8 CSCI 600, K. Kuzmin, A Milanova 6 4

5 Composite Pattern Good for part-whole relationships Can represent arbitrarily complex objects Client treats a composite object (a collection of objects) the same as a simple object (an atomic unit) Spring 8 CSCI 600, K. Kuzmin, A Milanova 7 Bicycle Example: Bicycle Wheel Skewer Hub Lever Body Cam Rod Acorn nut Spokes Frame Spring 8 CSCI 600, K. Kuzmin, A Milanova (due to Michael Ernst) 8 Example: Methods on Components abstract class BicycleComponent float cost(); Skewer is an atomic unit class Skewer extends BicycleComponent float price; float cost() return price; class Wheel extends BicycleComponent float assemblycost; Skewer skewer; Hub hub; float cost() return assemblycost+ Wheel is a collection of objects skewer.cost()+hub.cost()+ Spring 8 CSCI 600, K. Kuzmin, A Milanova (example due to Michael Ernst) 9 Even Better abstract class BicycleComponent float cost(); class Skewer extends BicycleComponent float price; float cost() return price; class Wheel extends BicycleComponent float assemblycost; BicycleComponent skewer; BicycleComponent hub; float cost() return assemblycost+ The skewer and hub are BicycleCompoenents, so we can use BicycleComponent! skewer.cost()+hub.cost()+ Spring 8 CSCI 600, K. Kuzmin, A Milanova 0 5

6 Another Example: Boolean Expressions A boolean expression can be Variable (e.g., x) Boolean constant: true, false Or expression (e.g., x or true) And expression (e.g., (x or true) and y) Not expression (e.g., not x, not (x or y)) And, Or, Not: collections of expressions Variable and Constant: atomic units Spring 8 CSCI 600, K. Kuzmin, A Milanova Using Composite to Represent Boolean Expressions abstract class BooleanExp abstract boolean eval(context c); class Constant extends BooleanExp private boolean const; Constant(boolean const) this.const=const; boolean eval(context c) return const; class extends BooleanExp String varname; (String var) this.varname = var; boolean eval(context c) return c.lookup(varname); Spring 8 CSCI 600, K. Kuzmin, A Milanova Using Composite to Represent Boolean Expressions class extends BooleanExp private BooleanExp leftexp; private BooleanExp rightexp; (BooleanExp left, BooleanExp right) leftexp = left; rightexp = right; boolean eval(context c) return leftexp.eval(c) && rightexp.eval(c); // analogous definitions for OrExp and NotExp Spring 8 CSCI 600, K. Kuzmin, A Milanova 3 Object Structure versus Class Diagram Expression (x or true) and y new ( new OrExp( new ( x ), new Constant(true) ), new ( y ) ) leftexp OrExp: x or true : (x or true) and y leftexp rightexp rightexp : y : x Constant: true Spring 8 CSCI 600, K. Kuzmin, A Milanova 4 6

7 Client Object Structure vs. Class Diagram BooleanExp Exercise: Object Structure Draw the object structure (a tree!) for expression x and true and (y or z) Constant NotExp OrExp Spring 8 CSCI 600, K. Kuzmin, A Milanova 5 Spring 8 CSCI 600, K. Kuzmin, A Milanova 6 Structure of Composite Another Option: Add Operations to Manage Composites Client Component operation() Client Component operation() add(component) remove(component) getchild(int) Leaf Composite Leaf Composite operation() operation() children operation() operation() add(component) remove(component) getchild(int) children 7 7

8 Wrapper Patterns A wrapper uses composition/delegation A wrapper is a thin layer over an encapsulated class Modify the interface Extend behavior Restrict access to encapsulated object The encapsulated object (delegate) does most work Adapter: modifies interface, same functionality Decorator: same interface, extends functionality Proxy: same interface, same functionality Spring 8 CSCI 600, K. Kuzmin, A Milanova (based on slide by Michael Ernst) 9 Decorator Pattern Another wrapper pattern Adapter is a wrapper, Composite is not Add functionality without changing the interface Problem, i.e., when to use: Add to existing method to do something in addition (while preserving the original specification and interface) Spring 8 CSCI 600, K. Kuzmin, A Milanova 30 Structure of Decorator Example Motivation: add small chunks of functionality without changing the interface ConcreteComponent Operation() ConcreteDecoratorA Operation() addedstate Component Operation() Decorator Operation() ConcreteDecoratorB Operation() AddedBehavior() component component->operation() component.operation() Decorator::Operation(); super.operation() AddedBehavior(); Spring 8 CSCI 600, K. Kuzmin, A Milanova 3 abstract class Component void draw(); class TextView extends Component public void draw() // Draw the TextView abstract class Decorator extends Component private Component component; public Decorator(Component c) this.component = c; public void draw() component.draw(); 3 8

9 Example Adds a border to the text view Example class BorderDecorator extends Decorator public BorderDecorator(Component c, int borderwidth) super(c); Calls Decorator.draw which redirects work to the enclosed component private void drawborder() public void draw() super.draw(); drawborder(); Adds a scroll bar to the text view class ScrollDecorator extends Decorator 33 public class Client public static void main(string[] args) TextView textview = new TextView(); Component decoratedcomponent = new BorderDecorator( new ScrollDecorator(textView),); decoratedcomponent.draw(); Spring 8 CSCI 600, K. Kuzmin, A Milanova 34 Java I/O Package InputStream: byte input streams Readers: character input streams InputStream Reader FileInputStream PipedInputStream FilterInputStream in StringReader PipedReader BufferdReader FilterReader in BufferedInputSteam #byte[] buf CheckedInputStream DataInputStream FilterInputStream is a Decorator. Enables the chaining of streams Each FilterInputStream redirects input action to the 35 enclosed InputStream PushbackReader FilterReader extends Reader Reader in; int read() return in.read(); Spring 8 CSCI 600, K. Kuzmin, A Milanova 36 9

10 Another Decorator Example public class UppercaseConvertor extends FilterReader public UppercaseConvertor(Reader in) super(in); public int read() throws IOException int c = super.read(); return ( c==-? c : Character.toUpperCase((char)c)); We also have LowercaseConverter extends FilterReader, which (surprise!) converts to lowercase 37 Another Decorator Example public static void main(string[] args) Reader f = new UppercaseConverter( new LowercaseConvertor( new StringReader(args[0]))); int c; while ((c = f.read())!= -) System.out.print((char)c); System.out.println(); What is the object structure of f? What does this code do? 38 Proxy Pattern Proxy Example: Manage Creation of Expensive Object Same interface and functionality as the enclosed class Control access to enclosed object Communication: manage network details when using a remote object Locking: serialize access by multiple clients Security: permit access only if proper credentials Creation: object might not yet exist (creation is expensive). Hide latency when creating object. Avoid work if object never used Spring 8 CSCI 600, K. Kuzmin, A Milanova (slide due to Michael Ernst) 39 Editor Image draw() getextent() store() load() ImageImp extent image Graphic draw() getextent() store() load() ImageProxy draw() getextent() store() load() filename extent if (image == null) // load image image.draw(); if (image == null) return extent; else return image.getextent(); 40 0

11 Proxy Example: Manage Details When Dealing with Remote Object Recovery from remote service failure in the Point-Of-Sale system When postsale is sent to an accounting service (remember, an AccountingAdapter), if connection cannot be established, failover to a local service Failover should be transparent to Register I.e., it should not know whether postsale was sent to the accounting service or to some special object that will redirect to a local service in case of failure Spring 8 CSCI 600, K. Kuzmin, A Milanova 4 Proxy Example: Manage Details When Dealing with Remote Object Register accounting:iaccountingadapter +makepayment() - compeletesalehandling() AccountingRedirectionProxy <<interface>> IAccountingAdapter + postsale( Sale ) - externalaccounting : IAccountingAdapter - localaccounting : IAccountingAdapter + postsale( Sale ) ExternalAccounting Adapter + postsale( Sale ) LocalAccounting Adapter + postsale( Sale ) Spring 8 CSCI 600, K. Kuzmin, A Milanova 4 Outline Design patterns Creational patterns, recap Subtypes: Factory method, Factory object, Prototype Sharing: Singleton and Interning Structural patterns Adapter, Composite, Decorator, Proxy Behavioral patterns for traversing composites Interpreter, Procedural and Visitor Spring 8 CSCI 600, K. Kuzmin, A Milanova 43 Traversing Composites Question: How to perform operations on composite objects (on all parts of the component)? E.g., evaluate a boolean expression, print a boolean expression The Interpeter, Procedural and Visitor patterns address this question Spring 8 CSCI 600, K. Kuzmin, A Milanova 44

12 Composite Objects Expression (x or true) and y new ( new OrExp( new ( x ), new Constant(true) ), new ( y ) ) leftexp We have a hierarchical structure: is top, OrExp and are below in the hierarchy, etc. OrExp: x or true : (x or true) and y leftexp rightexp rightexp : y Client Constant Composite Pattern: Class diagram BooleanExp NotExp OrExp : x Constant: true Spring 8 CSCI 600, K. Kuzmin, A Milanova 45 Atomic Units Composite Spring 8 CSCI 600, K. Kuzmin, A Milanova Objects 46 Perform Operations on boolean expressions Interpreter and Procedural Patterns Need to write code for each Operation/Object pair Objects Operations evaluate Constant OrExp NotExp Question: do we group together (in a class) the code for a particular operation or the code for a particular object Interpreter: groups code per object, spreads apart code for similar operations evaluate Procedural: groups code per operation, spreads apart code for similar objects evaluate prettyprint prettyprint prettyprint Spring 8 CSCI 600, K. Kuzmin, A Milanova 47 Spring 8 CSCI 600, K. Kuzmin, A Milanova (based on slide by Michael Ernst) 48

13 Interpeter Pattern evaluate prettyprint abstract class BooleanExp abstract boolean eval(context c); abstract String ; class extends BooleanExp boolean eval(context c); Add a method to each class String ; for each supported operation class extends BooleanExp boolean eval(context c); Dynamic dispatch chooses String ; right implementation at call myexpr.eval(c); Spring 8 CSCI 600, K. Kuzmin, A Milanova 49 Client Constant Interpreter Pattern BooleanExp boolean String NotExp OrExp Spring 8 CSCI 600, K. Kuzmin, A Milanova 50 Interpreter Pattern abstract class BooleanExp boolean eval(context c); class Constant extends BooleanExp private boolean const; Constant(boolean const) this.const=const; boolean eval(context c) class extends BooleanExp String varname; (String var) varname = var; boolean eval(context c) Spring 8 CSCI 600, K. Kuzmin, A Milanova 5 Interpreter Pattern class extends BooleanExp private BooleanExp leftexp; private BooleanExp rightexp; (BooleanExp left, BooleanExp right) leftexp = left; rightexp = right; boolean eval(context c) // analogous definitions for OrExp and NotExp Spring 8 CSCI 600, K. Kuzmin, A Milanova 5 3

14 Procedural pattern evaluate prettyprint // Classes for expressions don t have eval class Evaluate boolean evalconstexp(constant c) c.value(); // returns value of constant boolean eval( e) BooleanExp leftexp = e.leftexp; BooleanExp rightexp = e.rightexp; //Problem: How to invoke the right //implementation for leftexp and rightexp? Spring 8 CSCI 600, K. Kuzmin, A Milanova 53 Procedural pattern evaluate prettyprint // Classes for expressions don t have eval class Evaluate Context c; boolean evalexp(booleanexp e) if (e instanceof ) return eval(() e); else if (e instanceof Constant) return evalconstexp(() e); else if (e instanceof OrExp) return evalorexp((orexp)e); else What is the problem with this code? Spring 8 CSCI 600, K. Kuzmin, A Milanova 54 Visitor Pattern, a variant of the Procedural pattern Visitor helps traverse a hierarchical structure Nodes (objects in the hierarchy) accept visitors Visitors visit nodes (objects) class SomeBooleanExp extends BooleanExp void accept(visitor v) for each child of this node child.accept(v); n.accept(v) traverses the structure rooted at n, performing v s operation on every element class Visitor void visit(somebooleanexp e) do work on e Spring 8 CSCI 600, K. Kuzmin, A Milanova (based on slide by Michael Ernst) 55 Visitor Pattern class extends BooleanExp void accept(visitor v) class extends BooleanExp BooleanExp leftexp; BooleanExp rightexp; void accept(visitor v) leftexp.accept(v); rightexp.accept(v); class Evaluate implements Visitor void visit( e) //evaluate var exp void visit( e) //evaluate And exp class PrettyPrint implements Visitor Spring 8 CSCI 600, K. Kuzmin, A Milanova 56 4

15 The Interpreter Pattern BooleanExp Client String The Visitor Pattern BooleanExp Client accept(visitor v) Constant NotExp OrExp Constant accept (Visitor v) accept (Visitor v) NotExp accept (Visitor v) OrExp accept (Visitor v) accept (Visitor v) Spring 8 CSCI 600, K. Kuzmin, A Milanova The Visitor Pattern Visitor EvaluateVisitor visit(constant e) visit( e) visit(notexp e) visit( e) visit(orexp e) visit(constant e) visit( e) visit(notexp e) visit( e) visit(orexp e) PrettyPrintVisitor visit(constant e) visit( e) visit(notexp e) visit( e) visit(orexp e) Spring 8 CSCI 600, K. Kuzmin, A Milanova 59 Question class extends BooleanExp void accept(visitor v) class Constant extends BooleanExp void accept(visitor v) Why not move void accept(visitor v) up into superclass BooleanExp? class Evaluate implements Visitor void visit( e) void visit(constant e) void visit( e) 60 5

16 Exercise: Write Count Visitor which counts #subexpressions in a BooleanExp object class extends BooleanExp void accept(visitor v) class extends BooleanExp BooleanExp leftexp; BooleanExp rightexp; void accept(visitor v) leftexp.accept(v); rightexp.accept(v); class CounterVisitor implements Visitor int count = 0; void visit( e) void visit(constant e) void visit( e) 6 Starting the Visitor BooleanExp myexp = new ( new OrExp(new ( x ),new ( y )), new ( z ) ); Visitor v = new CounterVisitor(); myexp.accept(v); Spring 7 CSCI 600, A Milanova 6 Exercise: Write Evaluate Visitor which evaluates a BooleanExp object class extends BooleanExp void accept(visitor v) class extends BooleanExp BooleanExp leftexp; BooleanExp rightexp; void accept(visitor v) leftexp.accept(v); rightexp.accept(v); class EvaluateVisitor implements Visitor //?? void visit( e) //?? void visit(constant e) //?? void visit( e) //?? 63 Exercise: Write a Visitor that Computes the Cost of a Bicycle Component (Note: Cost of a composite is sum of costs of components + assembly cost) class Skewer extends BycicleComponent void accept(visitor v) class Wheel extends BycicleComponent BycicleComponent skewer; BycicleComponent hub; void accept(visitor v) skewer.accept(v); hub.accept(v); class CostVisitor implements Visitor void visit(skewer e) void visit(hub e) void visit(wheel e) 64 6

17 Visitor Pattern Must add definitions of visit (in Visitor hierarchy) and accept (in Object hierarchy) visit may do many different things: evaluate, count nodes, pretty print, etc. It is easy to add operations (a new Visitor class), hard to add nodes (must modify entire hierarchy of Visitors) Visitors are similar to iterators but different because they have knowledge of structure not just sequence Spring 8 CSCI 600, K. Kuzmin, A Milanova 65 Visitor Pattern s Double Dispatch evaluate prettyprint myexp.accept(v): we want to choose the right operation myexp.accept(v) // dynamically dispatch the right // implementation of accept, e.g.,.accept class void accept(visitor v) // at compile-time, chooses the // method family: visit(). At // runtime, dispatches the right implementation of // visit(), e.g., // EvaluateVisitor.visit() 66 Design Patterns Summary so Far Factory method, Factory class, Prototype Creational patterns: address problem that constructors can t return subtypes Singleton, Interning Creational patterns: address problem that constructors always return a new instance of class Wrappers: Adapter, Decorator, Proxy Structural patterns: when we want to change interface or functionality of an existing class, or restrict access to an object Spring 8 CSCI 600, K. Kuzmin, A Milanova 67 Design Patterns Summary so Far Composite A structural pattern: expresses whole-part structures, gives uniform interface to client Interpreter, Procedural, Visitor Behavioral patterns: address the problem of how to traverse composite structures Spring 8 CSCI 600, K. Kuzmin, A Milanova 68 7