Tentamen TDDB84 (Design Mönster) Examination TDDB84 (Design Patterns)

Transcription

1 eport o H07B84 rganization inköping University, weden ast odified 5 ovember 2007 odified by eter Bunus (petbu@ida.liu.se) entamen B84 (esign önster) xamination B84 (esign atterns) inköping University, epartment of omputer and nformation cience inköping weden hone: +46 (0) Fax:

2 able of ontents nformation...1 Betygsgränser...1 nformation...2 Grading...2 ask 1: rosswords uzzle (15 points)...3 ask 1: hort nswer hort otivation (15 points)...6 ask 3 oyota rius (20 points):...10 ask 4 ibrary tems (15 points):...11 ask 5: etris (20 points)...12 ask 6: ibrary ilemma (15 points)...13 inköping University, epartment of omputer and nformation cience inköping weden

3 1 entamen: B84 esign önster ( ) xaminator: eter Bunus nformation kriv namn, klass, och personnummer på samtliga lösa blad. oängavdrag kommer att göras om punkterna nedan inte åtföljs! 1) ndast ett svar per blad. nvänd endast framsidan (delfrågor kan vara på samma sida). 2) ortera inlämnade svar med avseende på uppgiftsnummer i stigande ordning. 3) varen får vara på svenska eller engelska. 4) ina svar skall tydligt visa lösningsmetod. nbart rätt svar kommer inte att ge poäng. det fall du är osäker på frågeställning, skriv ner din tolkning och lös uppgiften utifrån din tolkning. 5) om hjälpmedel är boken av GF eller liknande tillåtna och H-bilderna från föreläsningar. nteckningar i böcker är okej. Betygsgränser [0..50) poäng Betyg 2 [50..73) poäng Betyg 3 [73..85) poäng Betyg 4 [ ] poäng Betyg 5 ycka till! inköping University, epartment of omputer and nformation cience inköping weden hone: +46 (0) Fax:

4 2 entamen B xamination: B84 esign attern ( ) xaminer: eter Bunus nformation Write your name, class, and person number on all solution sheets. lease also observe the following; otherwise it might lead to subtraction of points: 1) Write only the answer to one task on one sheet. Use only the front side of the sheets (answers to subtasks can be on one page). 2) ort the solution sheets according to the task number. 3) nswers may be in nglish or wedish. 4) Your answers should clearly show solution methods, reasons, and arguments. hort answers should be motivated. f you have to a make an assumption about a question, write down the assumptions you make. 5) he GF book or other design pattern related book is allowed during the exam. he printed lecture slides are also allowed during the examination. Grading o pass the exam you have to do at least 50 points from 100 possible. [0..50) points F [50..59) points [59..70) points [70..78) points [78..85) points B [ ] points Good uck! Bonne chance! Viel Glück! ėkmės! 祝你好運祝福 inköping University, epartment of omputer and nformation cience inköping weden

5 3 ask 1: rosswords uzzle (15 points) s a warming up exercise solve this crosswords puzzle. Hand in this page with your final exam clipserossword.com inköping University, epartment of omputer and nformation cience inköping weden

6 4 entamen B cross 1. f want to implement an algorithm in a method deferring the definition of some steps of the algorithm so that subclasses can redefine them, this design pattern would be the most appropriate 5. ttaches additional responsibilities to an object dynamically 9. n adapter and a decorator can be said to an object 12. rue or false, dapters can only wrap one object 13. his pattern will let an object act on behalf of another object 14. f you intend to defer the decision of which class to instantiate you will use this pattern 15. attern that defines an interface that applies to individual objects and group of objects 18. bstract Factory creates a of objects 19. hristopher lexander's job 23. esign attern that distributes an operation so that each implementation applies to a different type of composition. 24. rich Gamma, ichard Helm, alph Johnson and John Vlissides 27. rogram to an interface, not an 28. entralize responsibility in a class that oversees how a set of other objects interact 29. llow the addition of new operation to a class or a class hierarchy without changing the class 30. class should have only one reason to do this own 2. ll factory patterns allow us to object creation 3. his pattern allows a request to escalade up a chain of objects until one handles it. 4. esign attern that encapsulates an operation, making implementation interchangeable 6. f you are going to study overseas you might need one of these to plug in your computer. 7. Favor over inheritance 8. on't call us, we'll call you is known as the rinciple 10. attern that constructs a family that share a trait 11. he template method is usually defined in a method as an method 16. attern that distributes an operation so that each class represent a different state. 17. You apply this pattern if you would like to centralize the responsibility in a single instance of a class 20. esign attern that encapsulates a method call in an object 21. rovides a way to access a collection of instances of a class that you create 22. attern that decouples an abstraction from its implementation so that the two can vary independently 25. attern that reconstructs an object from a dormant version that contains just the object's internal state. 26. attern that provides a simple interface into a collection of classes inköping University, epartment of omputer and nformation cience inköping weden

7 5 inköping University, epartment of omputer and nformation cience inköping weden clipserossword.com H F B Y H Y W H G Y F G V B F Y F W B Y B G U H F H G H F X Y Y G he solution to the crossword puzzle is given above. he crossword contained a little misspelling on own 1 field. t should be hain of esponsibility instead of hain of esponsibility as incorrectly was required in the crossword in order to match the cross 19 field. m sorry about this misspelling and have also noticed that the same misspelling slipped away in several slides as well.

8 6 entamen B ask 1: hort nswer hort otivation (15 points) lease answer the following questions. on t forget to motivate your answer. 1. GoF divided their patterns in the following categories: reational, tructural and Behavioral. lemental, tructural and Behavioral. lemental, eferencial and Behavioral. lemental, tructural and gnostic. one of the above. nswer: he correct answer here is the first option: reational, tructural and Behavioural. s you probably know creational patterns define mechanism for creating objects, structural patterns take care of the composition of objects to obtain new and varied functionality while the behaviour patterns covers the interactions between different objects. 2. s this a valid implementation of a singleton? xplain your answer. 1: package mytuff; 2: public class ingleton { 3: private static ingleton s_singleton = new ingleton(); 4: private ingleton() {} 5: private static ingleton getnstance() { 6: return s_singleton; 7: } 8: } nswer: his class is impossible to access because all the methods, including the constructor are private. he getnstance() method need to be public and static otherwise you have no chance to access the class. 3. want to add an "undo" feature to my project. Which pattern should use? ingleton Factory method * emento tate Visitor eteris aribus evert roxy nswer: he emento pattern captures and externalizes an object s internal state so that the object can be restored later to this saved state. his will make the emento pattern an ideal candidate for undo operation. inköping University, epartment of omputer and nformation cience inköping weden

9 7 4. uppose that you need to use a complex system, but you are only going to use a little part of it. ame the pattern you would use if you want to have a simplified interface to the system. nswer: Facade pattern however defines a general solution to encapsulate complex functionality. facade is an object that provides a simplified interface to a larger body of code, such as a class library. 5. f you would like to give another name to the nterpreter pattern what name would you like to choose from the possibilities enumerated below? lease motivate briefly your answer ranslator low executor * ittle language oo much looking any many interpretations one of the above nswer: nterpreter is mostly used for interpreting languages that have a very simple grammar, therefore the most appropriate name from the list would be little language 6. When you use the final modifier in a method, what design pattern are you explicitly forbidding in that method? xplain briefly your answer: terator bserver * emplate ethod one of the above nswer: nce you have used the final modifier you are explicitly forbidding the redefinition of the method. method that is declared final in a superclass cannot be overridden in a subclass. he emplate ethod design pattern defines the skeleton of an algorithm at the superclass level and lets the sublasses to redefine certain steps of an algorithm. he sublasses need to redefine the method but if the method in the superclass it is declared final it will be imposible to use this design pattern 7. uppose that you are experiencing high coupling and dependency between an abstraction of objects and their possible implementations. Which pattern allows you to decouple things? nswer: oupling is the factor of dependency of one class on another class. he factor of coupling varies between low and high. ystems having high coupling are unstable and requires more effort to modify. n a system with high coupling factor modification in one class forces a ripple of changes in other classes and thus increases the effort required for modification. he Bridge attern will decouple an abstraction from its implementation so that the two can vary independently. his pattern is very useful when you need to vary an interface and an implementation in different ways. inköping University, epartment of omputer and nformation cience inköping weden

10 8 entamen B Which of these statements are true? dapter extends target interface (interface that client expects). dapter holds instance of daptee. dapter implements interface of daptee. nswer: he adapter can be implemented in two ways: 1. he dapter class uses multiple inheritance to adapt one interface to another 2. he dapter relies on object composition n both cases dapter adapts the interface of the daptee to the arget interface. echnically the dapter is a subclass of the arget interface so one can say that the dapter extends the arget interface even if would prefer to use the word adapt in this case. his will make the first statement true When the dapter relies on object composition the statement thet the dpater holds an instance of the daptee is also true. Based on the assumptions that you made we can accept a false answer here if you mentioned that the dpater relies on multiple inheritance instead. When the adapter class uses multiple inheritance to adapt one interface to another it can override some of the daptee s behavior since daptor is a subclass of daptee. ne can say that the dapter implements the daptee s interface. hat will make the third statement true. However this is not possible for Java where multiple inheritance is not permitted. We accept here the false answer if you have correctly motivate it. 9. "lass explosion" is having too many classes, each with slightly different behavior. ach time you want to add some new method or some new instance variable, you need to update all your classes, doubling the number of classes each time. ne way to avoid the "lass xplosion" is to use one of these patterns: dapter * ecorator roxy ingleton nswer: he ecorator design pattern provides a more flexible way to add responsibilities to a class than by using inheritance. nheritance usually leads to class xplosion. he decorator allows customizing a class without creating subclasses high in the inheritance hierarchy so it will avoid the class explosion. 10. 'm building an application that connects to several servers, but the servers are sometimes unavailable, so decide to create stubs for the services and to able to switch at runtime between real services and stub services. Which pattern am using? ingleton Factory method inköping University, epartment of omputer and nformation cience inköping weden

11 9 ommand * trategy emento one of the above nswer: trategy defines a family of algorithms, encapsulate each one, and makes them interchangeable.o one can design a strategy that will interchange during run-time the real services and the stub services. 11. oes the following code fragment implement the Factory ethod design pattern? public class XeaderFactory { // his method returns an instance of a class // that implements the Xeader interface. // he specific class it creates and returns is // based on a system property. public static Xeader createxeader(); } public interface Xeader { public void setontenthandler(ontenthandler handler): public void parse(nputtream is); } nswer: ot really. he XeaderFactory it creates a class Xeader that is independent of the client but it does not define an abstract factory method so it does not follow literally the Factory ethod esign attern. 12, What pattern(s) would you use in combination with the Visitor attern? Why? nswer: omposite. he Visitor pattern allows you to add operations to a omposite structure without changing the structure itself 13. f a roxy is used to instantiate an object only when it is absolutely needed, does the roxy simplify code? nswer: roxy doesn't necessarily simplify code. he roxy is used to create an object when needed, allowing the application to run up until this point unimpeded by the burden of loading the proxy object, which likely encapsulates large amounts of data or an action that will take a long period of time to execute. 14. Would you ever create an dapter that has the same interface as the object which it adapts? Would your dapter then be a roxy? nswer: Yes why not. f create an daper that has the same interface as the object which it adapts will have the freedom of changing the adapted object any time. o if can anticipate that the adapted object will change in the future it is perfectly acceptable to do that. n this way can guard the clients against unexpected changes in the adapted object interface. ctually this trick is very often used by experienced programmers. inköping University, epartment of omputer and nformation cience inköping weden

12 10 entamen B How complex must a sub-system be in order to justify using a facade? nswer: t is not necessary to be very complex to justify the use of the Façade. he Facade can be used to provide a simplified interface to the underlying sub-system or perhaps a small subset of the sub-system. ask 3 oyota rius (20 points): oyota rius is the first hybrid-powered production car in the world. t is claimed to cut pollutants by 90%, fuel consumption by 50% and 2 emission by 50%. riving such a hybrid car will definitely make l Gore happy. fter finishing this course you might follow a promising career as an automotive engineer at oyota. You need take up the challenge to redesign a little bit the control panel of the car. Your new board panel system should provide the following functionality: - t should contain a traditional speed meter that shows the current speed of the car in km/h. - t should contain a digital speed meter that shows the speed in miles/hour. - he speed of the car should be obviously regulated by the acceleration pedal and the breaking pedal. he speed of the car can be also modified by pushing the cruise control buttons(the + button for increasing the speed and the - button for decreasing the speed) located on the steering wheel. ) elect the most appropriate design pattern to use implement the car control panel and show how it is applied. n particular, show an appropriate class diagram(s) and enough code fragments to illustrate your use of the pattern to solve the problem. he ediator attern is the most appropriate to use here inköping University, epartment of omputer and nformation cience inköping weden

13 11 ontrolanelirector 1 -panelirector aromponents oyotaontrolanelirector -digitalpeed : igitalpeedeter -speedeter : raditionalpeedeter -cruiselus : ncreasepeedbutton -cruiseinus : ecreasepeedbutton -break : Breakedal -gas : ccelerationedal raditionalpeedeter +howpeed() igitalpeedeter +howpeed() ruiseontrolbutton +hanged() 1 ncreasepeedbutton ecreasepeedbutton +hanged() +hanged() edal +hanged() Breakedal +hanged() ccelerationedal +hanged() he oyota rius hybrid system consists of a 1.5-litre lean burn engine and an.. induction electric motor and the car operates in the following modes: - G : owered by electric motor only. rius does not employ the engine during starting, because starting is a heavy load action which greatly increases the emission pollutant. - : When the car gets up to speed, the engine joins and provides power together with the electric motor. he engine provides the necessary power that pure electric motor cannot provide. n the other hand, electric motor help easing the load taken by the engine, so emission level remains low. - BKG / WH : his is the most important advantage of hybrid car. onventional car will eliminate the kinetic energy by braking that means transferring to heat loss. rius will make good use of the kinetic energy to recharge its batteries through electric motor (now act as generator) B) elect the most appropriate design pattern that will simulate the rius operating modes from above and show how it is applied. n particular, show an appropriate class diagram(s) by extending the class diagram that you previously created at section ) and enough code fragments to illustrate your use of the pattern to solve the problem. epending on the assumptions that you made here several design patterns could have been applied. he most obvious ones would be the trategy attern and the tate attern ask 4 ibrary tems (15 points): aking as starting point the class model depicted in the figure below select the most appropriate design pattern that will help you to compute the following for an instance of ibrary: ) total number of pages for all the items in it inköping University, epartment of omputer and nformation cience inköping weden

14 12 entamen B B) how many different authors there are? how an appropriate class diagram(s) and enough code fragments to illustrate your use of the pattern to solve the problem. nswer: he Visitor esign attern is the most appropriate pattern to be used here. ask 5: etris (20 points) magine, you have to design a etris game. his game uses a 20x10 scoreboard, in upright form. From its ceiling, blocks fall down. he blocks have different forms (rectangular, square, - shaped polygons). he player can rotate the blocks while they fall down. nce a block touches the ground, it is fixed where it landed. nly one block falls at any time. nce the current block has been fixed on the ground, a new block appears below the ceiling. ) esign the class model of the game. onsider a main class for the game, a game scoreboard, and a class hierarchy for blocks. (5 points) B) esign an bstractfactory with some concrete factories for the block hierarchy. etermine which concrete block type is created by using a random generator. (15 points) nswer: bstract Factory or Factory ethod design pattern could have been used in this case inköping University, epartment of omputer and nformation cience inköping weden

15 13 ask 6: ibrary ilemma (15 points) t the company for which work we are facing the following problem: We have a large amount of application software written using a particular library. et s call this library ibx. he provider of this library was Xndustries B 1. We wanted to extend the functionality of our application and for this reason we tried to contact Xndustries B to implement the necessary library functionality. o our surprise we found out that Xndustries B has gone out of business. Fortunately we found out that there is another company called Ynterprise B is producing iby that provides the same functionality as ibx but many of the classes have different interfaces. We don t have access to the source code of the old library ibx and neither to the source code of iby. What should we do? For 15 valuable points can you tell us how to proceed further by selecting the most appropriate design pattern that we should use and show how we should apply it. n particular, show an appropriate class diagram(s) and enough code fragments to illustrate your use of the pattern to solve the problem. simple usage example would be also valuable for our engineers. nswer: he dapter pattern should be used in this situation 1 he names used herein are fictional and any resemblance to real company or product names is purely coincidental inköping University, epartment of omputer and nformation cience inköping weden