CSC/ECE 517: Object-Oriented Languages and Systems Summer 2008 Test 2 with Answers

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1 CSC/ECE 517: Object-Oriented Languages and Systems Summer 2008 Test 2 with Answers This was a 100-minute open-book test. There were a total of 105 points on this test.. If you answered more than 100% worth of questions correctly, your score was 100%. Question 1. Using the following scenario, create CRC index cards using the CRC method. Scenario: Jack s Sue's boss his résumé. Note: In this scenario Jack obtains the boss s address by asking Sue for it (a) (4 points) The CRC method states that to determine the classes, you should search for all nouns in the scenario, and convert them to class names. Each class name is placed on a separate index card. What would be reasonable names for the classes in this scenario? Answer: Applicant, Employee, Boss, Résumé Applicant Employee Boss Résumé (b) (1 point) The CRC method states that the verb(s) in the scenario will represent the responsibilities. Add the responsibilities to the appropriate CRC cards. Answer: Applicant s a message (c) (4 points) The CRC method states that the collaborators are the other cards that must be accessed to carry out the responsibility. What two things does Jack need in order to carry out the responsibility to ? Answer: Jack needs access to his resume (Résumé card needed) He also needs the boss s address. (d) (3 points) Add responsibilities and collaborators on the appropriate cards to show how Jack can accomplish his objective in part (c). Answer: As was mentioned in the problem, the only way Jack can get the address is to ask Sue. (Employee card needed) This action gives Applicant another responsibility (Ask for boss s address), and a responsibility for Employee (Furnish address) Applicant s a message Ask for Boss s address Résumé, Boss Employee Employee Furnish address 1

2 Question 2. (3 points each) Consider the code segment below. Another portion of the code reads stock prices from a database into an array that holds prices for each stock for each day in the past month. Identify at least three examples of inelegance, according to how elegance is described in Skrien Chapter 4: Elegance and Methods. /****************************************************** * The class is responsible for holding information on a position * * in a given stock. * *******************************************************/ public class Stock { private String s; private float currpriceindollarsofaparticularstock; private float prevpriceindollarsofaparticularstock; private int numshares; /************************************************ * Constructor * * s -- the Stock symbol * * currpriceindollarsofaparticularstock * * -- the current price per share * * prevpriceindollarsofaparticularstock * * -- the previous day s price per share * * numshares is the number of shares currently owned * ************************************************/ public Stock(String s, float pps, int sh) { this.s = s; currpriceindollarsofaparticularstock = pps; prevpriceindollarsofaparticularstock = 0; numshares = sh; /************************************************ * This method returns the symbol of this stock * ************************************************/ public String getsymbol() { return s;... /************************************************ * This method returns the change in price per share. * ************************************************/ public int getpricechange() { return currpriceindollarsofaparticularstock prevpriceindollarsofaparticularstock; Answer 2. The following 3 answers are formatted as follows: Original code Changed code Explanation 1. private String s; private String symbol; the variable s is not descriptive enough (answer given is a good option). 2. private float currpriceindollarsofaparticularstock; private float pricepershare; the variable name is to long and should be shortened to something like the suggested answer. 3. private float prevpriceindollarsofaparticularstock. It is not clear that this variable is needed, if it is used only here. It could be replaced by an access to the appropriate array value. 3a. getpricechange() ==> this method should not be implemented in this class. It does not make sense to keep a separate variable for the previous day s price per share when the past 30 days of prices is needed for another portion of the system. The price change can easily be calculated using the structure used for storing the 30-day history of prices. 2

3 Question 3. (5 points each) Suppose you need to create a class that will keep track of service records for an automobile. Service records may include oil changes, tire replacements, tire maintenance (rotation, balance, flat repairs). (a) Modify the following code to adhere to the open-closed principle and explain. public class MaintenanceRecords{ public Object [] records; public int money; // money spent public addoilchange(oilc o); public addtirerepl(tirer r); public addtiremaint(timem m); Answer: To keep the class from being modified (closed) data should be kept private. private Object [] records; private int money; (b) From the standpoint of accessibility, this class is inelegant in other ways. What happens in case you decide to also keep track of repairs too? Explain. Answer: The add methods do essentially the same thing different kinds of maintenance and repairs. But a separate method is needed for each. This becomes cumbersome when there are a lot of potential items. Better we should create another abstract class Service that can hold data for any kind of service record. Then the 3 add methods can be replaced by public addservice(service c); Question 4. (6 points) Suppose you have a class with the following methods public class DatabaseParser { ParseDirectoryMapFile(); // parse directory map file ParsePasswordFile(); // parse user file ParseReviewDataFile(); // parse review data file ParseTeamFile(); // parse team file CalculateGrade(); CalulateRelativeGrade(); (a) Is this a well-formed class? Why or why not? Answer: No. A well-formed class should be responsible for doing one thing and do it well. A DatabaseParser class should do parser of only database file and nothing else, but it not only parsing the file and also calculate dividing and relative grade. Furthermore, it is not a good idea to have all of the Parse methods in a single class. They evidently relate to parsing different kinds of files, and thus, the implementation would better be placed in the classes for these particular files, so that the DatabaseParser class could simply execute mydirectorymapfile.parse(); mypasswordfile.parse();, etc. 3

4 (b) If this is an ill-formed class, tell how to make it into well-formed class. We should modify the DatabaseParser class by removing the calculategrade and calculaterelativegrade functions, and put them into a different class. public class DatabaseParser { ParseDirectoryMapFile(); // parse directory map file ParsePasswordFile(); // parse user file ParseReviewDataFile(); // parse review data file ParseTeamFile(); // parse team file public class Caculator { CalculateGrade(); CalulateRelativeGrade(); Question 5. (2 points per blank) There are many issues to consider in deciding whether to use an interface or an abstract class. The table below lists some of those issues. Fill in the blanks with a description of how an interface or an abstract class would affect the design. For example, in the first row, the blanks have been filled in by explaining how, if at all, an interface helps achieve code reuse, and how an abstract class helps achieve code reuse. Design consideration Interface Abstract class Facilitating code reuse Providing a common implementation for functionality in various classes Adding extra methods to each of the classes Since an interface by definition does not include any implementation, it does not facilitate code reuse. An interface provides/enforces only the signature of the methods within it, so the actual implementations of the methods may be completely different. This can be good, if the various implementers really are that different, or it can be bad if the divergence is unnecessary. Any class implementing an interface must implement all of the methods in the interface, or it will not compile. Therefore you cannot add any methods to the interface without breaking all of the classes which have already implemented it. To the extent that common functions for the class hierarchy are implemented in the superclass, code reuse is achieved. Classes implemented within the abstract class, if not overridden by its subclasses, will have the implementation intended by the designer of the abstract class. Of course, if the designer is not able to anticipate the likely uses of the class, this can become a limiting factor, requiring subclasses to override the "common" implementation. An abstract class can contain implementations for any/all of its methods. Therefore, if the system evolves so that it makes sense to add functionality to the class hierarchy, methods can be added to the abstract class without breaking classes that extend it. If it does not make sense to actually implement the new method, a "stub" can be provided in the superclass. Design consideration Interface Abstract class 4

5 Inheriting from other classes Hiding information from other classes (encapsulation) Implementing an interface does not consume the single inheritance opportunity that a Java class has. Thus an implementing class can aggregate the functionality of many interfaces without running into any inheritance limitations. Because the interface cannot contain any implementation and can only contain public static final variables (i.e. public constants), implementing it does not give the implementing class access to any of the internal data of the interface. Using an abstract class at the top of the hierarchy forces subclasses to use their sole inheritance opportunity to extend the abstract class. An abstract class can contain nonpublic members (methods and data). If the abstract class contains any such members, they are potentially exposed to all of the subclasses. This can be controlled by proper use of scope definitions. Question 6. Suppose a Java program includes these classes: public class A { public void m(a a) {... public class B extends A{... (a) (1 point) Is it legal, in class B, to define a method Yes or no? Answer: Yes. public void m(b b) {...? (b) (2 points) Is this a case of overloading or overriding? Answer: Overloading. (c) (4 points) Given this definition of m(b)in class B, is it possible to send a message to an object of class B (e.g., B myb = new B()), and have class A s method m(a) called? If so, give a statement that causes A s method m to be invoked on myb. Answer: Yes. myb.m(new A()) will do this. (d) (1 point) Assuming that method m(b) has been defined as in part (a), it also legal, in class B, to define a method Yes or no? Answer: Yes. public void m(a a) {...? 5

6 (e) (2 points) Is this a case of overloading or overriding? Answer: Overriding. (f) (4 points) Given these definitions of m(b)and m(b)in class B, is it possible to send a message to an object of class B (e.g., B myb = new B()), and have class A s method m(a) called? If so, give a statement that causes A s method m to be invoked on myb. Answer: No, it is no longer possible, because B defines its own methods for both parameter type A and parameter type B. Question 7. One of the Extreme Programming practices is collective code ownership. (a) (2 points each) List two things that are wrong with individual code ownership, and explain why they are bad. Answer: You have to negotiate with someone if you want to change the interface. There is pressure to get the interface done early so the rest of the project can proceed. If the programmer responsible for the code leaves the project, it is difficult to find someone competent to take over. The owner of a class may veto a change, only to have others design around it, which worsens the design in general. (b) (4 points) Explain how at least two other practices of XP make collective code ownership possible. Answer: Pair programming assists other programmers in learning the code. Test-first development assures that there will be ready-made tests for the code, so a programmer who changes it easily make sure that it still does what it s supposed to. Continuous integration prevents any programmer from taking code and changing it a lot without informing other members of the team. Question 8. Here is the requirements statement for a system. Requirements for the bus navigator system: The mission is to allow students to plan trips around NCSU and Raleigh by bus. Each rider specifies an origination stop and a destination stop. The system presents a list of departure times, arrival times, and transfer points, and a map of the trip. Each item on the list consists of one or more of trip segments. Each trip segment consists of a departure point and time, and an arrival point and time. If it would significantly reduce the time of the trip, the system may suggest walking between two nearby stops. Each segment may be on a Wolfline bus, a CAT bus, or a TTA bus. Each bus has a schedule, which is a set of trips, with each trip consisting of a set of stops and times. The fare is 0 on a Wolfline bus, $1 on a CAT bus, and $2 ($2.50 for express) on a TTA bus. The system calculates the total cost of the trip by adding the cost of the individual segments. From among the italicized nouns in the list above 6

7 (a) (1 point each) Choose four that constitute different key abstractions of the system, and should therefore be implemented as classes of the same name as the noun. Answer: Possibilities include stop, map, segment, walk, bus, any of the three kinds of buses, schedule, and trip. (b) (3 points each) Choose two that are not key abstractions and therefore should not be implemented as classes of the same name as the noun. Explain why they are not key abstractions. Answer: Student and rider are not key abstractions because there is no reason for them to be represented within the system (a client of the system may be a key abstraction, but there is no good reason for calling a client a student or rider, since a client is not necessarily either one). Departure point and arrival point are not key abstractions, because they are just particular stops. Time is not a key abstraction of the system, because the language undoubedly provides a way of representing it, which can simply be used by our system. Cost is not a key abstraction, because it is just a numeric value. Some people said that one term or another is not a key abstraction simply because it is an attribute of another class. This does not show that it s not a key abstraction. Abstractions such as Bus, Schedule, and Trip are all attributes of some class; otherwise it wouldn t be possible to reference them. (c) (2 points) Is any of your key abstractions a subclass of another key abstraction? Explain. Answer: Yes. Wolfline bus, CAT bus, and TTA bus are subclasses of bus, because each one has all the attributes of bus, but a different way of calculating a fare. (d) (3 points) If there is a subclass relationship in part (c), are there any abstract methods? In which class would they be implemented? Answer: fare, implemented in class Bus, would be an abstract method. (e) (4 points) Which one of the terms representing a key abstraction is used ambiguously in the requirements? Identify all the different senses in which the term is used. Answer: The term trip is used in two different senses: as a set of segments that a rider takes to get to the destination, and as a set of stops that a bus passes on its itinerary. Ambiguous is not the same as a synonym. Two terms that mean the same, e.g., stop and point may be synonyms, but they are not ambiguous. (f) (6 points; 3 points each) Suppose that our system includes a (bus) Trip class. Based on the requirement statement above, identify two responsibilities and the collaborator(s), if any, for each of them. (Use the back of the page if you need more room.) Answer: Responsibility Collaborator Know your stops Know your stopping times Stop There are also other possibilities. 7

8 Several students said that knowing the fare was a responsibility of the Trip class. But this is really a responsibility of the passenger s trip, not the bus s. On some buses, the fare varies according to distance traveled. So the Bus can t, in general, know what the fare is. (g) (5 points) Sketch a format for the output that the system might provide to the user. You don t need to draw a map, but do suggest how the list of departure and arrival points and times and transfer points should be presented to the user. Answer: Here is one possibility. From: College of Textiles To: UNC Hospital Fare: $2.00 Lv. Bus College of Textiles 7:20 AM Wolfline #2A-Centennial Campus 7:50 AM Wolfline #2A-Centennial Campus 8:20 AM Wolfline #2A-Centennial Campus Arr. Hill Library Lv. Hill Library Bus 7:33 AM 7:36 AM TTA #101A Raleigh to RTP 8:03 AM 8:06 AM TTA #101A Raleigh to RTP 8:33 AM 8:36 AM TTA #101A Raleigh to RTP Arr. RTP Bus Center Lv. RTP Bus Center Bus 8:10 AM 8:15 AM TTA 401B- RTP to Chapel Hill to Durham 8:40 AM 8:45 AM TTA 401B- RTP to Chapel Hill to Durham 9:10 AM 9:15 AM TTA 401B- RTP to Chapel Hill to Durham Arr. UNC Hospital 8:49 AM 9:19 AM 9:49 AM I leave the map to your imagination! Grade summary High 100 Mean Low 45 Std. Dev Quartiles (74, 83, 88) 8

What are the characteristics of Object Oriented programming language?

What are the characteristics of Object Oriented programming language? What are the various elements of OOP? Following are the various elements of OOP:- Class:- A class is a collection of data and the various operations that can be performed on that data. Object- This is