CS 215 Software Design Sample Midterm Questions

Transcription

1 Software Design 1. The administration at Happy Valley School District is redesigning the software that manages information about its students. It has identified an abstract class Student, with two subclasses: ElementaryStudent and MiddleSchoolStudent. MiddleSchoolStudent has a subclass called HighSchoolStudent. The basic organization of these classes is shown below. public abstract class Student { private String name; public class ElementaryStudent extends Student { // This array would have one entry for each year of elementary school // identifying the student s primary teacher private Teacher[] teachers; public class MiddleSchoolStudent extends Student { // All students take the 5 subjects shown below. This remembers who // taught each of those subjects in each year. private Teacher[] mathteacher; private Teacher[] englishteacher; private Teacher[] scienceteacher; private Teacher[] historyteacher; private Teacher[] languageteacher; public class HighSchoolStudent extends MiddleSchoolStudent { // In addition to the subjects in middle school, high school students // can take electives. We need to remember those teachers, too. private Teacher[] elective1teacher; private Teacher[] elective2teacher; private Teacher[] elective3teacher; Suggest another way that this information could be represented and explain what the advantages of your solution are. There is obviously a lot more information that you would want in such a system, but you should not concern yourself with that. Your redesign should keep the same basic information: which teachers did a student have, for which subjects and in which years. In particular, you do not need to model courses, grades, semesters, etc.!1

2 2. Please explain when each of the following ways of writing a method in a superclass and overriding it in a subclass are appropriate. When it says // Code here or // More code here, it means there would be actual code, not just the comment that I show. a. class Superclass { // Code here class Subclass extends Superclass { super.m(); // More code here b. class Superclass { // Code here class Subclass extends Superclass { super.m();!2

3 c. class Superclass { // Code here class Subclass extends Superclass { d. class Superclass { class Subclass extends Superclass { super.m(); // More code here e. class Superclass { public abstract void m(); class Subclass extends Superclass { super.m(); // More code here 3. The following questions concern preconditions, postconditions and inheritance. a. For the following class and its description, identify the preconditions and postconditions of the constructor and each method. public Counter() // Initializes a counter to 0 public int getvalue () // Returns the value of the counter public void increment () // Increases the value of the counter by 1 b. Consider the following subclass of Counter, called Counter2. Identify the preconditions and postconditions of its constructor and methods. Does Counter2 follow the rules about how preconditions and postconditions should change when sub typing? Why or why not? public Counter2() // Initializes a counter to 0 public void increment () // Doubles the value of the counter c. Consider the following subclass of Counter, called Counter3. Identify the preconditions and postconditions of its constructor and methods. Does Counter3 follow the rules about how preconditions and postconditions should change when sub typing? Why or why not? public Counter3() // Initializes a counter to 0 public void increment (int n) // If n > 0, adds n to the value of the // counter!3

4 4. Take a look at the Java API for Stack available online at 8/docs/api/. a. What problems do you see with where Stack is placed in Java s type hierarchy? b. Suggest an alternative design that avoids these problems. c. Discuss your alternative design. What are the advantages and disadvantages of your proposal compared to what Java provides? 5. A Date class consists of the following: public class Date { private int month; private int day; private int year; public Date (int month, int day, int year) { this.month = month; this.day = day; this.year = year; public void adddays (int numdays) { public boolean isleapyear () { a. Describe in English the class invariants for this class. b. Write a wellformed method to check the class invariants. c. What are the preconditions and postconditions for the constructor, adddays and isleapyear methods? d. If you wanted Date objects to be immutable, what would you change about the design above and your answers to the previous parts of this question? e. What are the advantages or disadvantages to having immutable Date objects?!4

5 6. Each of the following has a design flaw. Discuss what the flaw is and suggest a way to remove the flaw. a. public class KeypadLock { private int[] secretcode; public KeyPadLock(int[] code) { secretcode = code; public boolean checkcode (int[] code) { return Arrays.equals(secretCode, code); b. Employee -name -address HourlyEmployee - hourlyrate SalariedEmployee - annualsalary Retired c. public class DailySales { private ArrayList<Sale> sales; // Assume sales is appropriately set. public int totalsales () { int total = 0; for (Sale s : sales) { Payment payment = s.getpayment(); total = total + payment.getamount(); return total;!5

6 7. For each of the examples below indicate if this is a good use of inheritance or not. If it is not, briefly explain why not and what would be a better way to model the relationship between the classes. a. (3 points) An airplane has a model (like Boeing 747) and belongs to an airline. A flight has a starting and destination airport and a flight number and requires the use of an airplane. Should Flight be modeled as a subclass of Airplane? b. (3 points) An academic department has many types of employees including teaching assistants and faculty, with one designated faculty member who is the chair of the department. Should Chair be a subtype of Faculty? c. (3 points) A textbook is a book that has questions at the end of each chapter. Should Textbook be a subtype of Book? d. (3 points) A list is an ordered collection of values. An immutable list is a list whose values cannot change after the list is created. Should ImmutableList be a subclass of List? e. (3 points) Snickers is the name of my dog. Should Snickers be a subclass of Dog?!6

7 8. Answer the questions below about this UML class diagram. «abstract» BankAccount int balance deposit (int amount) withdraw (int amount) + Customer String name String address openaccount () closeaccount (Account act) CheckingAccount int insufficientfundsfee processcheck (Check c) SavingsAccount double interestrate depositinterest() a. (1 point) What are the names of the methods that can be called on any BankAccount? b. (1 point) What are the names of all the methods that can be called on a CheckingAccount? c. (1 point) How many Customers can be associated with a BankAccount? d. (1 point) How many BankAccounts can a Customer have? e. (1 point) Would the following statement be legal, assuming the constructor exists and the variable cust has type Customer? BankAccount acct = new CheckingAccount (cust);!7

8 f. (5 points) How would you change the design so that a married couple could share a BankAccount? Draw the revised UML diagram.!8

9 9. In Java s Collection interface, the add method is described as follows: boolean add(e e) Ensures that this collection contains the specified element. Returns true if this collection changed as a result of the call. Parameters: e - element whose presence in this collection is to be ensured Returns: true if this collection changed as a result of the call a. (2 points) What are the preconditions of Collection s add method? b. (2 points) What are the postconditions of Collection s add method? The List interface extends Collection. Its add method is describe as follows: boolean add(e e) Appends the specified element to the end of this list. Parameters: e - element to be appended to this list Returns: true c. (2 points) What are the preconditions of List s add method? d. (2 points) What are the postconditions of List s add method?!9

10 e. (2 points) Does List strengthen, weaken or not change Collection s precondition for add? f. (2 points) Does List strengthen, weaken or not change Collection s postcondition for add?!10

11 10.I would like you to reason about how Liskov s Substitution Principle helps us understand the subtyping rules associated with generics. The ArrayList class implements the List interface. For this question, focus on the behavior of the get and set methods in the List interface: Interface List<E> E get(int index) Returns the element at the specified position in this list. E set(int index, E element) Replaces the element at the specified position in this list with the specified element a. (2 points) Assuming that Java allows ArrayList<String> to be treated as a subtype of List<String>, does using List and ArrayList in the following way satisfy Liskov s Substitution Principle? Briefly explain your answer. List<String> mylist = new ArrayList<String>(); b. (2 points) Assuming that Java allows ArrayList<String> to be treated as a subtype of ArrayList<Object>, does using ArrayList in the following way satisfy Liskov s Substitution Principle? Briefly explain your answer. ArrayList<Object> mylist = new ArrayList<String>();!11