CS 132 Midterm Exam Spring 2004

Transcription

1 Date:22-Jun-2004 Time:7:00-9:00p.m. Permitted Aids:None CS 132 Midterm Exam Spring 2004 Please complete this page in ink. Last Name: Signature: First Name: ID: Please check off your Practicum Section: David Brown Jinbo Xu 101 2:30) :30) :30) 106 2:30) :30) unsure 104 2:30) unsure Instructions: (Read carefully before the exam begins): Question Max. Marks 1. Before you begin, make certain that you have one booklet with pages numbered 1-14 printed double-sided. 2. The marks assigned to each question are shown at the beginning of the question; use this information to organize your time effectively. Part I Marks Given Grader Initials 3. Place all your answers in the spaces provided on these pages. Use the spaces as an indication of the length of answer expected. Use the last page for extra space if you need it, but clearly indicate this if you do so. 4. You do not need to document your code unless it is specifically required by the question. 5. Questions will not be interpreted. Proctors will only confirm or deny errors in the questions. If you consider the wording of a question to be ambiguous, state your assumptions clearly and proceed to answer the question to the best of your ability. You may not trivialize the problem in your assumptions. 6. Cheating is an academic offense. Your signature on this exam indicates that you understand and agree to the University s policies regarding cheating on exams Total 107

2 Part I: Short Answer Questions Name: 1. Consider the following class: public class ShoppingList extends Object private String[] items; public ShoppingList() super(); this.items = new String[4]; this.items[1] = banana ; this.items[2] = apple ; this.items[3] = orange ; this.items[4] = grapefruit ; public String tostring() String str = ; for(int i=0;i<this.items.length;i++) str += this.items[i].tostring() + ; return str; Listed below are three error messages that might be received while compiling this class or running a program that uses an instance of this class. [A] ArrayIndexOutOfBoundsException [B] NullPointerException [C] method does not return a value Circle the lines of code of the above class that cause errors and for each line of code that you circle, indicate which error (A, B, or C) the line causes. 2. Consider the following class: public class SumTool extends Object public SumTool() super(); public static int sum(int[] a) int s = 0; for(int i=0; i <= a.length; i++) s += a[i]; return; Listed below are three error messages that might be received while compiling this class or running a program that uses an instance of this class. [A] ArrayIndexOutOfBoundsException [B] NullPointerException [C] method does not return a value Circle the lines of code of the above class that cause errors and for each line of code that you circle, indicate which error (A, B, or C) the line causes. Page 2 of 16

3 For each multiple choice question on this page, select the single best answer. 3. A softball team of 9 players plays 20 games in a season. Which of the following array declarations is best suited to record the team s score for each game played during the season? a) int[] games = new int[9]; b) int[] games = new int[20]; c) int[] games = new int[9*20]; d) int[][] games = new int[9][20]; e) none of the above 4. The same softball team wants to keep track of the number of runs each team member scores during a particular game throughout the season. Which of the following array declarations is best suited to store this information? 5. Use cases a) int[] games = new int[9]; b) int[] games = new int[20]; c) int[] games = new int[9*20]; d) int[][] games = new int[9][20]; e) none of the above a) capture some user-visible function. b) achieve a discrete goal for the user. c) may be large or small. d) satisfy all of the above. 6. Static methods a) cannot manipulate this. b) are attached to the class as a whole rather than a particular object c) can be called using the name of the class in which they are defined. d) satisfy all of the above. e) satisfy only two of the above. 7. Consider the class shown to the right. Suppose another class contains the code SomeClass sc = new SomeClass(); Which of the following could follow that line? a) sc.msg(); b) SomeClass.msg(); c) new SomeClass.msg(); d) A or B e) B or C public class SomeClass extends Object public static void msg() System.out.println( Msg from SomeClass ); Page 3 of 16

4 Part II: Coding Questions Name: 1. [10] The Employee class has the following instance variables and signatures for its constructor and public methods: public class Employee extends Object private String name; public Employee(String aname) public String getname() // constructs an Employee object // returns the employee s name The Salary class has the following instance variables and signatures for its constructor and public methods: public class Salary private Employee emp; private int rank; private double amt; private boolean hourly; public Salary(Employee anemployee, int arank, double anamount, boolean anhourly) public Employee getemployee() public int getrank() public double getamount() public boolean gethourly() // constructs a Salary object // returns the Employee object // returns the salary ranking // returns the salary amount // returns the type of salary Page 4 of 16

5 Write a main method in the Salary class that tests the constructor and get methods of the class. public static void main(string[] args) 2. [27] A data file contains information about professors and the courses they teach. The file is organized as follows: 1256 Burton Richard Francis BW100 Introduction to Basket Weaving PH345 Incomprehensible Philosophy 3457 Ing Li-meng CH267 Chemistry of Icky Things 6782 Kovac Ivan Boleslav CS132 Developing Programming Principles CS399 Programming Your X-Box CS198 Basic for Dummies etc. <id> <surname> <given names> <course name> <course description> <course name> <course description> <id> <surname> <given names> <course name> <course description> <id> <surname> <given names> <course name> <course description> <course name> <course description> <course name> <course description> etc. For the questions that follow assume that surnames consist of exactly one name, that a person may have multiple given names, that course names always begin with a letter, and that not all professors teach the same number of courses. Page 5 of 16

6 a. [3] Given the following instance variables, write a constructor for the Course class that reads a single course from an already open file: public class Course extends Object private String name; private String description;... b. [9] Given the following instance variables, write a constructor for the Professor class that reads a single professor and their course information from an already open file. Note that no professor has more than three courses per term. public class Professor extends Object private int id; private String surname; private String givennames; private Course[] courses = new Course[3]; private int numcourses;... Page 6 of 16

7 c. [6] Given the following instance variables, write a growarray() helper method that increases the size of the partially-filled professors array: public class ProfessorList extends Object private Professor[] professors = new Professor[0]; private int size = 0;... d. [9] Write the constructor for the ProfessorList class with the instance variables described in part c. The constructor reads data for all professors from a file given the filename. Page 7 of 16

8 3. [12] Given the class descriptions in Question 2, draw a class diagram showing the relations between the three classes (ProfessorList, Professor, and Course) and their instance variables and methods. Page 8 of 16

9 ProfessorList -Professor[] professors -int size +ProfessorList(String filename) -void growarray( ) Professor Course * -int id -String surname -String givennames -int numcourses -Course[] courses String name -String description +Course(TextInput in) +Professor(TextInput in) Page 9 of 16

10 4. [8] A certain class contains an instance variable that is a filled array of Strings named phrases. These Strings are of varying lengths and are in no particular order. Write a method getstringsoflength(int n) that returns a filled array of Strings from the array phrases where all Strings in the returned array have a length greater than or equal to n. Page 10 of 16

11 5. [6] Suppose we have the following class: Name: public class SomeClass extends Object private int[] values = new int[0]; private int numelements = 0;... where values is a partially-filled array of integers and numelements stores the number of integers in the partially-filled array. Write the method, delete, which deletes the value from the array values at position, pos. The order of the existing elements in the array must be preserved. public void delete(int pos) Page 11 of 16

12 6. [6] Consider the following class: public class A extends Object private int id; private static int tracker = 0; public A() super(); this.id = A.tracker; A.tracker++; public static int gettracker() return A.tracker; public int getid() return this.id; public static void main(string[] args) A a1 = new A(); System.out.println( x1 = + a1.gettracker()); System.out.println( y1 = + a1.getid()); A a2 = new A(); System.out.println( x2 = + a2.gettracker()); System.out.println( y2 = + a2.id()); System.out.println( x3 = + a1.gettracker()); System.out.println( y3 = + a1.getid()); What is the output to the console if the main method in this class is run? Fill in the appropriate blanks below. x1 = y1 = x2 = y2 = x3 = y3 = Page 12 of 16

13 7. [10] A palindrome is a word, phrase, or sentence that reads the same backwards and forwards. Write the method ispalindrome(..) of the StringFun class that takes a String message and determines whether or not the entire String is a palindrome. The method returns true if it is a palindrome and false otherwise. Assume that the message contains only lowercase letters. Some examples of palindromes are deed and racecar. Page 13 of 16

14 8. [19] Suppose we have a collection of text files. Their names are stored in an array of Strings, called filenames. The array, filenames, is an instance variable in the class FileManager.. [9] Write a helper method, calcfrequencies, for the FileManager class. This method calculates the frequency of occurrence of each letter of the alphabet in the file in and returns the result as an array of frequencies. Assume that any letters in the file are in uppercase. private double[] calcfrequencies(textinput in) Page 14 of 16

15 . [6] Write a public method, calcallfrequencies(), for the FileManager class using the helper method you implemented above. The method calculates the frequencies of all of the letters in all of the files with names stored in the array filenames. The frequencies are stored in an instance variable, allfreqs, which is a 2-Dimensional array. Each row of the 2-Dimensional array corresponds to the associated file in filenames and each column corresponds to a letter of the alphabet. You may assume that allfreqs is initialized to the appropriate size in the FileManager constructor.. [4] Write the constructor for the FileManager class given the description of this class in the previous parts of this question. public class FileManager extends Object private String[] filenames; private double[][] allfreqs; public FileManager(String[] files) Page 15 of 16

16 This space is available to continue answers started elsewhere in this booklet. Please label everything clearly, both here and where the question is asked so the marker will know where to look. Page 16 of 16