Name: CS 159 Practice Final Fall 2015

Transcription

1 Name: CS 159 Practice Final Fall 2015

2 CS 159, Fall 2015 Final Exam Section 02 Page 2 of Choose the best answer for each of the following multiple choice questions. (a) (2 points) What will happen when the following lines of code are executed? String [] names = new String [3]; System. out. println (" Length of 2: " + names [2]. length ()); An ArrayIndexOutOfBoundsException exception will be thrown. A NullPointerException exception will be thrown. "Length of 2: 0" will be printed. "Length of 2: 2" will be printed. (b) (2 points) What is the type of names[2] after the following line of code executes? String [][] names = new String [3][2]; String String[] String[][] Accessing numbers[2] will result in an ArrayIndexOutOfBoundsException. (c) (2 points) Which of the following best describes the class java.io.file? File is a subclass of OutputStream. A File object provides open, close and write methods that may be used to modify files. A File object represents the location of a file, including the file name and the directory path. All of the above. (d) (2 points) Which of the following could be a good reason to use an abstract class rather than an interface? You wish to declare a common set of methods that should be implemented by several otherwise unrelated classes. Logically, the class should not be instantiated, but you want to avoid code duplication by allowing other classes to inherit methods implemented in the class. You want to create your own type-safe collection, something like: YourCollection<T>. Any of the above. (e) (2 points) Which of the following would be a good reason to use an iterator directly (rather than using a traditional for loop or an enhanced for loop). Iterators can be used to remove elements from a collection during iteration. Iterator-based loops result in cleaner code that is easier to read. Iterator-based loops are usually the fastest way to iterate through a collection. There is no good reason to use an iterator directly. Iterators only exist to support the enhanced for loop. (f) (2 points) Which of the following operations is likely to execute significantly faster if mylist is a LinkedList rather than an ArrayList. mylist.contains(a); mylist.get(i); mylist.remove(0); mylist.remove(mylist.size() - 1); Page 2

3 CS 159, Fall 2015 Final Exam Section 02 Page 3 of (a) (4 points) Draw a UML diagram corresponding to the accompanying Named interface and the Employee, Hourly, Salaried and Movie classes. For full credit, you must use correct UML syntax, including arrow types, etc. Feel free to explicitly label italicized text. YOU DO NOT NEED TO INCLUDE INSTANCE VARIABLES OR METHODS IN YOUR DI- AGRAM. (b) (2 points) There are only six valid movie ratings in the United States: G, PG, PG- 13, R, NC-17 and NR. Given this fact, how could the design of the Movie class be improved? The ratings should be represented using an enumerated type. (c) (2 points) Read over the attached interface and classes. There is at least one opportunity to improve this design by reducing code duplication without changing the existing class hierarchy and without changing the functionality. What change should be made? The code relating to startyear should be moved into the Employee class. It is also reasonable to point out that the getpay method should be abstract in the Employee superclass instead of being implemented to return a nonsense value of -1. This isn t really a case of code duplication, just unnecessary code. Page 3

4 CS 159, Fall 2015 Final Exam Section 02 Page 4 of Consider the following UML diagram. (a) (2 points) Write the Encryptable interface in Java. public interface Encryptable void encrypt (); boolean equals ( Encryptable other ); (b) (4 points) Write the AudioFile class in Java. public abstract class AudioFile implements Encryptable protected int duration ; public AudioFile ( int duration ) this. duration = duration ; public int getduration () return duration ; public abstract void play (); Page 4

5 CS 159, Fall 2015 Final Exam Section 02 Page 5 of (a) (8 points) Given the attached Movie class, what will be printed by the following method? public class ReferenceMain public static void main ( String [] args ) Movie [] movies = new Movie [3]; Movie first, second ; first = new Movie (2003, " Elf "); movies [0] = first ; first. setrating ("R"); movies [1] = first ; first = new Movie (2012, " Ted "); second = first ; second. setrating ("PG -13 "); movies [2] = new Movie (1988, first. getname () + " 2"); second. setrating ("PG"); for ( Movie curmovie : movies ) System. out. println ( curmovie. tostring ()); System. out. println ( first. tostring ()); System. out. println ( second. tostring ()); Elf (2003) R Elf (2003) R Ted 2 (1988) NR Ted (2012) PG Ted (2012) PG (b) (4 points) Now assume that the Movie class has been modified to make the rating field static: public static String rating; What will be printed by the method above given this modification? Elf (2003) PG Elf (2003) PG Ted 2 (1988) PG Ted (2012) PG Ted (2012) PG Page 5

6 CS 159, Fall 2015 Final Exam Section 02 Page 6 of (8 points) Given the attached Employee, Hourly, and Salaried classes, what will be printed when the following main method is executed? public class BusinessDriver public static void main ( String [] args ) Employee [] workers = new Employee [2]; Salaried boss ; Hourly tech ; boss = new Salaried (" Lovelace, Ada ", , 2003); workers [0] = boss ; tech = new Hourly (" Hopper, Grace ", 15.00, 2007); tech. sethoursworked (3); workers [1] = tech ; printpayinfo ( boss ); printpayinfo ( tech ); for ( int i = 0; i < workers. length ; i ++) printpayinfo ( workers [i ]); public static void printpayinfo ( Employee employee ) System. out. print (" Employee Pay : "); System. out. println ( employee. getname () + ", " + employee. getpay ()); public static void printpayinfo ( Hourly employee ) System. out. print (" Hourly Pay : "); System. out. println ( employee. getname () + ": " + employee. getpay ()); public static void printpayinfo ( Salaried employee ) System. out. print (" Salary Pay : "); System. out. println ( employee. getname () + ": " + employee. getpay ()); Salary Pay: Lovelace, Ada: Hourly Pay: Hopper, Grace: 45.0 Employee Pay: Lovelace, Ada, Employee Pay: Hopper, Grace, 45.0 Page 6

7 CS 159, Fall 2015 Final Exam Section 02 Page 7 of (6 points) Given the attached files and the following main method: public class CompileDriver public static void main ( String [] args ) Named named ; Employee worker ; Hourly cook ; Salaried manager ; worker = new Salaried (" Jim ", ); // Code instantiating named, cook and manager not shown... // The code in the questions below will be inserted here... Indicate which of the following statements will: not compile (N), compile but generate an exception at runtime (C+X), or compile and run without generating an exception (C+R). You may assume that the named, cook and manager variables have been instantiated with appropriate types. Each statement should be considered independently. (a) C+R named = new Movie(1999, "The Matrix"); (b) N named = new Employee("Bob Doe"); (c) C+R named = worker; (d) C+R worker = cook; (e) N manager = worker; (f) N cook = named; (g) C+R String name = named.getname(); (h) C+R double pay = cook.getpay(); (i) N worker.sethours(5.0); (j) N named.name = "Bill"; (k) C+X cook = (Hourly)worker; (l) C+R manager = (Salaried)worker; Page 7

8 CS 159, Fall 2015 Final Exam Section 02 Page 8 of (10 points) Given the attached Movie class, write a statement (or statements) that does each of the following. Where possible, your answers must be type-safe. (a) Declares variables named forkids and gmovies as ArrayLists of Movie objects. ArrayList < Movie > forkids, gmovies ; (b) Instantiates forkids. forkids = new ArrayList < >(); (c) Declares a variable named ratingmap as a HashMap in which the keys are String objects and forkids could be a value. HashMap < String, ArrayList < Movie >> ratingmap ; (d) Instantiates ratingmap. ratingmap = new HashMap < String, ArrayList < Movie > >(); (e) Adds forkids to ratingmap using the key "G". ratingmap. put ("G", forkids ); (f) Declares a variable named it as an Iterator of Movie objects. Iterator < Movie > it; (g) Gets the value in ratingmap with the key "G" and assigns it to the variable named gmovies. gmovies = ratingmap. get ("G"); (h) Gets an Iterator of Movie objects from gmovies and assigns it to the variable named it. it = gmovies. iterator (); (i) Declares a variable named best as a Movie. Page 8

9 CS 159, Fall 2015 Final Exam Section 02 Page 9 of 17 Movie best ; (j) Gets the last Movie in forkids and assigns it to the variable best. (Your statement should not use the iterator object.) best = forkids. get ( forkids. size () - 1); (k) Prints the name and rating (separated by a space) for each movie accessible through the iterator it. while (it. hasnext ()) Movie movie = it. next (); System. out. println ( movie. getname () + " " + movie. getrating ()); Page 9

10 CS 159, Fall 2015 Final Exam Section 02 Page 10 of Consider the following recursive method: public static String func02 ( String s) String result ; if ( s. length () == 0) result = "_"; else String first = s. substring (0, 1); String rest = s. substring (1, s. length ()); result = "*" + first + func02 ( rest ) + first + "*"; return result ; (a) (3 points) Draw the activation records that will result from the following call to func1. Each activation record should be drawn as a box containing the local value of s. String answer = func02 (" pan "); "*p*a*n_n*a*p*" ^ "pan" < "a*n_n*a" > "an" < "*n_n*" > "n" < "_" > "" (b) (3 points) What value will be stored in answer after the line of code above has completed? "*p*a*n_n*a*p*" Page 10

11 CS 159, Fall 2015 Final Exam Section 02 Page 11 of Complete the recursive method described blow. (This question is borrowed from CodingBat.) We have triangle made of blocks. The topmost row has 1 block, the next row down has 2 blocks, the next row has 3 blocks, and so on. Compute recursively (no loops or multiplication) the total number of blocks in such a triangle with the given number of rows. triangle(0) 0 triangle(1) 1 triangle(2) 3 public int triangle ( int rows ) if ( rows == 0) return 0; else return rows + triangle ( rows - 1); Page 11

12 CS 159, Fall 2015 Final Exam Section 02 Page 12 of (8 points) Complete the following method of the Movie class so that it conforms to the Javadoc comments. /** * Return an ArrayList of sequels for this movie. Sequels are named by appending * an appropriate number to the name of the original movie. The first sequel * must be named using Roman numerals, while all subsequent sequels must be * named using Arabic numerals. For example, the sequels for " Rocky " must * be named " Rocky II", " Rocky 3", " Rocky 4" etc. * * Sequels appear at two - year intervals. For example, the movie " Rocky " * released in 1976 would have sequels released in 1978, 1980, etc. * All sequels must have the same rating as the original movie. * howmany How many sequels to create An ArrayList containing all sequels IllegalArgumentException If the number of sequels is less than 1 * or more than 6. */ public ArrayList < Movie > sequels ( int howmany ) if ( howmany < 1 howmany > 6) throw new IllegalArgumentException (); ArrayList < Movie > result = new ArrayList < >(); result. add ( new Movie ( year + 2, name + " II")); result. get (0). setrating ( getrating ()); for ( int i = 1; i < howmany ; i ++) result. add ( new Movie ( year + 2 * (i + 1), name + " " + i )); result. get (i). setrating ( getrating ()); return result ; Page 12

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14 public interface Named String getname (); public class Movie implements Named public int year ; public String name ; public String rating ; public Movie ( int year, String name ) this. year = year ; this. name = name ; rating = "NR"; public int getyear () return year ; public String getname () return name ; public String getrating () return rating ; public void setrating ( String rating ) this. rating = rating ; public ArrayList < Movie > sequels ( int howmany ) // Code not shown. public String tostring () return name + " (" + year + ") " + rating ; 14

15 public abstract class Employee implements Named protected String name ; public Employee ( String name ) this. name = name ; public String getname () return name ; public double getpay () return -1.0; public class Salaried extends Employee public static final int PAY_ PERIODS = 12; private double salary ; private int startyear ; public Salaried ( String name, double salary, int startyear ) super ( name ); this. salary = salary ; this. startyear = startyear ; public double getpay () return salary / PAY_ PERIODS ; public double getsalary () return salary ; public void setsalary ( double salary ) this. salary = salary ; public int getstartyear () return startyear ; 15

16 public class Hourly extends Employee private double hoursworked ; private double payrate ; private final int startyear ; public Hourly ( String name, double payrate, int startyear ) super ( name ); this. payrate = payrate ; this. startyear = startyear ; hoursworked = 0; public double getpay () return hoursworked * payrate ; public void sethoursworked ( double hoursworked ) this. hoursworked = hoursworked ; public void setpayrate ( double payrate ) this. payrate = payrate ; public int startyear () return startyear ; public String tostring () return " Name : " + name + " Rate : " + payrate ; 16

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