Com S 227 Spring 2013 Topics and review problems for Final Exam Wednesday, April 8, 7:00 9:00 pm in Hoover 2055 and 1213 ** You must bring your ID **

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1 General information Com S 227 Spring 2013 Topics and review problems for Final Exam Wednesday, April 8, 7:00 9:00 pm in Hoover 2055 and 1213 ** You must bring your ID ** This will be a 120-minute, timed, pencil-and-paper written exam. No books, no notes, no electronic devices, no headphones, no collaboration. The problems will primarily involve writing code or reading and interpreting code. The most important thing you can do to prepare for an exam like this is to practice solving problems and writing code. It is a good idea to write your solutions first on paper (since that is the format of the exam) and then check them by typing up what you ve written. In general you do not have to memorize methods from the Java API. You should know System.out.print and System.out.println by heart. You should be familiar with using String, Scanner, Random, File, and ArrayList objects, but for specific methods that might be needed, we ll provide you with the one-sentence descriptions from the API. Exam topics The final exam is comprehensive, though it will emphasize topics covered since Exam 2. A reasonable guess is that 40 to 60% of the exam will be on new material. The list below is a rough overview of the highlights for this new material. Numbers in parentheses refer to the text. Binary search (14.7) Using the String compareto method (5.2.3) Inheritance (Chapter 10) o Interfaces o Defining subclasses o Overriding methods o The class Object o Polymorphism and dynamic binding of methods o The declared type and the runtime type of a variable o The protected keyword o Abstract classes Recursion (13.1, 13.2) Exceptions reporting, propagating, and handling errors ( ) Sorting o Selection sort (14.1) o Merge sort (14.4) The following pages contain some practice problems related to the topics above. Remember that the review materials for Exams 1 and 2 are still relevant. You are encouraged to post your sample solutions on Piazza for discussion.

2 1. An interface is shown below that represents the players in a simple game. A move in the game consists of a player choosing an integer 0, 1, or 2 (like a Rock-Paper-Scissors game). Each player also keeps a history of past moves that can be examined with the getpreviousmove() method. Below you will find the code for two concrete implementations of IPlayer. Notice there is quite a bit of duplicated code. Modify the implementation so that most of the duplicated code is moved into an abstract base class. interface IPlayer int play(); //Returns the player's move, which is always 0, 1, or 2 int getpreviousmove(int movesago); // Returns a previous move class RandomPlayer implements IPlayer private Random rand = new Random(); private ArrayList<Integer> history = new ArrayList<Integer>(); public int play() int move = rand.nextint(3); // randomly chooses 0, 1, or 2 history.add(move); return move; public int getpreviousmove(int movesago) return history.get(history.size() - movesago); class AlternatingPlayer implements IPlayer private int state = 0; private Random rand = new Random(); private ArrayList<Integer> history = new ArrayList<Integer>(); public int play() // usually returns 0, but every third move randomly chooses 1 or 2 int move = 0; if (state % 3 == 2) move = rand.nextint(2) + 1; ++state; history.add(move); return move; public int getpreviousmove(int movesago) return history.get(history.size() - movesago);

3 2. A DataList is a subtype of ArrayList<Double> with an additional method analyze() for analyzing the stored values: public class DataList extends ArrayList<Double> public double analyze(analyzer analyzer) for (int i = 0; i < size(); ++i) analyzer.processnextvalue(get(i)); return analyzer.getresult(); a) Since there may be many different kinds of analyses we want to perform, the Analyzer type is an interface. Based on what you see above, write the Analyzer interface (remember there are no method bodies in an interface). b) Write an Analyzer called MeanAnalyzer that will find the average of the values. c) Write an Analyzer called MaxAnalyzer that will find the maximum value in the data set. 3. a) Given the method mystery below, determine the output produced by the call mystery(10). (It might be helpful to sketch the call stack as you go.) public static void mystery(int x) if (x == 0) System.out.println("pooh"); else if (x % 2 == 0) System.out.println(x); mystery(x / 2); else mystery(x - 1); a) What happens when you call mystery(-1)? Explain. 4. Rewrite the method mystery() from the previous problem so that it produces the same results for all positive numbers, but does not use recursion.

4 5. One way to efficiently calculate integer powers is by repeated squaring:,,, Write a recursive function pow(x, p) that uses this strategy. (Assume that p >= 0.) 6. Write a recursive method that, given a directory, returns a count of the total number of files beneath it. Note that the class java.io.file includes the methods: boolean isdirectory() returns true if this File represents a directory File[] listfiles() returns an array of all the files and directories contained in this one, provided that this File is a directory 7. What is the result of the method call trystuff(" skidoo 30 foo bar")? (Recall that Integer.parseInt() will throw a NumberFormatException if you attempt to parse any non-numeric string.) public static int trystuff(string text) int total = 0; int i = 0; Scanner scanner = new Scanner(text); while (scanner.hasnext()) try String s = scanner.next(); i = Integer.parseInt(s); total += i; catch (NumberFormatException nfe) total -= i; return total;

5 8. Recall the print method of the Printer class from Assignment 1. Here was one possible implementation. public void print(int numberofpages) int actualsheetsused = Math.min(sheets, numberofpages); double inkneeded = actualsheetsused * INK_USAGE; double actualinkused = Math.min(inkNeeded, ink); counter += actualsheetsused; sheets = actualsheetsused; ink = actualinkused; (Assume that ink, sheets, and counter are instance variables representing the current ink, current number of sheets of paper, and total pages printed for this printer.) Now suppose we want to change the specification for the print method so that the method throws an exception InkOutException if it runs out of ink during printing (it prints as many pages as possible before the ink runs out). Its complete specification is below. * Simulates printing pages in one sided mode, using * the appropriate number of sheets and a corresponding * quantity of ink. If there is not enough paper, * the printer will use up all remaining paper and * will only use the quantity of ink needed for the sheets * actually printed. If there is not enough ink, the * printer will use up all the ink, and will print * only as many sheets as possible before the ink runs out. * If the ink runs out, throws InkOutException. numberofpages * the number of pages to print InkOutException * if the ink runs out d a) Write the class InkOutException. It should be a checked exception type. b) Rewrite the print method according to the new specification above. c) Rewrite the following test class to handle the InkOutException. Specifically, the class should catch the exception, replace the ink, and print a warning indicating how many sheets were actually printed when the ink ran out. public static void main(string[] args) Printer p = new Printer(1000); p.print(1000); System.out.println(p.isInkOut()); System.out.println(p.getCurrentPaper());

6 9. Suppose that you are given a merge method with the following declaration: * Merges two sorted subarrays of a given array, storing the result back in * the given array. * * Precondition: arr[start] through arr[mid] is already sorted, and * arr[mid + 1] through arr[end] is already sorted * * Postcondition: arr[start] through arr[end] is sorted. private static void merge(int[] arr, int start, int end, int mid) Suppose that you also have a method: * Sorts the array. public static void mergesort(int[] arr) mergesortrec(arr, 0, arr.length - 1); Write the recursive helper method that will sort a given subarray using the merge sort algorithm: * Performs a recursive merge sort of the elements * arr[start] through arr[end]. private static void mergesortrec(int[] arr, int start, int end) 10. Suppose you also have a method that performs a selection sort on a specified subarray: * Performs a selection sort on the elements * arr[start] through arr[end]. selectionsortrange(int[] arr, int start, int end) Modify the base case for the merge sort algorithm so that for any subarray of length 5 or less, it uses a selection sort instead of making a recursive call. 11. Write the merge algorithm as specified in problem 9. Note: this is really just an exercise in array manipulation. The idea is to start by copying the first half of the array into a temporary array, and then you can safely merge the two halves back into arr. This minimizes the amount of extra memory needed by the merge sort algorithm. To try out your mergesort, look for Test.java in Week 14 of Write the method selectionsortrange of the problem above. 13. On the next two pages there are some declarations for a class hierarchy, along with a big box containing some code. For each line of code in the box, indicate one of the following:

7 If it is a compile error, comment out the line and state why; or if there is an exception at runtime, comment out the line and state which exception; or if the code works and produces output, indicate what the output is (assume that previous commented-out lines do not execute) (The first three lines are done for you as an example.) 14. Without changing the public API, modify the code from problem 13 so that the method getcheckoutperiod() is only written once. Item i = new Book("Treasure Island"); // OK System.out.println(i.getTitle()); // Output: Treasure Island // System.out.println(i.getCheckOutPeriod()); // Compile error, Item has no such method Book b = new ReferenceBook("How to Bonsai Your Pet"); System.out.println(b.getTitle()); System.out.println(b.getCheckOutPeriod()); LibraryItem li = null; li = new LibraryItem("Catch 22"); System.out.println(li.getTitle()); System.out.println(li.getCheckOutPeriod()); li = new DVD("Shanghai Surprise", 120); System.out.println(li.getTitle()); System.out.println(li.getCheckOutPeriod()); System.out.println(li.Duration()); i = b; b = i; System.out.println(i.getTitle()); ReferenceBook rb = (ReferenceBook) b; System.out.println(rb.getTitle()); rb = (ReferenceBook) new Book("Big Java"); System.out.println(rb.getTitle()); public interface Item String gettitle(); public abstract class LibraryItem implements Item private String title; public LibraryItem(String title) this.title = title; public String gettitle() return title; public abstract int getcheckoutperiod(); (continued on next page)

8 public class Book extends LibraryItem public Book(String title) super(title); public int getcheckoutperiod() return 21; // three weeks public class ReferenceBook extends Book public ReferenceBook(String title) super(title); public String gettitle() return "REF: " + super.gettitle(); public int getcheckoutperiod() return 0; // reference books don't circulate public class DVD extends LibraryItem private int duration; // duration in minutes public DVD(String title, int duration) super(title); this.duration = duration; public String gettitle() return "DVD: " + super.gettitle(); public int getcheckoutperiod() return 7; // DVDs check out for one week public int getduration() return duration;