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The questions contained in this AP ® Computer Science A Practice Exam are written to the content specifications of AP Exams for this subject. Taking this practice exam should provide students with an idea of their general areas of strengths and weaknesses in preparing \ for the actual AP Exam. Because this AP Computer Science A Practice Exam has never been administered as an operational AP Exam, statistical data are not available for calculating potential raw scores or conversions into AP grades. This AP Computer Science A Practice Exam is provided by the College Board for AP Exam preparation. Teachers are permitted to download the materials and make copies to use with their students in a class - room setting only. To maintain the security of this exam, teachers should collect all materials after their administration and keep them in a secure location. Teachers may not redistribute the files electronically for any reason. © 2008 The College Board. All rights reserved. College Board, Advanced Placement Program, AP, AP Central, SAT, and the acorn logo are registered trademarks of the College Board. PSAT/NMSQT is a registered trade - mark of the College Board and National Merit Scholarship Corporation. All other products and services may be trademarks of their respective owners. Visit the College Board on the Web: www.collegeboard.com. Practice Exam Advanced Placement Program AP ® Computer Science A
Contents Directions for Administration ........................................................................\ .................... ii Section I: Multiple-C hoice Questions ........................................................................\ ........ 1 Section II: Free-Resp onse Questions ........................................................................\ ...... 41 Quick Refe rence........................................................................\ ........................................ 53 Student Answer Sheet for Mu ltiple-Choice Section .......................................................69 Multiple-Choice Answer Key ........................................................................\ ................... 70 Free-Response Scori ng Guidelines........................................................................\ .......... 71 The College Board: Connecting Students to College Success The College Board is a not-for-profit membersh ip association whose mission is to connect students to college success and opportunity. Founded in 1900, the association is composed of more than 5, 000 schools, colleges, universiti es, and other educational organizations. Each year, the College Board serves seven million students and their parents, 23,000 high schools, and 3,500 college s through major programs and services in college admissions, guidance, assessment, fi nancial aid, enrollment, and teaching and learning. Among its best-kno wn programs are the SAT ®, the PSAT/NMSQT ®, and the Advanced Placement Program ® (AP ®). The College Board is co mmitted to the principles of excellence and equity, and that commitmen t is embodied in all of its programs, services, activities, and concerns. Visit the College Board on the Web: www.collegeboard.com. AP Central is the official online home fo r the AP Program: apcentral.collegeboard.com. -i-
AP ® Computer Science A Directions for Administration The AP Computer Science A Exam is three hours in le ngth and consists of a multiple-choice section and a free- response section. • The 75-minute multiple-choice section contains 40 ques tions and accounts for 50 percent of the final grade. • The 105-minute free-response section contains 4 questions and accounts for 50 percent of the final grade. Students should be given a 10-minute warning prior to the end of each section of the exam. A 10-minute break should be provided after Section I is completed. The actual AP Exam is administered in one session. Student s will have the most realistic experience if a complete morning or afternoon is available to administer this practice exam. If a schedule does not permit one time period for the entire practice exam administration, it would be acceptable to administer Section I one day and Section II on a subsequent day. Many students wonder whether or not to guess the answer s to the multiple-choice questions about which they are not certain. It is improbable that mere guessing will improve a score. However, if a student has some knowledge of the question and is able to eliminate one or more an swer choices as wrong, it may be to the student’s advantage to answer such a question. • The use of calculators, or any other electroni c devices, is not permitted during the exam. • It is suggested that the practice exam be completed using a pencil to simulate an actual administration. • Teachers will need to provide paper for the students to write their free-response answers. Teachers should provide directions to the students indicating how they wish the responses to be labeled so the teacher will be able to associate the student’s response with the question the student intended to answer. • The AP Computer Science A Exam Appendix is incl uded with the exam materials, and each student should have a copy of this document for use with both Section I and Section II. Previously used copies of the appendix should not be distributed for the practice exam administration because students should not have access to any notes that may have b een previously written into the appendix. • Remember that students are not allowed to remove any materials, including scratch work and the appendix, from the testing site. -ii-
Section I Multiple-Choice Questions -1-
GO ON TO THE NEXT PAGE. COMPUTER SCIENCE A SECTION I Time — 1 hour and 15 minutes Number of questions — 40 Percent of total grade — 50 Directions: Determine the answer to each of the following qu estions or incomplete statements, using the available space for any necessary scratch work. Then decide which is the best of the choices given and fill in the corresponding box on the student answer sheet. No credit will be given for anything written in the examination booklet. Do not spend too much time on any one problem. Notes : • Assume that the classes listed in the Quick Refere nce found in the Appendix have been imported where appropriate. • Assume that declarations of variables and methods appear within the context of an enclosing class. • Assume that method calls that are not prefixed with an object or class name and are not shown within a complete class definition appear within the context of an enclosing class. • Unless otherwise noted in the question, assu me that parameters in method calls are not null . -2-
GO ON TO THE NEXT PAGE. 1. Consider the following method. public static int mystery(int[] arr) { int x = 0; for (int k = 0; k < arr.length; k = k + 2) x = x + arr[k]; return x; } Assume that the array nums has been declared and initialized as follows. int[] nums = {3, 6, 1, 0, 1, 4, 2}; What value will be returned as a result of the call mystery(nums) ? (A) 5 (B) 6 (C) 7 (D) 10 (E) 17 -3-
GO ON TO THE NEXT PAGE. Questions 2-3 refer to the following information. Consider the following partial class declaration. public class SomeClass { private int myA; private int myB; private int myC; // Constructor(s) not shown public int getA() { return myA; } public void setB(int value) { myB = value; } } 2. The following declaration appears in another class. SomeClass obj = new SomeClass(); Which of the following code segm ents will compile without error? (A) int x = obj.getA(); (B) int x; obj.getA(x); (C) int x = obj.myA; (D) int x = SomeClass.getA(); (E) int x = getA(obj); 3. Which of the following changes to SomeClass will allow other classes to access but not modify the value of myC ? (A) Make myC public. (B) Include the method: public int getC() { return myC; } (C) Include the method: private int getC() { return myC; } (D) Include the method: public void getC(int x) { x = myC; } (E) Include the method: private void getC(int x) { x = myC; } -4-
GO ON TO THE NEXT PAGE. 4. Consider the following code segment. int x = 7; int y = 3; if ((x < 10) && (y < 0)) System.out.println("Value is: " + x * y); else System.out.println("Value is: " + x / y); What is printed as a result of executing the code segment? (A) Value is: 21 (B) Value is: 2.3333333 (C) Value is: 2 (D) Value is: 0 (E) Value is: 1 -5-
GO ON TO THE NEXT PAGE. 5. Consider the following method. public ArrayList mystery(int n) { ArrayList seq = new ArrayList(); for (int k = 1; k
GO ON TO THE NEXT PAGE. 6. Consider the following method that is intended to determine if the double values d1 and d2 are close enough to be considered equa l. For example, given a tolerance of 0.001 , the values 54.32271 and 54.32294 would be considered equal. /** @return true if d1 and d2 are within the specified tolerance , * false otherwise */ public boolean almostEqual(double d1, double d2, double tolerance) { /* missing code */ } Which of the following should replace /* missing code */ so that almostEqual will work as intended? (A) return (d1 - d2) = tolerance; (E) return Math.abs(d1 - d2)
GO ON TO THE NEXT PAGE. 7. Consider the following class declaration. public class Person { private String myName; private int myYearOfBirth; public Person(String name, int yearOfBirth) { myName = name; myYearOfBirth = yearOfBirth; } public String getName() { return myName; } public void setName(String name) { myName = name; } // There may be instance variables, constr uctors, and methods that are not shown. } Assume that the following declaration has been made. Person student = new Person("Thomas", 1995); Which of the following statements is th e most appropriate for changing the name of student from "Thomas" to "Tom" ? (A) student = new Person("Tom", 1995); (B) student.myName = "Tom"; (C) student.getName("Tom"); (D) student.setName("Tom"); (E) Person.setName("Tom"); -8-
GO ON TO THE NEXT PAGE. 8. Consider the following class declaration. public class Student { private String myName; private int myAge; public Student() { /* implementation not shown */ } public Student(String name, int age) { /* implementation not shown */ } // No other constructors } Which of the following declarations will compile without error? I. Student a = new Student(); II. Student b = new Student("Juan", 15); III. Student c = new Student("Juan", "15"); (A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III -9-
GO ON TO THE NEXT PAGE. 9. Consider the following method that is intended to return the sum of th e elements in the array key . public static int sumArray(int[] key) { int sum = 0; for (int i = 1; i
GO ON TO THE NEXT PAGE. Questions 10-11 refer to the following information. Consider the following instance variable and methods. You may assume that data has been initialized with length > 0. The methods are intended to retu rn the index of an array element equal to target , or -1 if no such element exists. private int[] data; public int seqSearchRec(int target) { return seqSearchRecHelper(target, data.length - 1); } private int seqSearchRecHelper(int target, int last) { // Line 1 if (data[last] == target) return last; else return seqSearchRecHelper(target, last - 1); } 10. For which of the following test cases will the call seqSearchRec(5) always result in an error? I. data contains only one element. II. data does not contain the value 5. III. data contains the value 5 multiple times. (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 11. Which of the following should be used to replace // Line 1 in seqSearchRecHelper so that seqSearchRec will work as intended? (A) if (last = 0) -11-
GO ON TO THE NEXT PAGE. 12. Consider the following method. public String mystery(String input) { String output = ""; for (int k = 1; k < input.length(); k = k + 2) { output += input.substring(k, k + 1); } return output; } What is returned as a result of the call mystery("computer") ? (A) "computer" (B) "cmue" (C) "optr" (D) "ompute" (E) Nothing is returned because an IndexOutOfBoundsException is thrown. -12-
GO ON TO THE NEXT PAGE. 13. Consider the following code segment. int[] arr = {7, 2, 5, 3, 0, 10}; for (int k = 0; k < arr.length - 1; k++) { if (arr[k] > arr[k + 1]) System.out.print(k + " " + arr[k] + " "); } What will be printed as a result of executing the code segment? (A) 0 2 2 3 3 0 (B) 0 7 2 5 3 3 (C) 0 7 2 5 5 10 (D) 1 7 3 5 4 3 (E) 7 2 5 3 3 0 -13-
GO ON TO THE NEXT PAGE. 14. Consider the following interface and class declarations. public interface Vehicle { /** @return the mileage traveled by this Vehicle */ double getMileage(); } public class Fleet { private ArrayList myVehicles; /** @return the mileage traveled by all vehicles in this Fleet */ public double getTotalMileage() { double sum = 0.0; for (Vehicle v : myVehicles) { sum += /* expression */ ; } return sum; } // There may be instance variables, constr uctors, and methods that are not shown. } Which of the following can be used to replace /* expression */ so that getTotalMileage returns the total of the miles traveled for all vehicles in the fleet? (A) getMileage(v) (B) myVehicles[v].getMileage() (C) Vehicle.get(v).getMileage() (D) myVehicles.get(v).getMileage() (E) v.getMileage() -14-
GO ON TO THE NEXT PAGE. 15. Consider the following method, isSorted , which is intended to return true if an array of integers is sorted in nondecreasing order and to return false otherwise. /** @param data an array of integers * @return true if the values in the array appear in sorted (nondecreasing) order */ public static boolean isSorted(int[] data) { /* missing code */ } Which of the following can be used to replace /* missing code */ so that isSorted will work as intended? I. for (int k = 1; k < data.length; k++) { if (data[k - 1] > data[k]) return false; } return true; II. for (int k = 0; k < data.length; k++) { if (data[k] > data[k + 1]) return false; } return true; III. for (int k = 0; k < data.length - 1; k++) { if (data[k] > data[k + 1]) return false; else return true; } return true; (A) I only (B) II only (C) III only (D) I and II only (E) I and III only -15-
GO ON TO THE NEXT PAGE. 16. Consider the following incomplete method that is intended to return an array that contains the contents of its first array parameter followed by the conten ts of its second array parameter. public static int[] append(int[] a1, int[] a2) { int[] result = new int[a1.length + a2.length]; for (int j = 0; j < a1.length; j++) result[j] = a1[j]; for (int k = 0; k < a2.length; k++) result[ /* index */ ] = a2[k]; return result; } Which of the following expressions can be used to replace /* index */ so that append will work as intended? (A) j (B) k (C) k + a1.length - 1 (D) k + a1.length (E) k + a1.length + 1 -16-
GO ON TO THE NEXT PAGE. 17. Consider the following code segment. int[] arr = {1, 2, 3, 4, 5, 6, 7}; for (int k = 3; k < arr.length - 1; k++) arr[k] = arr[k + 1]; Which of the following represents the contents of arr as a result of executing the code segment? (A) {1, 2, 3, 4, 5, 6, 7} (B) {1, 2, 3, 5, 6, 7} (C) {1, 2, 3, 5, 6, 7, 7} (D) {1, 2, 3, 5, 6, 7, 8} (E) {2, 3, 4, 5, 6, 7, 7} -17-
GO ON TO THE NEXT PAGE. 18. Assume that myList is an ArrayList that has been correctly constructed and populated with objects. Which of the following expressions produces a valid random index for myList ? (A) (int)( Math.random() * myList.size() ) - 1 (B) (int)( Math.random() * myList.size() ) (C) (int)( Math.random() * myList.size() ) + 1 (D) (int)( Math.random() * (myList.size() + 1) ) (E) Math.random(myList.size()) -18-
GO ON TO THE NEXT PAGE. 19. Assume that a and b have been defined and initialized as int values. The expression !(!(a != b ) && (b > 7)) is equivalent to which of the following? (A) (a != b) || (b < 7) (B) (a != b) || (b
GO ON TO THE NEXT PAGE. 20. Consider the following method. public static void arrayMethod(int nums[]) { int j = 0; int k = nums.length - 1; while (j < k) { int x = nums[j]; nums[j] = nums[k]; nums[k] = x; j++; k--; } } Which of the following describes what the method arrayMethod() does to the array nums ? (A) The array nums is unchanged. (B) The first value in nums is copied to every location in the array. (C) The last value in nums is copied to every location in the array. (D) The method generates an ArrayIndexOutOfBoundsException . (E) The contents of the array nums are reversed. -20-
GO ON TO THE NEXT PAGE. Questions 21-25 refer to the code from the GridWorld ca se study. A copy of the code is provided in the Appendix. 21. Consider the design of a Grasshopper class that extends Bug . When asked to move, a Grasshopper moves to a randomly chosen empty adjacent location that is within the grid. If there is no empty adjacent location that is within the grid, the Grasshopper does not move, but turns 45 degrees to the right without changing its location. Which method(s) of the Bug class should the Grasshopper class override so that a Grasshopper can behave as described above? I. act() II. move() III. canMove() (A) I only (B) II only (C) I and II only (D) II and III only (E) I, II, and III -21-
GO ON TO THE NEXT PAGE. 22. Assume that gus has been defined and initialized as a Bug object in a class that contains the following code segment. int numTurnsMade = 0; for (int k = 1; k
GO ON TO THE NEXT PAGE. 23. Consider the following method that is intended to return an ArrayList of all the locations in grd that contain actors facing in direction dir . public ArrayList findLocsFacingDir(int dir, Grid grd)\ { ArrayList desiredLocs = new ArrayList(); for (Location loc : grd.getOccupiedLocations()) { if ( /* expression */ == dir ) desiredLocs.add(loc); } return desiredLocs; } Which of the following can be used to replace /* expression */ so that findLocsFacingDir will work as intended? (A) loc.getDirection() (B) getDirection(loc) (C) ((Actor) loc).getDirection() (D) grd(loc).getDirection() (E) grd.get(loc).getDirection() -23-
GO ON TO THE NEXT PAGE. 24. A ColorChangingCritter behaves like a ChameleonCritter but does not turn when it moves. A partial declaration for the ColorChangingCritter class is as follows. public class ColorChangingCritter extends ChameleonCritter { public void makeMove(Location loc) { /* missing code */ } } Which of the following replacements for /* missing code */ will correctly implement the desired behavior? I. moveTo(loc); II. super.super.makeMove(loc); III. int dir = getDirection(); super.makeMove(loc); setDirection(dir); (A) I only (B) III only (C) I and II only (D) I and III only (E) I, II, and III -24-
GO ON TO THE NEXT PAGE. 25. A MunchingCritter acts by selecting one adjacent actor of any type, eating it (removing it from the grid), and moving to occupy its location. If there is no adjacent actor, the MunchingCritter moves like a normal critter. Consider the following three implementations of MunchingCritter . Implementation I public class MunchingCritter extends Critter { private Location eatLoc; // Remember location of critter that was eaten public void processActors(ArrayList actors) { if (actors.size() == 0) eatLoc = null; else { Actor selected = actors.get(0); eatLoc = selected.getLocation(); selected.removeSelfFromGrid(); } } public Location selectMoveLocation(ArrayList locs) { if (eatLoc == null) return super.selectMoveLocation(locs); else return eatLoc; } } Implementation II public class MunchingCritter extends Critter { private Location eatLoc; // Remember location of critter that was eaten public void processActors(ArrayList actors) { if (actors.size() == 0) eatLoc = null; else { Actor selected = actors.get(0); eatLoc = selected.getLocation(); selected.removeSelfFromGrid(); } } public void makeMove(Location loc) { if (eatLoc == null) moveTo(loc); else moveTo(eatLoc); } } -25-
GO ON TO THE NEXT PAGE. Implementation III public class MunchingCritter extends Critter { private boolean hasEaten; // Remember if this critter ate something during this step public void processActors(ArrayList actors) { if (actors.size() == 0) hasEaten = false; else { Actor selected = actors.get(0); Location moveLoc = selected.getLocation(); selected.removeSelfFromGrid(); moveTo(moveLoc); hasEaten = true; } } public void makeMove(Location loc) { if (!hasEaten) moveTo(loc); } } Which of the implementations would be considered to be well designed, in that they satisfy the postconditions in Critter.java ? (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III -26-
GO ON TO THE NEXT PAGE. 26. Assume that the array arr has been defined and initialized as follows. int[] arr = /* initial values for the array */ ; Which of the following will correctly pr int all of the odd integers contained in arr but none of the even integers contained in arr ? (A) for (int x : arr) if (x % 2 == 1) System.out.println(x); (B) for (int k = 1; k < arr.length; k++) if (arr[k] % 2 == 1) System.out.println(arr[k]); (C) for (int x : arr) if (x % 2 == 1) System.out.println(arr[x]); (D) for (int k = 0; k < arr.length; k++) if (arr[k] % 2 == 1) System.out.println(k); (E) for (int x : arr) if (arr[x] % 2 == 1) System.out.println(arr[x]); -27-
GO ON TO THE NEXT PAGE. Questions 27-28 refer to the following method. public static int mystery(int n) { int x = 1; int y = 1; // Point A while (n > 2) { x = x + y; // Point B y = x – y; n--; } // Point C return x; } 27. What value is returned as a result of the call mystery(6) ? (A) 1 (B) 5 (C) 6 (D) 8 (E) 13 28. Which of the following is true of method mystery ? (A) x will sometimes be 1 at // Point B . (B) x will never be 1 at // Point C . (C) n will never be greater than 2 at // Point A . (D) n will sometimes be greater than 2 at // Point C . (E) n will always be greater than 2 at // Point B . -28-
GO ON TO THE NEXT PAGE. 29. Consider the following code segment. for (int k = 1; k
GO ON TO THE NEXT PAGE. 30. Consider the following method. public static String scramble(String word, int howFar) { return word.substring(howFar + 1, word.length()) + word.substring(0, howFar); } What value is returned as a result of the call scramble("compiler", 3) ? (A) "compiler" (B) "pilercom" (C) "ilercom" (D) "ilercomp" (E) No value is returned because an IndexOutOfBoundsException will be thrown. -30-
GO ON TO THE NEXT PAGE. 31. Consider the following method. public void mystery(int[] data) { for (int k = 0; k < data.length - 1; k++) data[k + 1] = data[k] + data[k + 1]; } The following code segment appears in another method in the same class. int[] values = {5, 2, 1, 3, 8}; mystery(values); for (int v : values) System.out.print(v + " "); System.out.println(); What is printed as a result of executing the code segment? (A) 5 2 1 3 8 (B) 5 7 3 4 11 (C) 5 7 8 11 19 (D) 7 3 4 11 8 (E) Nothing is printed because an ArrayIndexOutOfBoundsException is thrown during the execution of method mystery . -31-
GO ON TO THE NEXT PAGE. 32. Consider the following method. public int compute(int n, int k) { int answer = 1; for (int i = 1; i
GO ON TO THE NEXT PAGE. 33. Consider the following code segment. int sum = 0; int k = 1; while (sum < 12 || k < 4) sum += k; System.out.println(sum); What is printed as a result of executing the code segment? (A) 6 (B) 10 (C) 12 (D) 15 (E) Nothing is printed due to an infinite loop. -33-
GO ON TO THE NEXT PAGE. 34. Consider the following class declarations. public class Point { private double x; // x-coordinate private double y; // y-coordinate public Point() { x = 0; y = 0; } public Point(double a, double b) { x = a; y = b; } // There may be instance variables, constr uctors, and methods that are not shown. } public class Circle { private Point center; private double radius; /** Constructs a circle where (a, b) is the center and r is the radius. */ public Circle(double a, double b, double r) { /* missing code */ } } Which of the following replacements for /* missing code */ will correctly implement the Circle constructor? I. center = new Point(); radius = r; II. center = new Point(a, b); radius = r; III. center = new Point(); center.x = a; center.y = b; radius = r; (A) I only (B) II only (C) III only (D) II and III only (E) I, II, and III -34-
GO ON TO THE NEXT PAGE. 35. Consider the following code segment. int num = 2574; int result = 0; while (num > 0) { result = result * 10 + num % 10; num /= 10; } System.out.println(result); What is printed as a result of executing the code segment? (A) 2 (B) 4 (C) 18 (D) 2574 (E) 4752 -35-
GO ON TO THE NEXT PAGE. 36. Consider the following method. public void test(int x) { int y; if (x % 2 == 0) y = 3; else if (x > 9) y = 5; else y = 1; System.out.println("y = " + y); } Which of the following test data sets would test each possible output for the method? (A) 8, 9, 12 (B) 7, 9, 11 (C) 8, 9, 11 (D) 8, 11, 13 (E) 7, 9, 10 -36-
GO ON TO THE NEXT PAGE. 37. Consider the following code segment. int x = 1; while ( /* missing code */ ) { System.out.print(x + " "); x = x + 2; } Consider the following possible replacements for /* missing code */ . I. x < 6 II. x != 6 III. x < 7 Which of the proposed replacements for /* missing code */ will cause the code segment to print only the values 1 3 5 ? (A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III -37-
GO ON TO THE NEXT PAGE. 38. Assume that x and y have been declared and initialized with int values. Consider the following Java expression. (y > 10000) || (x > 1000 && x < 1500) Which of the following is equivalent to the expression given above? (A) (y > 10000 || x > 1000) && (y > 10000 || x < 1500) (B) (y > 10000 || x > 1000) || (y > 10000 || x < 1500) (C) (y > 10000) && (x > 1000 || x < 1500) (D) (y > 10000 && x > 1000) || (y > 10000 && x < 1500) (E) (y > 10000 && x > 1000) && (y > 10000 && x < 1500) -38-
GO ON TO THE NEXT PAGE. 39. Consider the following recursive method. public int recur(int n) { if (n
40. Consider the following recursive method. public static void whatsItDo(String str) { int len = str.length(); if (len > 1) { String temp = str.substring(0, len – 1); whatsItDo(temp); System.out.println(temp); } } What is printed as a result of the call whatsItDo("WATCH") ? (A) WATC WAT WA W (B) WATCH WATC WAT WA (C) W WA WAT WATC (D) W WA WAT WATC WATCH (E) WATCH WATC WAT WA W WA WAT WATC WATCH END OF SECTION I IF YOU FINISH BEFORE TIME IS CALLED, YOU MAY CHECK YOUR WORK ON THIS SECTION. DO NOT GO ON TO SECTION II UNT IL YOU ARE TOLD TO DO SO. -40-
Section II Free-Response Questions -41-
GO ON TO THE NEXT PAGE. COMPUTER SCIENCE A SECTION II Time — 1 hour and 45 minutes Number of questions — 4 Percent of total grade — 50 Directions: SHOW ALL YOUR WORK. REMEMBER THAT PROGRAM SEGMENTS ARE TO BE WRITTEN IN JAVA. Notes : • Assume that the classes listed in the Quick Refere nce found in the Appendix have been imported where appropriate. • Unless otherwise noted in the question, assume that parameters in method calls are not null and that methods are called only when their preconditions are satisfied. • In writing solutions for each question, you may use any of the accessible methods that are listed in classes defined in that question. Writing significant amounts of code that can be replaced by a call to one of these methods may not receive full credit. 1. Consider the following partial declaration for a WordScrambler class. The constructor for the WordScrambler class takes an even-length array of String objects and initializes the instance variable scrambledWords . public class WordScrambler { private String[] scrambledWords; /** @param wordArr an array of String objects * Precondition: wordArr.length is even */ public WordScrambler(String[] wordArr) { scrambledWords = mixedWords(wordArr); } /** @param word1 a String of characters * @param word2 a String of characters * @return a String that contains the first half of word1 and the second half of word2 */ private String recombine(String word1, String word2) { /* to be implemented in part (a) */ } /** @param words an array of String objects * Precondition: words.length is even * @return an array of String objects created by recombining pairs of strings in array words * Postcondition : the length of the returned array is words.length */ private String[] mixedWords(String[] words) { /* to be implemented in part (b) */ } // There may be instance variables, constructors, and methods that are not shown. } -42-
GO ON TO THE NEXT PAGE. (a) Write the WordScrambler method recombine . This method returns a String created from its two String parameters as follows. • take the first half of word1 • take the second half of word2 • concatenate the two halves and return the new string. For example, the following table shows some results of calling recombine. Note that if a word has an odd number of letters, the second half of the word contains the extra letter. word1 word2 recombine(word1, word2) "apple" "pear" "apar" "pear" "apple" "peple" Complete method recombine below. /** @param word1 a String of characters * @param word2 a String of characters * @return a String that contains the first half of word1 and the second half of word2 */ private String recombine(String word1, String word2) (b) Write the WordScrambler method mixedWords . This method creates and returns a new array of String objects as follows. It takes the first pair of strings in words and combines them to produce a pair of strings to be included in the array returned by the method. If this pair of strings consists of w1 and w2 , the method should include the result of calling recombine with w1 and w2 as arguments and should also include the result of calling recombine with w2 and w1 as arguments. The next two strings, if they exist, would form the next pair to be processed by this method. The method should continue until all the strings in words have been processed in this way and the new array has been filled. For example, if the array words contains the following elements: {"apple", "pear", "this", "cat"} then the call mixedWords(words) should return the following array. {"apar", "peple", "that", "cis"} In writing mixedWords , you may call recombine . Assume that recombine works as specified, regardless of what you wrote in part (a). Complete method mixedWords below. /** @param words an array of String objects * Precondition: words.length is even * @return an array of String objects created by recombining pairs of strings in array words * Postcondition : the length of the returned array is words.length */ private String[] mixedWords(String[] words) -43-
GO ON TO THE NEXT PAGE. 2. An array of positive integer values has the mountain property if the elements are ordered such that successive values increase until a maximum value (the peak of the mountain) is reached and then the successive values decrease. The Mountain class declaration shown below contains methods that can be used to determine if an array has the mountain property. You will implement two methods in the Mountain class. public class Mountain { /** @param array an array of positive integer values * @param stop the last index to check * Precondition: 0 £stop < array.length * @return true if for each j such that 0 £j < stop , array[j] < array[j + 1] ; * false otherwise */ public static boolean isIncreasing(int[] array, int stop) { /* implementation not shown */ } /** @param array an array of positive integer values * @param start the first index to check * Precondition: 0 £start < array.length - 1 * @return true if for each j such that start £j < array.length - 1 , * array[j] > array[j + 1] ; * false otherwise */ public static boolean isDecreasing(int[] array, int start) { /* implementation not shown */ } /** @param array an array of positive integer values * Precondition: array.length > 0 * @return the index of the first peak (local maximum) in the array, if it exists; * -1 otherwise */ public static int getPeakIndex(int[] array) { /* to be implemented in part (a) */ } /** @param array an array of positive integer values * Precondition: array.length > 0 * @return true if array contains values ordered as a mountain; * false otherwise */ public static boolean isMountain(int[] array) { /* to be implemented in part (b) */ } // There may be instance variables, constructors, and methods that are not shown. } -44-
GO ON TO THE NEXT PAGE. (a) Write the Mountain method getPeakIndex . Method getPeakIndex returns the index of the first peak found in the parameter array, if one exists. A peak is defined as an element whose value is greater than the value of the element immediately before it and is also greater than the value of the element immediately after it. Method getPeakIndex starts at the beginning of the array and returns the index of the first peak that is found or -1 if no peak is found. For example, the following table illustrates the results of several calls to getPeakIndex. arr getPeakIndex(arr) {11, 22, 33, 22, 11} 2 {11, 22, 11, 22, 11} 1 {11, 22, 33, 55, 77} -1 {99, 33, 55, 77, 120} -1 {99, 33, 55, 77, 55} 3 {33, 22, 11} -1 Complete method getPeakIndex below. /** @param array an array of positive integer values * Precondition: array.length > 0 * @return the index of the first peak (local maximum) in the array, if it exists; * -1 otherwise */ public static int getPeakIndex(int[] array) -45-
GO ON TO THE NEXT PAGE. (b) Write the Mountain method isMountain . Method isMountain returns true if the values in the parameter array are ordered as a mountain; otherwise, it returns false. The values in array are ordered as a mountain if all three of the following conditions hold. • There must be a peak. • The array elements with an index smaller than the peak’s index must appear in increasing order. • The array elements with an index larger than the peak’s index must appear in decreasing order. For example, the following table illustrates the results of several calls to isMountain. arr isMountain(arr) {1, 2, 3, 2, 1} true {1, 2, 1, 2, 1} false {1, 2, 3, 1, 5} false {1, 4, 2, 1, 0} true {9, 3, 5, 7, 5} false {3, 2, 1} false In writing isMountain , assume that getPeakIndex works as specified, regardless of what you wrote in part (a). Complete method isMountain below. /** @param array an array of positive integer values * Precondition: array.length > 0 * @return true if array contains values ordered as a mountain; * false otherwise */ public static boolean isMountain(int[] array) -46-
GO ON TO THE NEXT PAGE. 3. This question involves reasoning about the code from the GridWorld case study. A copy of the code is provided as part of this exam. A Grub is a Critter that burrows from one location to another. A Grub knows how far away it can burrow and randomly chooses the direction in which to burrow. It burrows down from its current location and burrows up at some target location. If the target location is empty or contains a Flower, the Grub moves to this location. If the target location contains any other type of object, the Grub gets stuck underground and dies. You will implement three of the methods in the following Grub class. public class Grub extends Critter { private int maxDistance; public Grub(int distance) { maxDistance = distance; } /** @return one of the eight direction constants from the Location class */ public int getRandomDirection() { /* to be implemented in part (a) */ } /** Gets a list of possible locations for the next move. These locations must be valid * in the grid of this Grub. Implemented to return all locations in a random direction * up to and including the maximum distance that this Grub can burrow. * Postcondition : The state of all actors is unchanged. * @return a list of all locations within the maximum distance in a randomly selected direction */ public ArrayList getMoveLocations() { /* to be implemented in part (b) */ } /** Selects the location for the next move. * Postcondition : (1) The returned location is an element of locs, this critter's current location, * or null . (2) The state of all actors is unchanged. * @param locs the possible locations for the next move * @return the location that was selected for the next move, or null to indicate * that this Grub should be removed from the grid. */ public Location selectMoveLocation(ArrayList locs) { /* to be implemented in part (c) */ } // There may be instance variables, constructors, and methods that are not shown. } -47-
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