CSCI 1226 Second Midterm Test

Transcription

1 CSCI 1226 Second Midterm test General Instructions Read and follow all directions carefully. CSCI 1226 Second Midterm Test Wednesday, November 21, 2018 Name: Student #: You may use Sop and Sopln as abbreviations for System.out.print/println. You do not need to add comments to code unless I ask for them. You may use magic numbers in your code unless I say otherwise. There are 70 points available. You have 75 minutes to complete it. Good luck. Question Score Out of TOTAL 70 1 of 8

2 1. [16] Declare the following. You do not need to provide any constructors or method definitions. a) An array to hold up to 200 numbers with decimal points. b) The instance variables of a class to hold information on an element its name, chemical symbol, and weight. For example: hydrogen is named Hydrogen, has the symbol H and a weight of public class Element {... c) The constants and/or variables required for a Time class: the number of hours per day (24), minutes per hour (60), and seconds per minute (60). public class Time {... d) An array holding the names of the Three Stooges (Larry, Curly and Moe you may substitute Shemp in the appropriate place, if you prefer). 2 of 8

3 2. [12] Suppose we have a Car class where the car objects track the distance they ve traveled and the amount of gas they have left. (For examples, see the two Car objects to the right.) a) Write a getter for the variable distance. b) Write a setter for the variable gas. Don t accept requests to change the gas to a negative value. (For example, if the client tries to set the gas level to -1.5, the setter does nothing.) c) Write a tostring method for this class. The method returns a String with both instance variables. For example, mycar.tostring() returns Car: 10.0km 25.5L and yourcar.tostring() returns Car: 30.2km 0.6L. 3 of 8

4 3. [8] Consider the following partial class definition: public class Student { private int yearadmitted; private String name; private String address; private int numcoursestaken; private String[] coursestaken; ; /* more for you to provide below */ Write a constructor for the class it takes a year, a name, and an address and sets the corresponding fields of the new object. It initializes the number of courses taken to zero, but makes space for up to ten courses to be added later. 4. [4] Suppose we have given the following declarations. public class MyList { public int numentries; // NOTE: public, not private public String[] entry; // NOTE: public, not private /* remainder of class definition hidden */ ; MyList[] arr = new MyList[100]; /* arr initialized and filled in details hidden */ Give the data type of each of the following expressions. a) arr[5] b) arr[5].numentries c) arr[5].entry[4] d) arr[5].entry[4].startswith( A ) 4 of 8

5 5. [4] What does the array arr contain after the following code is executed? int[] arr = new int[5]; int j = 3; for (int i = 4; i >= 0; i--) { arr[i] = j; j++; 6. [8] Write a Java method called zerocount that takes an array of integer values as its parameter and returns a count of the number of zeros in the array. For example, given the array myarray as defined below, zerocount should return 2 since the number of zeros in myarray is equal to 2. The output of the following code fragment should also be 2. int [] myarray = {4, 5, 6, 0, 2, 1, 0; int count = zerocount(myarray); System.out.println(count); 5 of 8

6 7. [8] Write a method fillarray which takes an integer value n and a double value d and returns an array of n elements, each initialized to d. For example: double[] da1 = ArrayOperations.fillArray(5, 2.3); double[] da2 = ArrayOperations.fillArray(3, -5.0); After running that code, the following code System.out.println(Arrays.toString(da1)); System.out.println(Arrays.toString(da2)); should print out [2.3, 2.3, 2.3, 2.3, 2.3] [-5.0, -5.0, -5.0] 8. [10] Multiple Choice: select the best available answer from the options shown. If we create an array with 200 elements, those components will be numbered from a) 0 to 199. b) 0 to 200. c) 1 to 199. d) 1 to 200. e) 1 to 201. An array consists of several variables of the same type. Each of those variables is called of the array. a) a component b) an element c) an index d) (a or b) e) (b or c) 6 of 8

7 CSCI 1226 Second Midterm test If MyList is a class, and list1 is a variable of type MyList, then list1 is called a) a class variable. b) a field. c) an instance variable. When a variable is declared private, that means that a) anyone can access that variable. d) an object. e) a record. b) each object of that class has its own copy of that variable. c) it is shared by all instances of this class. d) its value will never change. e) no other class can use it. What is the output of the following code? Car car1 = new Car(2010, Toyota, Prius, green ); Car car2 = new Car(1987, Ford, Pinto, black ); Car car3 = car1; car3.setcolour( blue ); System.out.print(car1.getColour() + + car2.getcolour() + + car3.getcolour()); a) blue black blue b) blue black green c) green black blue d) green black green e) green blue blue Which of the following variables should be declared private? a) arguments b) instance variables c) local variables d) parameters e) (all of the above should be declared private.) For a Student object we need to track both their percentage grade and their letter grade. However, the letter grade can easily be calculated from the percentage grade (but not the other way around). Which instance variables should we create? a) Both the letter grade and the percentage grade. b) Just the letter grade. c) Just the percentage grade. d) One variable to hold both values. e) (Any of the above would be OK.) 7 of 8

8 What happens if we try to run the following code? int[] a = new int[10]; for (int i = 0; i <= 10; i++) { System.out.println(a[i]); a) it crashes with an ArrayIndexOutOfBoundsException. b) it prints ten lines, each with a 0 (zero) on it. c) it prints the numbers 0 to 9 each on its own line. d) nothing happens, because the array has not been initialized. e) nothing happens, because there is a syntax error. If we want to assign consecutive student numbers to Student objects, then the Student class should contain the declarations: a) int NUMBER = 1++; b) private final int NUMBER; public static int nextnumber = 1; c) private static int NUMBER; public final int nextnumber = 1; d) public final int NUMBER; private static int nextnumber = 1; e) public static int NUMBER; private final int nextnumber = 1; What is wrong with the following code? Student[] myclass = new Student[NUM_STUDENTS]; for (int i = 0; i < NUM_STUDENTS; i++) { System.out.println(myClass[i].getName()); a) myclass[i].getname() should be replaced by myclass.getname[i]. b) NUM_STUDENTS should be replaced by myclass.length in the first line. c) NUM_STUDENTS should be replaced by myclass.length in the second line. d) The < sign should be replaced by <= in the second line. e) The Student objects in the array need to be initialized before they can be printed. 8 of 8