Midterm Solutions. Instructor: Scott Kristjanson CMPT 125/125 SFU Burnaby, Fall 2013

Transcription

1 Midterm Solutions Instructor: Scott Kristjanson CMP 125/125 SU Burnaby, all 2013

2 2 Question 1 1 Mark if checked something that should be checked 1 Mark if left unchecked, something that should not be checked Work out the clues: Asked to identify epressions, not Statements Cannot use the operator with Strings Casting (int), (double) are value epression operators Conditional Epressions are valid epressions he first one is a boolean epression with the logical OR operator Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 2

3 3 Question 2 (A &&!B &&!C) Work out the logic one step at a time: rue when only one is true So if A is true, B or C must be false => A && ( B==ALSE && C==ALSE) => A && (!B &&!C) => A &&!B &&!C Similarly, if B is the one that is true => B &&!A &&!C Similarly, if C is the one that is true => C &&!A &&!B rue if and only if one of these is true Combine the three epressions using OR => A &&!B &&!C B &&!A &&!C C &&!A &&!B Check precedence! Need to add parenthesis? No! But will add them to make the logic clearer (!A && B &&!C) (!A &&!B && C) 1 Mark 1 Mark 1 Mark 1 Mark 1 Mark A B C Validate using a ruth able (A&&!B&&!C) (!A&&B&&!C) (!A&&!B&&C) Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 3

4 4 Question 3 3 Marks Polymorphism means of many forms 2 Marks he method invoked is determined using late binding based on the object being referenced, rather than the class of the reference itself. Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 4

5 5 Question 4 Multiple Choice 2 Marks if got the one right answer 1 Mark if got the right answer but also circled a wrong answer 0 Marks if did not get the right answer, or circled more than 2 Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 5

6 6 Question 4 Multiple Choice 2 Marks if got the one right answer 1 Mark if got the right answer but also circled a wrong answer 0 Marks if did not get the right answer, or circled more than 2 Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 6

7 7 Question 4 Multiple Choice Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 7

8 8 Question 5 low of Control 5 Marks Java ignores the indenting, so should you! Match up the if-then-else statements No {} around second if as implied by indenting => first Else matches up with second if, not the first! hird if-then-else is part of the 2 nd if too Since (a < b) is false, every if-then-else gets skipped => Print1, Print2 and Print3 do not get eecuted Print4 is not part of ANY if statement ({} missing) => Print4 must get printed Print4 is not doing addition, so not 111 Print4 doing String Concatenation => will print => Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 8

9 9 Question 6 - Create a String concat Method public static String concat(string a, String b) { int alen = a.length(); // Only need check for double-chars if both have 1 or more chars if ((alen >0) && (b.length()>0)) // Check if last char in a equals first char in b if (a.substring(alen-1,alen).equals(b.substring(0,1))) // o eliminate the double-char, remove last char of a a = a.substring(0,alen-1); return a+b; } Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 9

10 10 Question 7 What is the output? public class est { public static void main (String[] args){ int a = 0; int[] b = {0, 1, 2}; int[] c = {5, 6, 7}; helper(a,b,c); System.out.println(" a = " + a ); System.out.println(" b[2] = " + b[2]); System.out.println(" c[2] = " + c[2]); } private static void helper (int, int[]y, int z[]) { } = 100; y[2] = 201; z = new int[3]; z[2] = 301; } a = 0 b[2] = 201 c[2] = 7 5 Marks for each correct output line Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 10

11 11 Question 8 Working with 2-Dimensional Arrays Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 11

12 12 Question 8 Working with 2-Dimensional Arrays // mat is an object reference, so caller will see any // changes made here. herefore we should return void. public static void addoirstcol (int intval, int[][] mat) { for (int i=0; i<mat.length; i++) if ((mat[i]!= null) && (mat[i].length>0)) mat[i][0] += intval; } Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 12

13 13 Question 9 What is the output? public class ZooKeeper { public static void main(string args[]) { Animal bear = new Animal("Bear", "Grrrr!"); Animal[] animals = {new Bee(), new Lion(), new MountainLion()}; bear.speak(); for(int i=0; i<animals.length; i++) animals[i].speak(); }} public class Animal { protected String animalype; private String sound; public Animal(String animalype, String sound) { this.animalype = animalype; Bears say Grrrr! Bees say Buzz Buzz Big Cats ROAR! Lions purr this.sound = sound; } public void speak() { System.out.println(animalype + "s say " + sound);}} 5 Marks for each correct output line public class Bee etends Animal { public Bee() { super("bee", "Buzz Buzz"); } } public class Lion etends Animal { public Lion () { super("lion", "Meow"); } public void speak() { System.out.println("Big Cats ROAR!"); }} public class MountainLion etends Lion { public MountainLion() {super();} public void speak() {System.out.println(animalype + "s purr");}} Scott Kristjanson CMP 125/126 SU Midterm Solutions Slide 13