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

2. something that should not be checked
Question 1 √ x x √
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 expressions, not Statements
Cannot use the – operator with Strings
Casting (int), (double) are value expression operators
Conditional Expressions are valid expressions
The first one is a boolean expression with the logical OR operator

3. (A&&!B&&!C)||(!A&&B&&!C)||(!A&&!B&&C)
Question 2
(A && !B && !C)
|| (!A && B && !C)
|| (!A && !B && C)
1 Mark
1 Mark
1 Mark
1 Mark
1 Mark
Validate using a Truth Table
Work out the logic one step at a time:
True when only one is true
So if A is true, B or C must be false
=> A && ( B==FALSE && C==FALSE)
=> 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
True if and only if one of these is true
Combine the three expressions 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)||(!A&&B&&!C)||(!A&&!B&&C) F T

4. Question 3 3 Marks – Polymorphism means “of many forms”
2 Marks – The method invoked is determined using late binding based on the object being referenced, rather than the class of the reference itself.

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

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

7. Question 4 – Multiple Choice

8. Question 5 – Flow of Control 5 Marks2 x
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!
Third if-then-else is part of the 2nd if too
Since (a < b) is false, every if-then-else gets skipped
=> Print1, Print2 and Print3 do not get executed
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 “100”+”10” +”1”=> “100101” 5 √ x x 4 3 x x

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)))
// To eliminate the double-char, remove last char of a
a = a.substring(0,aLen-1);
return a+b; }

10. Question 7 – What is the output?
public class Test {
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 x, int[]y, int z[]) {
x = 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

11. Question 8 – Working with 2-Dimensional Arrays

12. Question 8 – Working with 2-Dimensional Arrays
// mat is an object reference, so caller will see any
// changes made here. Therefore we should return void.
public static void addToFirstCol (int intVal, int[][] mat) {
for (int i=0; i<mat.length; i++)
if ((mat[i] != null) && (mat[i].length>0))
mat[i][0] += intVal; }

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 animalType;
private String sound;
public Animal(String animalType, String sound) {
this.animalType = animalType;
this.sound = sound; }
public void speak() {
System.out.println(animalType + "s say " + sound);}}
public class Bee extends Animal {
public Bee() { super("Bee", "Buzz Buzz"); } }
public class Lion extends Animal {
public Lion () { super("Lion", "Meow"); }
public void speak() { System.out.println("Big Cats ROAR!"); }}
public class MountainLion extends Lion {
public MountainLion() {super();}
public void speak() {System.out.println(animalType + "s purr");}}
Bears say Grrrr!
Bees say Buzz Buzz
Big Cats ROAR!
Lions purr
5 Marks for each correct output line

14. Grades
This is the grade distribution for the class Midterm: