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CS 240 Exam 1
Page 1
Computer Science II
Spring 2012, Wednesday, 2/8/12
100 points
Name _____________________________
2/8/12
1. Miscellaneous, basic Java concepts. Fill in the blanks.
[10 pts]
The source code of the Java class Test1 should be found in a file called ________________________ and its
compiled bytecodes are found in _______________________________.
A subclass Test1a of Test1 would have a header line public class Test1a ____________ Test1 {
All top level user defined Java classes inherit from the class called ___________.
The automatic recovery of memory used by objects no longer accessible through any references is called
____________________________
Give two other Java primitive data types other than int: __________________ and ____________________.
The method used to lexicographically compare Strings (i.e., determine order) is ___________________.
The method inherited from that top level class that should be overridden to provide some status/value
presentation for printing, usually, is called _________________.
2. Number the steps of the software development cycle (Waterfall method) from 1 to 6 in correct order.
[5 pts]
_____ Implementation
_____ Design
_____ Maintenance
_____ Requirements identification
_____ Testing
_____ Analysis
3. Object-oriented programming design principles. (True/False)
[10 pts]
a. ____ An Abstract Data Type hides its data structure organization and exposes its operations.
b. ____ Reusability is a quality software goal in object-oriented class design.
c. ____ A method that returns an object can be called as part of a cascade of method calls.
d. ____ The this instance reference in a method can always be omitted.
e. ____ A constructor in a subclass must call its superclass constructor using super.
f.
____ The information hiding principle prevents the user of a class from knowing the details of its
implementation and the implementer generally does not know how the class is used.
g. ____ A precondition defines what must initially hold for a method to correctly carry out its task.
h. ____ A class may only define one method for each unique method name.
i.
____ A protected instance variable is like a private variable, but accessible by the subclass.
j.
____ A variable designated as static in a class makes it a class variable of which there is only one instance.
CS 240 Exam 1
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4. Arrays. For this question just give segments of Java code; not entire methods or classes.
[18 pts]
a. Assume there exists an array of Sound objects called soundClips[ ]. Give a Java code segment to 1) create
a second array of Sound objects called reversedSounds with soundClips.length elements and 2) use a
for loop to initialize each element of reversedSounds to the reversed sound clips in the soundClips
array using the reverse() method (it returns a Sound object). [9]
b. Give a Java code segment to print the position/index of the longest soundClips object using the getLength()
method. [9]
5. Fill in Java code in the method makeDarker() below to modify a Picture object that darkens the original. This
is done by halving the red, green, blue levels.
[15 pts]
public _____________ makeDarker() {
Pixel [] pixels = _______ . getPixels();
for(Pixel pix : _________ ){
____.setRed ( pix.___________() __
____.setGreen ( pix.___________() __
____.setBlue ( pix.___________() __
________ );
________);
________);
}
}
Would a call to makeLighter() that doubles the levels effectively restore the Picture object?
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6. Complete the method mirror() that flips the image as a mirror image. The pixels on a horizontal line are
swapped.
[10 pts]
public void mirror(){
Pixel leftPix; //need to hold pixels to swap
Pixel rightPix;
Pixel temp;
int width = this.___________();
for( _____ y = 0; y<this.getHeight(); y++){
//swap left half with right half
for( ________ x = 0; x<this. ___________ / 2 ; x++){
leftPix = this.getPixel(x,y);
rightPix = this.getPixel (width _____________ , y);
//swap their colors
temp = _____________________;
rightPix. _____________
( __________________. _____________());
leftPix.setColor(rightPix.getColor());
}
}
}
7. Below is a main program to draw something random with turtles.
a. Draw a possible scenario from executing this code to the right. This is testing your code reading.
[10 pts]
public class SomethingByTurtles {
private static final int N = 4;
public static void main(String[] args){
World myWorld = new World();
Turtle [] myTurtles = new Turtle[N];
Turtle p,s; //temporary references
int head = 0;
for (int i=0; i < N; i++) {
myTurtles[i] = new Turtle(myWorld);
}
for (int i=0; i < N; i++) {
head += ((int) (80 * Math.random())+10);
s = myTurtles[i];
s.turn(head);
s.penDown();
s.forward(100);
}
for (int j=0; j < N-1; j++) {
s = myTurtles[j];
p = myTurtles[j+1];
s.turnToFace(p);
int dist =(int)(s.getDistance(p.getXPos(),p.getYPos()));
s.forward(dist);
s.forward(-dist);
}
}
}
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b. The assignments s = myTurtles[j] and p = myTurtles[j+1] are using shallow copying? Why is this the
preferred mode? What would be the effect of the program if deep copying were being used, if any?
[10 pts]
8. Below is the SlowWalkingTurtle class. Answer the questions about this code listed below.
[12 pts]
class SlowWalkingTurtle extends Turtle {
public SlowWalkingTurtle (ModelDisplay modelDisplay) {
super(modelDisplay);
}
public void forward(int pixels) {
super.forward(pixels);
try {
Thread.sleep(500); // Wait a half sec
} catch (Exception e) {
System.err.println(“Exception: “ + e.getMessage());
System.err.println(“Stack Trace is:”);
e.printStackTrace();
}
}
}
a. What is the constructor doing?
b. Explain the super.forward(pixels) statement.
c. Why is the Thread.sleep(500) in a try-catch block?
d. What are the statements of the catch block imitating?