Computer Science 112a

Object Oriented Programming and Data Structures
Ileana Streinu
Fall 1997
Midterm examination
October 9, 1995

Name:

112a-ax account:
The exam is open book: you may consult your notes, books, even computer 
programs that you have written before. However you may not consult with 
your colleagues. All the work should be yours.

The exam has two parts. Each should take you roughly one hour. 

Part 1 is to be done in class without the use of a computer (10:30-11:50).
Part 2 involves writing and running Java applets.  You will do this in in the 
Seelye 212 lab in the afternoon, 1:00 to 2:50. You will logon to hermite and type 
in, compile and debug  three short Java applets. Name them according to the 
instructions and submit by turning the permissions on at 2:50pm.

Do first the problems that you consider to be the easiest, then allow time for 
the most difficult  ones. Show all your work: you may get partial credit. Add 
extra sheets if necessary.

Part 1. (50 points)
1. (6 points) The following are parts of a Java program:

final int circle = 1;
final int square = 2;
final int rectangle = 3;
final int line = 4;

.....

if (shape =  1) g.drawOval(10, 10, 5, 5);
else if (shape = 2) g.drawRect(10, 10, 10, 10);
else if (shape = 3) g. drawRect(10, 10, 20, 30);
else g.drawLine(10, 10, 20, 20);

Replace the if... else if... sequence with a switch statement, so that the 
program will still be doing the same thing.






2.  (12 points) You are given the following Java applet (some details are 
missing, but you should be able to infer the global structure):

import java.awt.*;
import java.applet.*;

public class myGizmo extends Applet{

int shape;
Color myColor;
Color yourColor;

public void init()
{
myColor = Color.red;
System.out.println(ÒInitializing...Ó);
}

...

void colorIt(Color myColor)
{
...
yourColor = myColor;
System.out.println(ÒColoring....Ó);
...
}

void eraseColor()
{
...
Color myColor = yourColor;
System.out.println(ÒErasing...Ó);
...
}


}


(6 points) For each occurrence of the myColor  variable, write next to it 
whether that is referring to: 
1. an instance variable ; 
2. a parameter to some method; 
3. a local variable  (and explain: local to what method?)






(3 points) When is the  init method executed? (One sentence suffices).






(3 points) Where are printed the values in the System.out.println(...) 
statements? (Two words suffice!).






3.(6 points)  Write down the global structure of a Java applet (instance 
variables and methods that have to be overridden) which contains a button. 
When the user clicks on the button, some graphics is drawn. You do not need to 
be too precise about syntax - I only want to see the global structure. Make sure 
you include all the methods needed to implement this sort of functionality, and 
no more than this.































4. (6 points) Extending the class hierarchy. The following example of a 
class definition in Java is taken directly from your textbook: you are familiar 
with it!

class Motorcycle {

  String make;
  String color;
  boolean engineState;

  void startEngine() {
     if (engineState == true)
         System.out.println("The engine is already on.");
     else {
         engineState = true;
         System.out.println("The engine is now on.");
     }
  }
  
  void showAtts() {
    System.out.println("This motorcycle is a "
        + color + " " + make);
    if (engineState == true)
        System.out.println("The engine is on.");
    else System.out.println("The engine is off.");
  }

  public static void main (String args[]) {
    Motorcycle m = new Motorcycle();
    m.make = "Yamaha RZ350";
    m.color = "yellow";
    System.out.println("Calling showAtts...");
    m.showAtts();
    System.out.println("--------");
    System.out.println("Starting engine...");
    m.startEngine();
    System.out.println("--------");
    System.out.println("Calling showAtts...");
    m.showAtts();
    System.out.println("--------");
    System.out.println("Starting engine...");
    m.startEngine();
  }
}

Your task now is to define a subclass of the Motorcycle class (letÕs call it 
Harley) which:

a. inherits from Motorcycle the instance variables.
b. overrides the startEngine method with another one, which, in addition to 
what this one is already doing, prints a message such as ÒBRRRRRÓ indicating 
noise when the engine is started.
3. has a new method showOff, which prints a message ÒLook at me!Ó if the color 
of the motorcycle is ÒredÓ.

You do not have to worry about details such as the public/private modifiers or 
syntax. I just want to see the structure of the new program. Write it in the 
space below. Do not include the main method, just the new class Harley.


















































5. (10 points) Write a short Java application where everything happens in the 
main method. There, do the following things:

a. define a variable of type array of integers, named myIntegers.
b. define a variable of the same type named mySquares.
b. allocate space for them, in the amount of 20 units.
c. use a for loop to initialize myIntegers to the first 20 integers from 1 to 20 
(careful, there is a slight subtlety here!).
d. now compute in mySquares the squares of these numbers (i.e. 1, 4, 9, 16, 
etc...). Use ultiplication x*x to get the square of a number.
e. print (using System.out.println) the square values one after another.











































6.(6 points) With reference to the previous problem. Expand now your 
program with a method that will copy one array into another one. This method 
should be called copyArray, should take two parameters, sourceArray and 
destArray, and should just copy - element by element - the contents of 
sourceArray into destArray. In the main method, use this method to copy the 
contents of the array myArray into a new one (which you will have to create!) 
called newArray. 

I am interested in the global structure, so do not worry about details such as 
how parameters are passed, for instance.












































7. (6 points)  What is the following Java applet doing? (A short sentence 
suffices!)

import java.awt.*;
import java.applet.*;

public class whatDoing extends Applet{

public void init(){
setLayout(new GridLayout(3,2);
add(new Button(ÒOneÓ));
add(new Button(ÒTwoÓ));
add(new Button(ÒThreeÓ));
add(new Button(ÒFourÓ));
add(new Button(ÒFiveÓ));
add(new Button(ÒSixÓ));
}
}








What would happen if you dropped the first statement setLayout(...)? (One 
sentence suffices.)


























Computer Science 112a

Object Oriented Programming and Data Structures
Ileana Streinu
Fall 1997
Midterm examination
October 9, 1995

Name:

112a-ax account:
The exam is open book: you may consult your notes, books, even computer 
programs that you have written before. However you may not consult with 
your colleagues. All the work should be yours.

Part 2.  (50 points)

Create a directory Midterm1 in your public_html directory. Make sure 
permissions are set up correctly. Create three subdirectories Pb1, Pb2 and Pb3, 
corresponding to the three problems I ask you to solve. At 2:50, turn on the 
permissions for the java source files: that is the method of submission. Do NOT 
touch these files after the exam period is over, or else they will not count.

Pb1. (15 points) Work in a directory Pb1. Write a Java applet with two buttons 
on it, b1 and b2. When you click on b1, the text ÒHello!Ó is drawn on the screen. 
When you click on b2, the text ÒHello!Ó is printed on the Java console. Name 
your applet TwoButtons.java. Create an html file for it. To get full credit, the 
applet should work, but if you spend more than 30 minutes on this problem 
without making it work, I suggest that you include some comments in the java 
file about what its status is (compiles or not, and what the bug seems to be) and 
move on to the next problem.




Pb2. (15 points) Work in a directory Pb2.  Write a Java applet with two 
buttons on it, for opening and closing a window. The window should have a 
white background and two buttons on it, labeled 0 and 1. When you click on 
the buttons in the window, the appropriate character (0 or 1) is drawn on 
the window screen (but not one on top of the other: rather, one left and one 
right of the screen). Same comments as above: do what you can do in 40 
minutes or so, then move on to the next problem.






Pb3. (20 points) Work in directory Pb3.  Copy into this directory the java file 
myGizmo.java which I have shown in class on Tuesday,  from the address:

http://cs.smith.edu/~112a/Lab5/V6/V6.3/myGizmo.java

Perform the following modifications on it:

a. Allow at most 25  graphical objects to be created.
b. Include a new background color, yellow. Make sure you make all the relevant changes 
(i.e., in the Background menu and in the method(s) dealing with it).


Wrap Up: turn on the permissions at 2:50 sharp, then leave.

Have a nice October break! See you in a week. Ileana.