PRELIM 2 SAMPLE QUESTIONS

Prelim 2 concentrates on what we have covered since Prelim 1 (up through Program 3) but will also include material covered by Prelim 1.  Arrays are NOT covered.

1. a) Draw a picture of the "world,” i.e., all objects, variables and methods at the moment the following program reaches the line indicated. Use the diagramming technique illustrated in lecture:

· Draw objects as rectangles with dashed borders containing the object's instance variables.

· Each object should contain two compartments separated by a dashed line.

· The upper compartment should contain the object's instance variables; the lower compartment should show the object's instance methods.

· Each variable should be drawn as a solid rectangle. The name of the variable should appear to the left of the rectangle, and the contents of the variable should be written in the rectangle.

· Draw object references as arrows pointing to the objects to which they refer.

b) What is the output of method "main" below?

public class Q1 {
private static int a = 1;
private int b;
private Q1 c = null;

public Q1() {
        b = a;
        a = 2*a;
    }

public String toString() {
        String s = "null";
if (c != null) s = "" + c.b;
return a + " " + b + " " + s;
    }

public void magic(int a) {
        Q1 now = this;
while (now != null) {
            System.out.println(a + " " + now);
            now = now.c;
        }
    }

public static void main (String[] args) {
        Q1 a = new Q1();
        Q1 b = new Q1();
        Q1 c = new Q1();
        c = a;
        a.c = b;
        c.c.b++;
    // draw a picture of "the world" as it exists at THIS LINE
        c.magic(7);
    }
}

2. a) In the code for Q1 above, label each variable declaration with a unique integer. Then label each variable use throughout the code with the integer of the corresponding declaration. This will illustrate your understanding of the scope of names in Java.

b) Does method "toString()" above return a value or print a value?  How --Java willing-- would you change the code for method "toString()" to make it do the other (print instead of return or vice versa)?
c) Define the term "class".
d) Define the term "method".
e) Explain the difference between public and private.
f) Explain the differences between static and non-static.

3. a) What is the pattern for repeatedly doing something N times?  The pattern for processing an input sequence of pairs of values terminated by a pair of stopping values?

b) Androids Trurl and Klapaucius perform a random walk on a grid.  Both begin at square (0,0).  At each step, they independently move one square to the right and one square up or down, e.g., from (0,0), each android will move independently to either (1,1) or (1,-1).  Trurl's probability of moving up is 60%; Klapaucius' probability of moving is 45%.

Write a code segment to simulate 200 steps of their random walk and count how many times they meet during their walk (how often they are standing on the same square).  Output this number to the screen.  Do not include their starting position in your count.

HINT: The expression Math.random() <= 0.4 will be true with probability 40%.  Function Math.random() returns a random number strictly between 0.0 and 1.0.

4. Recall that the minute hand of a clock and the two digits for the minutes on a digital clock represent minutes modulo 60: numbers go from 0..59 and then "wrap around".  Let us implement a class MM60 (short for "minutes modulo 60", for convenience) with integer fields *tens* and *ones* for the tens digit and a ones digit, respectively, where we (you) maintain the condition

0<= tens <=5 and 0<= digits <= 9

because any quantity of minutes can be converted to an MM60 quantity in the range 0..59.  Examples: 

28 minutes:	tens = 2, ones = 8
125 minutes = 5 minutes modulo 60:	tens = 0, ones = 5
-17 minutes = 43 minutes modulo 60:	tens = 4, ones = 3

5. a) Write all the code (show us everything that would go into file MM60.java) for a class MM60 with integer fields *tens*, *ones* as described above and two methods *swap* and *add*:

• Method *swap* is used to swap the contents (fields) of two MM60 objects.

• Method *add* is used to take two MM60's and return their MM60 sum.

• NO LOOPS.

• Remember to use the (default) constructor for instances of class MM60.

• The two methods are independent of each other.

• You may do parts (b) and (c) under the assumption that part (a) is right, even if (a) is incomplete or wrong.

• Be aware that the remainder r = a%b could be negative when a < 0 (but satisfies 0 <= |r| < |b|, where |r|, |b| are the absolute values of r, b).

6. b)  Use calls to your *swap* method from (a) to write a code segment (outside of class MM60) to "shift" (rearrange) the values of MM60 variables a, b, c as follows:

• a ends up with b's original value

• b with c's original value, and

• c with a's original value.

• You may NOT use any addiitional variables

• You may NOT reference the fields (instance variables) *tens*, *ones*.

7. c) Use the (default) constructor for MM60 to write a short code segment (outside of class MM60) equivalent to a = -x; where *x* is an integer variable and *a* is an MM60 variable.
   
   d) Use your *add* method from (a) to write a short expression (outside of class MM60) equivalent to 2*a + c, where a and c are MM60 variables.  (Note: since you are writing an expression, you may NOT reference the fields *tens*, *ones* and you may NOT declare any more variables. )