CS 10: Introduction to Computer Programming

Final Examination

Closed-Book, 3 hours, Fall 1997

General Instructions

Before you answer any questions, write your name and perm number on your answer booklet.
Read each question carefully. Make sure that you clearly understand a question before answering it.
Please put your answer to each question on its own page in your blue-book.
Answers that do not show work will not get partial credit.
You may wish to work out an answer on scratch paper before writing it on your answer page; answers that are difficult to read may lose points for that reason.
Each question is worth 10 points.

1. Describe the difference between public, private, and protected. Ignore differences having to do with packages.

Solution:

public class members are accessible by name outside the class;

private class members are not accessible by name outside the class;

protected class members are accessible by name within the defining class and its subclasses.

2. What is the meaning of the keyword this? Illustrate the meaning with a class definition that uses it.

Solution:

The keyword this is used when one wants an object to refer to itself . Here is a simple example where an object's method returns the object itself (look at the Solar System applet on the Lectures page for another example):

public class Complex {

private double r, i;

public Complex() { r = i = 0; }

public double getR(){ return r; }

public void setR(double R){ r = R; }

        public Complex add(Complex c) {
            r += c.r;
            i += c.i;
            return this;
        }
    }

3. Describe what it means when a data member is declared static. Illustrate the meaning with a class definition that uses it.

Solution:

When a data member is declared static, only one instance of it exists for the whole class, no matter how many of the class's objects have been instantiated. This is useful, for example, when a class constant is created. Since only one is needed for the whole class, there is no need to instantiate such a constant for every object in the class. Another example is when you want to characterize something about the state of the class, such as a count of how many of it's objects exist at any point in time. Here is a simple example.

public class Student {

static int count;

private String name;

            public Student(String NAME) {
                name = NAME;
                count++;
            }

public void finalize() { count--; }

// other methods go here
}

4. Give an example of an overloaded constructor (of your own design or from Java's class libraries).

Solution:

One example is the Color constructors. One Color constructor takes 3 int arguments (each of whose values are in [0, 255]), and another that takes 3 float arguments (each of whose values are in [0,1]).

5. Give a method that takes a 2-dimensional array of double as an argument, and puts a random number in [0, 1) in each element.

Solution:

void summer(double a[][]) {

for (int i = 0; i < a.length; i++)

for (int j = 0; j < a[i].length; j++)

a[i][j] = Math.random();

}

6. Give a method that takes a 3-dimensional array of int as an argument, and returns the sum of the elements whose indices have a sum that is evenly divisible by 3. For example, if the array has element [1][1][1], then that element's value is part of the sum, because 1 + 1 + 1 is evenly divisible by 3. On the other hand, if the array has element [1][1][2], then that element's value is not part of the sum, because 1 + 1 + 2 is not evenly divisible by 3.

Solution:

int summer(int a[][][]) {

int sum = 0;

        for (int i = 0; i < a.length; i++)
        for (int j = 0; j < a[i].length; j++)
        for (int k = 0; k < a[i][j].length; k++)
            if ((i + j + k) % 3 == 0) sum += a[i][j][k];
        return sum;
    }

7. Give a method that has 2 int array arguments. Assume that they each have at least 2 elements. The method puts the largest element of the first array into the first element of the second array, and the second-largest element of the first array into the second element of the second array.

Solution

public void largest2(int a[], int b[]) {

        if (a[0] < a[1]) {
            b[0] = a[1];
            b[1] = a[0];
        }
        else {
            b[0] = a[0];
            b[1] = a[1];
        }
        for (int i = 2; i < a.length; i++)
            if (a[i] > b[0]) {
                b[1] = b[0];
                b[0] = a[i];
            }
            else if (a[i] > b[1])
                b[1] = a[i];
    }

8. Give an applet that draws a 8 X 8 checker board (of black and red squares), where each square on the board has an edge length of 10 pixels. The upper left corner of the board has pixel coordinates (0, 0).

Solution

import java.awt.*;

import java.applet.*;

public class test extends Applet {

    public void paint(Graphics g) {
        g.fillRect(0, 0, 80, 80);
        g.setColor(Color.red);
        for (int i = 0; i < 8; i++)
        for (int j = i % 2; j < 8; j +=2)
            g.fillRect(10*j, 10*i, 10, 10);
    }
}

9. As applied to methods, what is the meaning of the keyword super? Illustrate the meaning with some class definitions. (Hint: at least 2 class definitions are needed).

Solution

When a subclass method overrides a superclass method, the superclass method may be accessed from the subclass by preceding the superclass method name with super followed by the dot operator. A superclass constructor can be invoked simply by substituting

super

for the name of the superclass.

import java.awt.*;

public class Box {
protected int x, y, edge;

        public Box(int X, int Y, int E) {
            x = X;
            y = Y;
            edge = E;
        }

        public void draw(Graphics g) {
            g.setColor(Color.blue);
            g.fillRect(x, y, edge, edge);
        }
}

import java.awt.*;

public class OutlinedBox extends Box {
    public OutlinedBox(int X, int Y, int E) {
        super(X, Y, E);
    }

    public void draw(Graphics g) {
        super.draw(g);
        g.setColor(Color.black);
        g.drawRect(x, y, edge, edge);
    }
}

10. Give an example of call-by-value and call-by-reference. Describe the difference.

Solution

Call-by-value is when a copy of the value of an argument is passed to a method. The method cannot change the value of the argument (only its copy). Call-by-reference is when an argument is referred to directly by the method; in this case, the method can change the value of the argument. Below, the int variable i is passed call-by-value; the int array a is passed call-by-reference. The method sum changes the caller's int array argument; it does not change the caller's the int argument.

public void sum(int i, int a[]) {

a[i++]++;

a[i]++;

}