Answers to Midterm Exam.

1. Answers to Midterm Exam. • 1. (a) What does the “plateform independence” mean? How is it implemented in Java? (4) • (b) If we have not defined a constructor in a class, how can an instance of the class be constructed? (2) • (c) Specify the difference between class methods and instance methods. (2) • (d) Specify the meaning of the key words “public”, “final” and “abstract” for classes. (2)

2. 2. Write a Java program to compute the following function (20) • h(N) = f(N) + g(N), • where f(N) = 3*f(N-3) + 4*f(N-2) - 5*f(N-1) with f(0) = 1, f(1) = 1, and f(2) = 3, and g(N) = g(N-3) *g(N-2) - 5*g(N-1) with g(0) = 1, g(1) = 2, and g(2) = 4. • class Computation { • public static int f(int x) { • if (x== 0 || x==1) return 1; • else if (x==1) return 3; • else return 3*f(x-3) + 4*f(x-2) – 5*f(x-1); • } • public static int g(int y) { • if (x== 0 ) return 1; • else if (x==1) return 2; • else if (x==2) return 4; • else return g(x-3)*g(x-2)– 5*g(x-1); • } • }

3. public class Computing { • public static void main(String arg[]) { • int a = Factorial.factorial(Integer.parseInt(arg[0])); • System.out.println(Computation.f(a) + Computation.g(a)); • } • }

4. 3. Consider the following class. If we use the statement: (15) • A e = new A("gggggg");' to create an object of this class, what is the value of e.d? Explain your answer. • class A { • String a; • double b, c, d; • public A(String a, double b, double c, double d) { • this.a = a; this.b = b; this.c = c; this.d = d; • } • public A(String a, double b, double c) { • this(a, b, c, 1.0); • } • public A(String a, double b) { • this(a, b, 2.0); • } • public A(String a) { • this(a, 3.0); • } • } • e.d = 1.0

5. 4. Analyze the following two programs and show the outputs of the programs, respectively. •  Program I: (10) • class Note { • int value; • Note(int val) {value = val;} • public static final Note • middle_c = new Note(0), • c_sharp = new Note(1), • b_flat = new Note(2); • } • class Instrument { • public void play(Note n) { • System.out.println(“Instrument.play()”); • }}

6. class Wind extends Instrument { • public void play(Note n) { • System.out.println(“Wind.play()” + “ “ + n.value); • } • public static void tune(Instrument i) { • i.play(Note.middle_c); • } • public static void main(String[] args) { • Wind flute = new Wind(); • tune(flute); • }}

7. Program II: (10) • class Shape { • void draw() {} • void erase() {} • } • class Circle extends Shape { • void draw() { • System.out.println(“Circle.draw()”); • } • void erase() {System.out.println(“Circle.erase()”);}} • class Square extends Shape { • void draw() { • System.out.println(“Square.draw()”); • } • void erase() {System.out.println(“Square.erase()”);}}

8. class Triangle extends Shape { • void draw() { • System.out.println(“Triangle.draw()”); • } • void erase() {System.out.println(“Triangle.erase()”);}} • public class Shapes { • public static Shape randShape(int j) { • switch(j) { • case 0: return new Circle(); • case 1: return new Square(); • case 2: return new Triangle(); • default : return new Circle();}} • public static void main(String[] args) { • Shape[] s = new Shape[3]; • for (int i = 0; i < s.length; i++) • s[i] = randShape(i); • for (int i = 0; i < s.length; i++) s[i].draw();}}

9. 4. Program I: Wind.play() 0 Program II: Circle.draw() Square.draw() Triangle.draw()

10. Fig. 1 5. Write a Java program which draws a graph shown in Fig. 1 on an applet. (15)

11. 5. import java.applet.*; import java.awt.*; class DrawGraph extends Applet { public paint(Graphics g) { int x[] = {50, 25, 25, 75, 75}; int y[] = {50, 75, 100, 100, 75}; int k = x.length; g.drwaPolygon(x, y, k); g.drawLine(50, 50, 25, 100); g.drawLine(50, 50, 75, 100); } }

12. 6. Write a Java program which computes all non-primes less than 50. (20) • Import java.lang.*; • public class Sieve { • public static void main(String[] args) { • int max = 20; //Assign a default value • try {max = Integer.parseInt(arg[0]);} • catch (Exception e) {} //Silently ignore exceptions. • //Create an array that specifies whether each number is prime or not. • Boolean[] isprime = new boolean[max+1]; • //Assume that all numbers are primes, until proven otherwise. • for (int i = 0; i <= max; i++) isprime[i] = true; • //We know that that 0 and 1 are not prime. Make a note of it. • isprime[0] = isprime[1] = false;

13. //To compute all primes less than max, we need to rule out multiples of all • //integers less than the square root of max. • int n = (int) Math.ceil(Math.sqrt(max)); • for (int i = 0; i <= n; i++) { • if (isprime[i]) { int k = 2; • for (int j = k*i; j <= max; j = (k ++)*i) • isprime[j] = false; } • } • int j; • for (j = ma0; j < 50; j++); //empty loop body • if (isprime[j] == false) System.out.println(j + “is not a prime} • }