Unit 1: Java programming exercises

• Single-file launch: java Program.java compiles and runs in one step (Java 11+).

javapublic class MathDemo {
    public static void main(String[] args) {
        int a = 17, b = 5;
        System.out.println("Sum " + (a + b) + " Quotient " + (a / b) + " Remainder " + (a % b));
        System.out.println("a > b ? " + (a > b) + "  a & b = " + (a & b) + "  a << 2 = " + (a << 2));
        System.out.println(Math.sqrt(144) + " " + Math.pow(2, 10) + " " + Math.abs(-7));
        System.out.println(Math.max(a, b) + " " + Math.round(4.6) + " " + Math.ceil(4.1) + " " + Math.floor(4.9));
        double r = 7;
        System.out.printf("Area of circle = %.2f%n", Math.PI * r * r);
        System.out.println("Random 1–6: " + (int) (Math.random() * 6 + 1));
    }
}

javaimport java.util.Scanner;
public class Numbers {
    static boolean isPrime(int n) { if (n < 2) return false; for (int i = 2; i * i <= n; i++) if (n % i == 0) return false; return true; }
    static long fact(int n) { return n <= 1 ? 1 : n * fact(n - 1); }
    public static void main(String[] a) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        System.out.println(n + (isPrime(n) ? " is prime" : " is not prime"));
        System.out.println("Factorial = " + fact(n));
        int x = 0, y = 1; for (int i = 0; i < n; i++) { System.out.print(x + " "); int t = x + y; x = y; y = t; }
        for (int i = 1; i <= 10; i++) System.out.println(n + " x " + i + " = " + n * i);
        double A = 1, B = -3, C = 2, d = B * B - 4 * A * C;   // quadratic roots
        if (d >= 0) System.out.println(((-B + Math.sqrt(d)) / (2 * A)) + ", " + ((-B - Math.sqrt(d)) / (2 * A)));
        else System.out.println("Imaginary roots");
    }
}

• Also: average of n numbers, volume of cube, cylinder and sphere (V = 4/3 πr³), weekday from a number using switch.

javafor (int i = 0; i < 5; i++) {
    int c = 1;
    System.out.print(" ".repeat(5 - i));
    for (int j = 0; j <= i; j++) { System.out.print(c + " "); c = c * (i - j) / (j + 1); }
    System.out.println();
}

javaint[] a = {5, 2, 9, 1};
java.util.Arrays.sort(a);                                   // sorting
int k = 1, n = a.length; int[] r = new int[n];
for (int i = 0; i < n; i++) r[(i + k) % n] = a[i];          // rotate right by k
int[][] p = {{1, 2}, {3, 4}}, q = {{5, 6}, {7, 8}}, m = new int[2][2];
for (int i = 0; i < 2; i++) for (int j = 0; j < 2; j++) for (int t = 0; t < 2; t++) m[i][j] += p[i][t] * q[t][j];
int zeros = 0; for (int[] row : p) for (int v : row) if (v == 0) zeros++;
boolean sparse = zeros > (p.length * p[0].length) / 2;      // sparse check

• Sum and difference use m[i][j] = p[i][j] ± q[i][j].

javaString s = "Bathinda".toLowerCase(); int v = 0, c = 0;
for (char ch : s.toCharArray()) if (Character.isLetter(ch)) { if ("aeiou".indexOf(ch) >= 0) v++; else c++; }
System.out.println("Vowels " + v + ", consonants " + c);   // 3, 5

javainterface Printable { void print(); }
interface Showable  { void show(); }
class Doc implements Printable, Showable {
    public void print() { System.out.println("print"); }
    public void show()  { System.out.println("show"); }
}

• Demonstrate private, default, protected and public by accessing members from the same class, same package, a subclass in another package, and an unrelated class.

java// shapes/Shape.java
package shapes;
public interface Shape { double area(); default String name() { return "Shape"; } }

// shapes/Circle.java
package shapes;
public class Circle implements Shape {
    double r; public Circle(double r) { this.r = r; }
    public double area() { return Math.PI * r * r; }
    public String name() { return "Circle"; }
}

// Main.java
import shapes.*;
public class Main {
    public static void main(String[] args) {
        Shape s = new Circle(3);
        System.out.printf("%s area %.2f%n", s.name(), s.area());
    }
}

bashjavac -d . shapes/Shape.java shapes/Circle.java Main.java
java Main

javaimport java.util.Scanner;
class AgeException extends Exception { AgeException(String m) { super(m); } }
public class ExDemo {
    static void checkAge(int age) throws AgeException {
        if (age < 18) throw new AgeException("Not eligible to vote");
        System.out.println("Eligible");
    }
    public static void main(String[] args) {
        try {
            int[] x = new int[2];
            System.out.println(10 / Integer.parseInt(args.length > 0 ? args[0] : "0"));
            x[3] = 1;
        } catch (ArithmeticException e) { System.out.println("Divide by zero");
        } catch (ArrayIndexOutOfBoundsException e) { System.out.println("Bad index");
        } finally { System.out.println("finally runs"); }
        try { checkAge(16); } catch (AgeException e) { System.out.println(e.getMessage()); }
    }
}

javaclass Counter { int c; synchronized void inc() { c++; } }
public class ThreadDemo {
    public static void main(String[] a) throws InterruptedException {
        Counter k = new Counter();
        Runnable r = () -> { for (int i = 0; i < 1000; i++) k.inc(); };
        Thread t1 = new Thread(r, "T1"), t2 = new Thread(r, "T2");
        t1.setPriority(Thread.MAX_PRIORITY); t2.setPriority(Thread.MIN_PRIORITY);
        t1.start(); t2.start(); t1.join(); t2.join();
        System.out.println(k.c);   // always 2000 with synchronized
    }
}

• Applets no longer run in browsers; run applet code with appletviewer on JDK 8, or draw the same output in a Frame as below.

javaimport java.awt.*;
public class Drawing extends Frame {
    Drawing() { setTitle("Graphics"); setSize(420, 320); setVisible(true); }
    public void paint(Graphics g) {
        g.setColor(Color.red);    g.drawLine(30, 60, 380, 60);
        g.setColor(Color.blue);   g.fillRect(30, 80, 100, 60);
        g.setColor(Color.green);  g.fillOval(150, 80, 80, 80);
        g.setColor(Color.black);  g.drawArc(250, 80, 100, 100, 0, 180);
        g.drawPolygon(new int[]{60, 110, 10}, new int[]{200, 280, 280}, 3);
        g.drawString("Java Graphics Lab", 150, 260);
    }
    public static void main(String[] a) { new Drawing(); }
}

javaimport javax.swing.*;
public class Adder {
    public static void main(String[] a) {
        JFrame f = new JFrame("Adder");
        JTextField x = new JTextField(5), y = new JTextField(5); JLabel out = new JLabel("=");
        JButton b = new JButton("Add");
        b.addActionListener(e -> out.setText("= " + (Integer.parseInt(x.getText()) + Integer.parseInt(y.getText()))));
        JPanel p = new JPanel(); p.add(x); p.add(y); p.add(b); p.add(out);
        f.add(p); f.setSize(300, 100); f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); f.setVisible(true);
    }
}

java@WebServlet("/visit")
public class Visit extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
        HttpSession s = req.getSession();
        Integer n = (Integer) s.getAttribute("n"); n = (n == null) ? 1 : n + 1; s.setAttribute("n", n);
        res.addCookie(new Cookie("lastUser", req.getParameter("user")));
        res.setContentType("text/html");
        res.getWriter().println("Hello " + req.getParameter("user") + ", visit " + n);
    }
}