Unit 3: Exception handling and applet/graphics programming

javatry {
    int[] a = new int[3];
    int x = Integer.parseInt("12a");     // NumberFormatException
    a[5] = 10 / 0;
} catch (ArithmeticException | NumberFormatException e) {   // multi-catch
    System.out.println("Error: " + e.getMessage());
} catch (Exception e) {
    System.out.println("Other error");
} finally {
    System.out.println("Always runs — close files here");
}

try (java.io.FileReader fr = new java.io.FileReader("a.txt")) {   // try-with-resources
    System.out.println((char) fr.read());
} catch (java.io.IOException e) { e.printStackTrace(); }

javaclass InsufficientBalanceException extends Exception {
    InsufficientBalanceException(String msg) { super(msg); }
}
class Account {
    double balance = 5000;
    void withdraw(double amt) throws InsufficientBalanceException {   // declares it
        if (amt > balance) throw new InsufficientBalanceException("Balance too low");
        balance -= amt;
    }
}
// caller
try { new Account().withdraw(8000); }
catch (InsufficientBalanceException e) { System.out.println(e.getMessage()); }

• throw raises an exception object; throws declares exceptions a method may pass on.

• Applet: a small Java program embedded in a web page and run by a browser plug-in or appletviewer. Applets are now obsolete — browsers removed plug-in support, appletviewer was dropped in JDK 11 and the Applet API is deprecated — but the concepts carry over to GUI programming and remain in the syllabus.

javaimport java.applet.Applet;
import java.awt.*;
public class Welcome extends Applet {
    String name;
    public void init() {
        name = getParameter("student");          // read PARAM
        if (name == null) name = "Guest";
        setBackground(Color.yellow);
    }
    public void paint(Graphics g) { g.drawString("Welcome, " + name, 40, 50); }
}

html<!-- Welcome.html -->
<applet code="Welcome.class" width="300" height="120">
  <param name="student" value="Simran">
</applet>

• Run with appletviewer Welcome.html (JDK 8 or earlier). The same drawing code can run today inside a JFrame or JPanel by overriding paint or paintComponent.

javapublic void paint(Graphics g) {
    g.setColor(Color.blue);
    g.drawLine(10, 10, 150, 10);                 // x1, y1, x2, y2
    g.drawRect(10, 30, 100, 50);                 // x, y, width, height
    g.fillRoundRect(130, 30, 100, 50, 20, 20);
    g.drawOval(10, 100, 80, 80);                 // circle: equal width and height
    g.drawOval(110, 100, 120, 60);               // ellipse
    g.drawArc(250, 100, 80, 80, 0, 180);         // start angle 0, sweep 180
    int[] xs = {300, 350, 400}, ys = {250, 180, 250};
    g.fillPolygon(xs, ys, 3);                    // triangle
    g.setFont(new Font("Arial", Font.BOLD, 16));
    g.drawString("Shapes", 10, 220);
}

javapublic void paint(Graphics g) {
    int[] sales = {40, 75, 60, 90, 55};
    String[] yr = {"2021", "2022", "2023", "2024", "2025"};
    g.drawLine(40, 250, 360, 250);               // x-axis
    g.drawLine(40, 250, 40, 40);                 // y-axis
    for (int i = 0; i < sales.length; i++) {
        int x = 60 + i * 60, h = sales[i] * 2;
        g.setColor(Color.orange);
        g.fillRect(x, 250 - h, 30, h);           // bar
        g.setColor(Color.black);
        g.drawString(yr[i], x, 265);
        if (i > 0) {                             // line graph joining bar tops
            int px = 60 + (i - 1) * 60 + 15, ph = sales[i - 1] * 2;
            g.drawLine(px, 250 - ph, x + 15, 250 - h);
        }
    }
}