Unit 4: AWT, event handling and I/O

• AWT (Abstract Window Toolkit) uses native, heavyweight components; Swing (JFrame, JButton) adds lightweight, richer components and is preferred for new desktop code.

javaimport java.awt.*;
import java.awt.event.*;

public class AddFrame extends Frame implements ActionListener {
    TextField t1 = new TextField(10), t2 = new TextField(10);
    Label result = new Label("Result:            ");
    Button add = new Button("Add");

    AddFrame() {
        super("Adder");
        setLayout(new FlowLayout());
        add(new Label("A")); add(t1); add(new Label("B")); add(t2); add(add); add(result);
        add.addActionListener(this);                         // ActionListener
        addMouseListener(new MouseAdapter() {                 // MouseListener via adapter
            public void mouseClicked(MouseEvent e) { setTitle("Clicked at " + e.getX() + "," + e.getY()); }
        });
        t2.addKeyListener(new KeyAdapter() {                  // KeyListener
            public void keyTyped(KeyEvent e) { if (!Character.isDigit(e.getKeyChar())) e.consume(); }
        });
        addWindowListener(new WindowAdapter() {               // WindowListener
            public void windowClosing(WindowEvent e) { dispose(); }
        });
        setSize(420, 120); setVisible(true);
    }
    public void actionPerformed(ActionEvent e) {
        int s = Integer.parseInt(t1.getText()) + Integer.parseInt(t2.getText());
        result.setText("Result: " + s);
    }
    public static void main(String[] args) { new AddFrame(); }
}

• Standard streams: System.in (InputStream), System.out and System.err (PrintStream). The File class gives file information (exists, length, delete, list).

javaimport java.io.*;
// copy any file byte by byte
try (FileInputStream in = new FileInputStream("photo.jpg");
     FileOutputStream out = new FileOutputStream("copy.jpg")) {
    int b;
    while ((b = in.read()) != -1) out.write(b);
} catch (IOException e) { System.out.println(e.getMessage()); }

java// write and read a text file line by line
try (BufferedWriter bw = new BufferedWriter(new FileWriter("students.txt", true))) {  // append
    bw.write("101,Aman,82"); bw.newLine();
}
try (BufferedReader br = new BufferedReader(new FileReader("students.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        String[] f = line.split(",");
        System.out.println(f[1] + " scored " + f[2]);
    }
}

• RandomAccessFile reads and writes at any position using a file pointer — modes "r" and "rw"; methods seek(), getFilePointer(), length(), readInt(), writeInt(), readUTF(), writeUTF().

javatry (RandomAccessFile raf = new RandomAccessFile("marks.dat", "rw")) {
    for (int m : new int[]{78, 85, 90, 66}) raf.writeInt(m);   // each int = 4 bytes
    raf.seek(2 * 4);                    // jump to the third record
    System.out.println(raf.readInt());  // 90
    raf.seek(2 * 4);
    raf.writeInt(95);                   // update in place
}