The document discusses implementing listeners and overriding methods in Java. It shows code to add an item change listener to a combo box that prints the selected item to a text field. It also demonstrates overriding the paint() method to draw shapes on a panel and overriding the getCgpa() method to calculate grade point averages differently in subclasses.

1. Lecture 9
More on implementing Listeners
Object Oriented Programming
Eastern University, Dhaka
Md. Raihan Kibria

2. Add a combo box
We will add an
item change
listener and print
to console.
See next page
for code

3. public class ComboDemo extends JFrame {
ComboDemo(){
super.setBounds(0, 0, 500, 400);
super.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
super.setLayout(new FlowLayout());
final JTextField text = new JTextField("Select an item");
class MyItemListener implements ItemListener{
JComboBox combo;
MyItemListener(JComboBox combo){
this.combo = combo;
}
public void itemStateChanged(ItemEvent e) {
text.setText(combo.getSelectedItem().toString());
}
}
JComboBox combo = new JComboBox(new String[]{"Square", "Circle",
"Triangle"});
combo.addItemListener(new MyItemListener(combo));
super.add(combo);
super.add(text);
super.setVisible(true);
}
public static void main(String[] args){
new ComboDemo();
}
}

4. Output

5. Overriding paint method of
Component
public class OverridePaintDemo {
public static void main(String[] args) {
JFrame f = new JFrame();
f.setBounds(0, 0, 500, 400);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setLayout(new FlowLayout());
f.add(new MyFirstPanel());
f.setVisible(true);
}
}
class MyFirstPanel extends JPanel{
MyFirstPanel(){
setBackground(Color.GRAY);
setPreferredSize(new Dimension(100, 100));
}
}

6. Output
We want to add a circle to the panel above

7. public class OverridePaintDemo {
public static void main(String[] args) {
JFrame f = new JFrame();
f.setBounds(0, 0, 500, 400);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setLayout(new FlowLayout());
f.add(new MyFirstPanel());
f.setVisible(true);
}
}
class MyFirstPanel extends JPanel{
MyFirstPanel(){
setBackground(Color.GRAY);
setPreferredSize(new Dimension(100, 100));
}
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.RED);
g.drawOval(5, 5, 90, 90);
}
}

8. Output

9. Now we want to add a circle, a
rectangle and a triangle
public class OverridePaintDemo {
public static void main(String[] args) {
JFrame f = new Jframe();
f.setBounds(0, 0, 500, 400);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE );
f.setLayout(new FlowLayout());
f.add(new MyFirstPanel());
f.add(new MySecondPanel());
f.add(new MyThirdPanel());
f.setVisible(true);
}
}
More code next..

10. class MyFirstPanel extends JPanel{
MyFirstPanel(){
setBackground(Color.GRAY);
setPreferredSize(new Dimension(100, 100));
}
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.RED);
g.drawOval(5, 5, 90, 90);
}
}
class MySecondPanel extends JPanel{
MySecondPanel(){
setBackground(Color.GRAY);
setPreferredSize(new Dimension(100, 100));
}
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.RED);
g.drawRect(10, 15, 80, 60);
}
}
More code next..

11. class MyThirdPanel extends JPanel{
MyThirdPanel(){
setBackground(Color.GRAY);
setPreferredSize(new Dimension(100, 100));
}
public void paint(Graphics g) {
super.paint(g);
g.setColor(Color.RED);
g.drawLine(10, 90, 90, 90);
g.drawLine(10, 90, 90, 10);
g.drawLine(90, 10, 90, 90);
}
}

13. More on overriding
public class OverrideMethodDemo {
public static void main(String[] args) {
System.out.println("CGPA by class I: " + new I().getCgpa());
System.out.println("CGPA by class J: " + new J().getCgpa());
}
}

14. class I{
int[] gpas = new int[]{3, 4, 2, 4, 5};
int calculateSum(){
int sum = 0;
for (int i=0; i<gpas.length; i++)
sum += gpas[i];
return sum;
}
public float getCgpa(){
int sum = calculateSum();
return sum/gpas.length;
}
}
class J extends I{
public float getCgpa() {
int sum = calculateSum();
return (float)sum / (float)gpas.length;
}
}

15. Output
CGPA by class I: 3.0
CGPA by class J: 3.6