1. Modular Event Handling
CSIS 3701: Advanced Object Oriented Programming

2. Simple Event Handling
public class Main extends JFrame implements ActionListener { private JButton b; public Main() { b = new JButton(“press”); b.addActionListener(this); … } public void actionPerformed( ActionEvent e) {
… } }
JButton object -- stores reference to Main object in list of type ActionListener -- calls actionPerformed method of that Main object when pressed

3. Simple Event Handling
Can use getSource to find which component caused event
public void actionPerformed(ActionEvent e) {
String temp;
if (e.getSource() == leftButton) { temp = rightField.getText(); leftField.setText(temp); rightField.setText(""); }
if (e.getSource() == rightButton) { temp = leftField.getText(); rightField.setText(temp); leftField.setText(""); } }

4. Simple Event Handling Awkward if many event components
Would need separate if for each
Difficult to write, maintain
12 if statements

5. Arrays of Components
Can store groups of similar components as array private compType[] comps = new compType[size];
Use loop to construct, add listeners, add to panel for (int i = 0; i < size; i++) { comps[i] = new compType[parameters]; comps[i].addActionListener(this); panel.add(comps[i]); }

6. Keypad Example
public class Keypad1 extends JFrame implements ActionListener {
private JTextField keyField;
private static final int KEYS = 12;
private JButton[] keys = new JButton[KEYS];
private String[] labels = new String[] {"1","2","3","4","5","6","7","8","9","*","0","#"};
Will use this array to add different labels to each button

7. Keypad Example public Keypad1() {
JPanel buttonPanel = new JPanel(new GridLayout(4, 3));
for (int i = 0; i < KEYS; i++) {
keys[i] = new JButton(labels[i]);
keys[i].addActionListener(this);
buttonPanel.add(keys[i]); }
Construct button from ith label in array
Listen for its events
Add it to the panel

8. Loops for Event Handling
Can iterate through array to handle events
Loop through array in actionPerfrormed
Compare each component to source of event
public void actionPerformed(ActionEvent e) { for (int i = 0; i < size; i++) if (e.getSource() == comps[i]) { … code to handle event on that object … }

9. Loops for Event Handling
public void actionPerformed(ActionEvent e) {
for (int i = 0; i < KEYS; i++) {
if (e.getSource() == keys[i]) {
keyField.setText(keyField.getText()+labels[i]); }
Since we know the ith button caused the event, add the corresponding ith label to the output

10. Event Handler Classes Can create separate classes to handle events
Constructed by application
Passed to buttons using addActionListener
Its actionPerformed called by buttons when pressed
Application
JButton
button
Listeners
Passes reference to handler
Handler
actionPerformed
constructs

11. Event Handler Objects Must implement ActionListener interface
Application no longer has to implement ActionListener
Has actionPerformed method to handle event
Handles event independently from application
Often passed information to constructor from application
Example: KeypadHandler
One for each button on application
Passed string corresponding to number on its button by constructor
Displays number when it button pressed

12. KeypadHandler Class public class Handler2 implements ActionListener {
private String key;
public Handler2(String k) { key = k; }
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(null, key);
String corresponding to label on its button
Set in constructor
Displayed when that button calls its actionPerformed method

13. Using KeypadHandler public Keypad() {…
for (int i = 0; i < KEYS; i++) {
keys[i] = new JButton(labels[i]);
keys[i].addActionListener( new KeypadHandler(labels[i])); }
For each button with a given label
Construct a KeypadHandler with that label
Pass it to the button
KeypadHandler
key
KeypadHandler
key 3 4

14. Accessing Application from Handler
What if handler must access visual components on application?
Example: KeypadHandler should append to textfield instead of popping up dialog
Application can also pass reference to visual component the handler must manipulate
KeypadHandler
key
field 4

15. KeypadHandler Class public class Handler3 implements ActionListener {
private String key;
private JTextField field;
public Handler3(String k, JTextField f) { key = k;
field = f; }
public void actionPerformed(ActionEvent e) {
field.setText(field.getText()+key);
Reference to component to append to
Set in constructor
Manipulate that component when its actionPerformed method is called by its button

16. Event Handlers as Inner Classes
Event handler classes often implemented as inner classes of application
Automatically have access to all components of the application
Less modular/reusable, but easier to implement
Application
Visual component
Inner class
reference to visual component

17. KeypadHandler Inner Class
public class Keypad4 extends JFrame { …
private class Handler4 implements ActionListener {
private String key;
private Handler4(String k) {key = k;}
public void actionPerformed(ActionEvent e) {
keyField.setText(keyField.getText()+key); }
} // end inner class …
private JTextField keyField = new JTextField(15);
direct access