Transcript Text Field Events

Event-Driven Programming
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Listener or Event handler
Example: Handling Button Events
Example: Handling Mouse Events
Example: Handling Keyboard Events
Adapter Classes
Example: Handling Window Events
Example: Handling Text Field Events
Exercises
Learning Outcomes
o Extend the example programs to write more interesting GUI
o Use nested classes and adapter classes to write medium-sized
applications.
Unit 11
1
Listener or Event handler
• Listener: An object interested in being notified when an event
occurs in a given component.
• A Listener object registers with a component to be notified of
events generated by it.
• Listener must implement the event listener interface associated
with events for which it registered.
• Programming a Listener for an event means:
• Implementing the interface associated with the event type.
• Registering it as an event listener on the appropriate event
source.
Unit 11
2
Events Revisited
• Some events are generated only by certain components.
• But we can set up a listener on any component for any of the
following events:
– Component event - changing a component's size, position, or
visibility.
– Focus event - gaining or losing the keyboard focus.
– Key event - pressing, releasing, or clicking keyboard keys.
– Mouse event - clicking the mouse button and moving the
mouse onto and off of a component.
– Mouse motion event - moving or dragging a mouse over a
component.
Unit 11
3
Events and Event listeners
Some Events and Their Associated Event Listeners
Act that Results in the Event
Listener Type
User clicks a button, presses Enter while typing
in a text field, or chooses a menu item
ActionListener
User closes a frame (main window)
WindowListener
User presses a mouse button while the cursor is
over a component
User moves the mouse over a component
Component becomes visible
MouseListener
MouseMotionListener
ComponentListener
Component gets the keyboard focus
FocusListener
Any property in a component changes such as
the text on a label
PropertyChangeListener
When an item such as a check box is changed
ItemListener
Unit 11
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Event Handling
• Each event is represented by an object
– Contains information about the event
• Event source
– Generates event
– Usually a GUI component
• Event listener
– Responds to the event
– Can be any Java class that implements correct interface
• User clicks a button -- ActionListener
• User closes a frame -- WindowListener
• User presses a mouse button -- MouseListener
• User moves the mouse -- MouseMotionListener
• Component becomes visible -- ComponentListener
• A single source can have multiple listeners
• A single listener can listen to multiple sources
Unit 11
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Event Handling
– Every event handler requires three pieces of code:
• Code that specifies that the class implements a listener
interface.
public class MyClass implements ActionListener {
• Code that registers an instance of the event handler class
as a listener upon one or more components.
someComponent.addActionListener(instance Of MyClass);
• Code that implements the methods in the listener
interface. For example:
public void actionPerformed(ActionEvent e) {
...//code that responds to the event... }
Unit 11
6
Example 1: Adding Components to Containers
1 import java.awt.*;
2 import javax.swing.*;
3 public class AddingComponents extends JFrame{
4 JButton button = new JButton("Press Me");
5 JLabel label = new JLabel( "Running Total:");
6 JTextField textField = new JTextField(10);
7 Container cp = getContentPane();
8 public AddingComponents() {
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super("A Container With Components");
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setSize(300,100);
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cp.setLayout(new FlowLayout());
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cp.add(label);
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cp.add(textField);
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cp.add (button);
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setVisible(true); }
16 public static void main(String args []) {
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new AddingComponents();
18 } }
Unit 11
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Handling Button Events
• This example builds on Example 1 of the preceding section.
• Notice that when the button is pushed in that example, nothing
happens.
• We will add some code to respond to the button pushes.
• When the mouse is pushed it generates and ActionEvent.
• Thus, we will be implementing the
corresponding ActionListener interface.
• ActionListener consists of the method:
Unit 11
8
Example 2: Button Events
1 import java.awt.*;
2 import java.awt.event.*;
3 class ButtonEventTest extends AddingComponents
4 implements ActionListener{
5 private int sum;
6 public ButtonEventTest() {
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button.addActionListener(this);
8 }
9 public void actionPerformed(ActionEvent ae) {
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sum += 1;
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textField.setText(sum+"");
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Toolkit.getDefaultToolkit().beep();
13 }
14 public static void main(String args []) {
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new ButtonEventTest();
16 }
17 }
Unit 11
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Handling Mouse Events
• This example illustrates how mouse events can be responded to.
• It also shows how a single listener can register with many sources.
• The event listener in this case will implement the MouseListener
interface.
• MouseListener consists of five methods:
• The program is given in the following page.
Unit 11
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Example 3: Mouse Events
1 import java.awt.*; import java.awt.event.*;
2 public class MouseEventTest extends ButtonEventTest{
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public MouseEventTest(){
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class LightUpListener extends MouseAdapter {
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public void mouseEntered(MouseEvent e) {
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Component c = (Component)e.getSource();
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c.setBackground(Color.green);
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}
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public void mouseExited(MouseEvent e) {
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Component c = (Component)e.getSource();
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c.setBackground(Color.red);
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}
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}
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MouseListener listener = new LightUpListener();
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button.addMouseListener(listener);
a single listener can register
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textField.addMouseListener(listener);
with many sources
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cp.addMouseListener(listener);
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}
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public static void main(String[] args) {
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new MouseEventTest();
Unit 11
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} }
Handling Keyboard Events
• This example illustrates how keyboard events can be responded to.
• To receive KeyEvent, a component must have keyboard focus.
• We will be implementing the KeyListener interface.
• KeyListener consists of three methods:
• Notice that when you press a key, at least two events are
generated.
Unit 11
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Example 4: Keyboard Events
1 import java.awt.*; import java.awt.event.*;
2 import javax.swing.JApplet;
3 public class KeyEventTest extends JApplet implements KeyListener{
4 private String msg = "";
5 private int startX = 10, startY = 10;
6 public void keyPressed(KeyEvent ke){
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showStatus ("Key Down");
8 }
9 public void keyReleased(KeyEvent ke) {showStatus("Key Up"); }
10 public void keyTyped(KeyEvent ke){
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msg += ke.getKeyChar();
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repaint();
13 }
14 public void init(){
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requestFocus();
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addKeyListener(this);
17 }
18 public void paint(Graphics g){
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g.drawString(msg,startX,startY);
20 }
Unit 11
21 }
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Introduction to Adapter Classes
• From previous examples, listener interfaces can have several methods.
• A particular listener may not be interested in all the methods.
• Nevertheless, the listener must implement all methods in the interface.
• Java provides adapter classes for implementing handlers selectively.
• Most listener interfaces with two or more methods have matching adapter
classes.
• Java provides a collection of abstract event adapter classes.
• These adapter classes implement listener interfaces with empty, do-nothing
methods.
• To implement a listener class, we extend an adapter class and override only
methods needed.
Unit 11
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Handling Window Events
• This example shows how window events can be handled.
• The listener should implement the WindowListener interface.
• WindowListener consists of seven methods:
• We will extend the corresponding WindowAdapter in this
example.
Unit 11
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Example 5: Window Events
1 import javax.swing.*;import java.awt.event.*;
2 class WindowEventTest extends JFrame{
3 private String msg = "Are you sure you want to Quit Window?";
4 public WindowEventTest() {
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super("Window Event Test"); setSize(300,300);
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addWindowListener(new WindowAdapter(){
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public void windowClosing(WindowEvent we) {
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WindowEventTest obj = WindowEventTest.this;
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int result = JOptionPane.showConfirmDialog(obj, msg);
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if (result == JOptionPane.YES_OPTION)
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System.exit(0);
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else {
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int keepOpen = WindowConstants.DO_NOTHING_ON_CLOSE;
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setDefaultCloseOperation(keepOpen);
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}
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}});
17 }
18 public static void main(String args [] ) {
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WindowEventTest wt = new WindowEventTest();
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wt.setVisible(true);
Unit 11
21 } }
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Handling Text Field Events
• This example shows how texfield events are generated and
handled.
• It also illustrates the use of multiple handlers.
• Two text fields are shown handling an ActionEvent in
different ways.
• The program implements Celcius to Fahrenheit temperature
conversions.
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You enter a temperature value in one text
field and get the equivalent in the other.
• The complete program follows in the next page.
Unit 11
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Example 6: Text Field Events
1 import java.awt.*;
2 import java.awt.event.*;
3 import javax.swing.*;
4 class TextFieldEventTest extends JFrame{
5 JTextField celcius = new JTextField(10);
6 JTextField fahrenheit = new JTextField(10);
7 Container c = getContentPane();
8 TextFieldEventTest(){
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c.setLayout(new FlowLayout());
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c.add(new JLabel("Celcius"));
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c.add(celcius);
12 celcius.addActionListener(new ActionListener() {
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public void actionPerformed(ActionEvent ae){
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String cString = celcius.getText();
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double cValue = Double.parseDouble(cString.trim());
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double fValue = cValue*9.0/5.0+32.0;
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fahrenheit.setText((int)fValue+"");
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}});
Unit 11
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Text Field Events – Cont’d
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c.add(new JLabel("Fahrenheit"));
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c.add(fahrenheit);
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fahrenheit.addActionListener(new ActionListener() {
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public void actionPerformed(ActionEvent ae){
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String fString = fahrenheit.getText();
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double fValue = Double.parseDouble(fString.trim());
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double cValue = (fValue-32.0)*5.0/9.0;
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celcius.setText((int)cValue+"");
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}
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});
30 } // end of constructor
31 public static void main(String [] args){
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TextFieldEventTest t = new TextFieldEventTest();
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t.pack();
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t.show();
35 }
36 }
Unit 11
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Review Exercises
1.
Extend Example 1 by adding a Reset Total button. When the Reset
Total button is pushed, the running total should be reset to zero.
2.
Modify the program in Example 2 to work as a stand-alone application.
3.
Modify the program in Example 3 to display its output on the
application’s JFrame window.
4.
Modify the program in Example 4 to use an anonymous inner class to
implement the windowClosing() handler method.
5.
Extend Example 5 to validate the data entered in both text fields to avoid
the spurious exceptions currently raised when invalid characters are
included in the input.
Unit 11
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Review Exercises (cont’d)
6.
A student has written a working applet. Write a step-by-step procedure
that guides the student to make his applet work as an application also.
7.
Consider the program in Example 1. Write down all the events generated
from the time the frame is displayed up to the time the user pushes the
PushMe button. You may restrict your answer to the events covered in this
section.
Unit 11
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