EVENTS

1
EVENTS

• CSC 171 FALL 2004
• LECTURE 16

2
Traditional Input

• In console applications, user input is under
  control of the program
• The program asks the user for input in a specific
  order

3
Event based input

• The user is in control of the sequencing
• Mouse
• Keyboard
• pull down menus
• click buttons
• etc.
• IN ANY ORDER!

4
Java event handling

• The Java window toolkit has a sophisticated
  mechanism that allows a program to specify the
  event
• Whenever the user of a graphical program types
  characters or uses the mouse the Java window
  managers sends a notification to the program that
  an event has occurred

5
Events

• User interface events include key presses, mouse
  moves, button presses, etc.
• As the programmer, you supply event listeners
  (objects) for the events you care about.
• You need to implement methods for handling the
  events in the event listener you supply
• You construct listener objects.

6
Event Listeners

• Event Listeners are objects of a class
• You construct listener objects and attach them to
  the event sources
• registration
• Event sources are things like text boxes,
  buttons, etc
• Sources of events keep track of the type of
  things they listen for

7
Event Notification

• Event notifications happen in event listener
  classes.
• An event listener class implements an event
  listener interface
• When an event occurs the event source will call
  the methods you supplied in the listener
• The invocation of the method will be supplied
  with information about the event in an event
  class object

8
The listener is an object

• Sometimes an object can be two things
• an applet
• a listener
• Sometimes, we make an entirely different class

9
MouseListener

• public interface MouseListener void
  mousePressed(MouseEvent event)
•  
• void mouseReleased(MouseEvent event)
• void mouseClicked(MouseEvent event)
• void mouseEntered(MouseEvent event)
• void mouseExited(MouseEvent event)

10
MouseSpy
11

• public class MouseSpy implements MouseListener
• public void mousePressed(MouseEvent event)
• System.out.println("Mouse pressed . x "
• event.getX() " y "
• event.getY())
• public void mouseReleased(MouseEvent event)
• System.out.println("Mouse released . x "
• event.getX() " y "
• event.getY())

12

• public void mouseClicked(MouseEvent event)
• System.out.println("Mouse clicked . x "
• event.getX() " y "
• event.getY())

13

• public void mouseEntered(MouseEvent event)
• System.out.println("Mouse entered . x "
• event.getX() " y "
• event.getY())
• public void mouseExited(MouseEvent event)
• System.out.println("Mouse exited . x "
• event.getX() " y "
• event.getY())

14

• public class Lec14 extends Applet
• public Lec14()
• MouseSpy listener new MouseSpy()
• addMouseListener(listener)

15
Components

• Java supports things like buttons and fields
• The java awt can vary from machine to machine
• Swing is more platform independent

16
Event Registration
textField.addActionListener(handler)

17
Example Applet

• lthtmlgt
• ltheadgt
• lttitlegt Ted Pawlicki's CSC 171 Applet
• lt/titlegt
• lt/headgt
• ltbodygt
• ltapplet codeLec14b.class" width 512 height
  512 gt
• lt/htmlgt

HOW DOES IT WORK?
18

• import java.awt.
• import java.applet.
• import java.awt.event.
• public class Lec14b extends Applet
• implements ActionListener
• private Label prompt1,
• prompt2, prompt3, greeting
• private TextField firstname, lastname

WHAT IS THIS THING?
19

• public Lec14b()
• setLayout(null)
• prompt1 new Label("Welcome to Ted Pawlicki's
  CSC 171 Lec 14b Applet")
• prompt2 new Label("Please enter your first
  name")
• prompt3 new Label("Please enter your last
  name")
• greeting new Label()
• firstname new TextField()
• lastname new TextField()

20

• add(prompt1)
• prompt1.setBounds(150,50,350,32)
• add(prompt2)
• prompt2.setBounds(150,100,150,32)
• add(firstname)
• firstname.setBounds(150,150,150,32)
• add(prompt3)
• prompt3.setBounds(150,200,150,32)
• add(lastname)
• lastname.setBounds(150,250,150,32)

21

• firstname.addActionListener(this)
• lastname.addActionListener(this)

22

• public void actionPerformed (ActionEvent
  event)
• greeting.setText("Nice to meet you, "
• firstname.getText() " "
• lastname.getText())
• add(greeting)
• greeting.setBounds(150,300,350,32)
• doLayout()

WHAT IS GOING ON?
23

• public void actionPerformed (ActionEvent
  event)
• greeting.setText("Nice to meet you, "
• firstname.getText() " "
• lastname.getText())
• System.out.println("Event " event)//
• add(greeting)
• greeting.setBounds(150,300,350,32)
• doLayout()

WHAT IS THE EVENT?
24
Inner Classes
25
Inner Classes

• public JButton makeButton(String label, final int
  dx,final int dy) JButton button new
  JButton(label) class ButtonListener implements
  ActionListener public void
  actionPerformed(ActionEvent event)
  box.translate(dx, dy) repaint()
  ButtonListener listener new
  ButtonListener() button.addActionListener(listen
  er) return button

26

• panel.add(makeButton("Left",-BOX_WIDTH,0))
• panel.add(makeButton("Right",BOX_WIDTH,0))
• panel.add(makeButton("Up",0,-BOX_HEIGHT))
• panel.add(makeButton("Down",0,BOX_HEIGHT))