1. Lecture 5
Event Handling

2. Introduction
Event handling is at the core of successful applet and AWT programming
Most commonly events are generated by mouse, keyboard, and various controls, such as a push button
Events are supported by java.awt.event package
The way in which events are handled changed significantly in the newer versionof Java

3. The Delegation Event Model
Source generates an event and sents it to one or more listeners
Listeners simply waits until it receives an event. Once received, they process the event and returns
Event processing code is cleanly separated from user interface code
Listeners must register with a source in order to receive an event notification. So, notifications are sent only to listeners that want to receive them

4. Events Event is an object that describes a state change in a source
Activities that cause to generate events are pressing a button, entering a character via keyboard, selecting an item in a list, clicking the mouse
Events can be generated without user interaction, i.e. timer expired, counter exceeded a value, s/w or h/w failure

5. Event sources
A source is an object that generates an event (due to change of internal state)
A source may generate more than one type of event
A source must register listeners in order to notify them about a specific type of event
publc void addTypeListener(TypeListener el)
i.e. addKeyListener(), addMouseMotionListener()
When an event occurs, all registered listeners are notified and receive a copy of the event object (called multicasting)

6. Event sources (contd.)
Some sources allow only one listener to register (unicasting):
publc void addTypeListener(TypeListener el)
throws java.util.TooManyListenersException
A source also allows a listener to unregister
publc void removeTypeListener(TypeListener el)
i.e. removeKeyListener()

7. Event Listeners
A listener is an object that is notified when an event occurs
Responsibility of a listener:
Register with one or more sources
Implement methods to receive and process these notifications
i.e. MouseMotionListener interface defines two methods to receive notifications when mouse is dragged or moved

8. Event classes
EventObject (in java.util package) is the superclass for all events
EventObject (Object src)
Its getSource() method returns the source of the event and toString() method returns the string equivalent of the event
AWTEvent (in java.awt package) is a subclass of EventObject and superclass of all AWT-based events
Its getID() method can be used to determine the type of the event

9. Main event classes in java.awt.event
ActionEvent
Generated when a button is pressed, list item is double-clicked, or a menu item is selected
Adjustment Event
When a scroll bar is manipulated
ComponentEvent
A component is hidden, moved, resized or become visible
ContainerEvent
A component is added to or removed from a container
FocusEvent
A component gains or loss keyboard focus
InputEvent
Superclass of all input event classes
KeyEvent
Input is received from the keyboard
MouseEvent
Mouse is dragged, moved, clicked, released

10. Main event classes in java.awt.event (contd.)
ItemEvent
A check box, list item is clicked or choice selection is made
TextEvent
The value of a text area or text field is changed
WindowEvent
Window is actiated, closed, deactivated, opened or quit

12. Sources of events Button Checkbox Choice List Menu Item Scrollbar
Generates action event when it is pressed
Checkbox
Generates item events when the check box is selected or deselected
Choice
Generates item events when the choice is changed
List
1. Action events when an item is double-clicked
2. Item events when an item is selected/deselected
Menu Item
1.Action events when a menu item is selected
2.Item events when checkable menu item selected
Scrollbar
Adjustment events when scroll bar is manipulated
Text box
Text events when user enters a character
Window
Window events when window is activated, closed, deactivated, opened etc

13. Event Listener Interfaces
Listeners are created by implementing one or more of the interfaces defined by java.awt.event package
When a event occurs, the event source invokes the appropriate method defined by the listener and provides an event object as its argument

14. Commonly used Event Listener Interfaces
ActionListener
Defines one method to receive action events
AdjustmentListener
One method to receive adjustment events
ComponentListener
Four methods to recognize when a component is hidden, moved, resized or shown
ContainerListener
Two methods, added and removed
FocusListener
Two methods, focusGained, focusLost
ItemListener
One method, itemStateChanged
KeyListener
Three methods, KeyPressed, KeyReleased, KeyTyped

15. Commonly used Event Listener Interfaces (contd.)
MouseListener
Five methods, mouseClicked, mouseEntered, mouseExited, mousePressed, mouseReleased
MouseMotion
Listener
Two methods, mouse is dragged or moved
TestListener
One method, textChanged
WindowEvent
Seven events, windowActivated, windowClosed, windowClosing, windowDeactivated, windowDeiconified, windowicinified, windowOpened

16. Examples Chapter 20, listing 1, MouseEvents
Chapter 20, listing 2, SimpleKey

17. Adapter classes
Adapter classes simplify the creation of event handlers in certain situations
It allows to receive and process only some of the events that are handled by a particular event listener interface
We can define a new class to act as an event listener by extending one of the adapter classes and implementing only necessary events. Other unimplemented methods are handled automatically by adapter interfaces

18. Adapter classes example
MouseMotionAdapter class has two methods, like MouseMotionListener:
mouseDragged()
mouseMoved()
We can extend MouseMotionAdapter and implement only mouseDragged() in my own way.
Empty implementation of mouseMoved() would handle the mouse events for us

20. Introducing the AWT: Working with Windows, Graphics, and Text

21. Introduction
The Active Window Toolkit (AWT) contains numerous classes and methods that allow us to create and manage windows
AWT can be used to create both applet windows and stand-alone windows for GUI environment
AWT classes are contained in the java.awt package
Some AWT classes: Table 21-1 (i.e. Button, BorderLayout, Canvas, Checkbox, Choice, Color, Component, Container, FlowLayout, Frame, Font, Image, Label, List, Menu, Panel, Scrollbar)

22. Window Fundamentals
The AWT defines windows according to a class hierarchy that adds functionality and specificity with each level
Two most common windows are derived from
Panel (used by Applet)
Frame (creates standard windows)

23. The class hierarchy Component Container MenuContainer Window Panel
Interface
Frame

24. Component
An abstract class that encapsulates all of the attributes of a visual component
All user interface elements are subclasses of Components
Provides methods for managing events (i.e. mouse and keyboard input), positioning and resizing the window and repainting
Remember current foreground and background color and current text font

25. Container Allow other component objects to be nested within it
Other Container objects can be stored inside of a Container and thus provide multilevel containment system
Also responsible for laying out components that it contains (by the use of layout manager)

26. Panel A concrete subclass of Container and superclass for Applet
When screen output is directed to an applet, it is drawn on the surface of a Panel object
It does not contain title bar, menu bar or border (applet viewer provides title and border from itself)
Components can be added to a Panel by add() method (inherited from Container)
Components can be positioned and resized by setLocation(), setSize() and setBounds()

27. Window Window class creates a top-level window
A top-level window is not contained within any other object
It sits directly on the desktop
Generally we don’t create Window objects directly, rather we create subclass of Window called Frame

28. Frame It has title bar, menu bar, borders, and resizing corners
Frame object can be created within an applet, but that is not well practiced, as applet application can do some malicious things in the background
When a frame window is created by a program rather than an applet, a normal window is created

29. Working with Frame windows
Frame can be used as a child window within applet, a top-level window or child windows for applications
Frame() // standard window that does not contain title
Frame(String title)
The size of the window can be mentioned during construction, rather it is set afterwards
setSize(int newWidth, int newHeight)
setSize(dimension newSize)
getSize() method is used to obtain the current size
setVisible(boolean visibleFlag) is used to show or hide a window, specially when a window is closed
setTitle(String newTitle) is used to set the new title
To intercept a window-close event, we have to implement the windowClosing() method of WindowListener interface

30. Creating a frame window in an Applet
Generally we don’t create Frame objects directly but create a subclass of Frame and override Frame’s methods and handle events
Creating an object of subclass of a frame does not make initially visible. We make visible by setVisible() method
When created, the window is given a default height and width and can be changed at anytime
Example: AppletFrame

31. Handling Events in a Frame Window
We can use and manage a frame window that we create just like we manage our applet’s main window (as both are subclass of Component)
Whenever an event occurs in a window, the event handlers defined by that window will be called
Each window handles its own events
For applet window, we don’t need to register window listener, as window closing event is automatically handled by applet
Example: WindowEvents

32. Creating a Windowed Program
We can create a stand-alone AWT based applications
To do so, we need to create an instance of the window or windows inside main()
After creating, we need to set its visibility equals to true
Example: AppWindow

33. Working with Graphics All graphics are drawn relative to a window
The origin of each window is the top-left corner and is 0,0.
Coordinates are specified in pixels
All output to a window takes place through a graphics context, defined in Graphics class
Objects are drawn and filled in the currently selected graphics color, which is black by default

34. Working with Graphics (contd.)
Graphics class defines a number of drawing functions:
drawLine(int startX,int startY,int endX,int endY)
drawRect(int top,int left,int width, int height)
fillRect(int top,int left,int width, int height)
drawRoundRect(int top,int left,int width, int height,int xDiam,int yDiam)
fillRoundRect(int top,int left,int width, int height,int xDiam,int yDiam)
Example: Lines and Rectangles

35. Drawing functions: Example: Ellipses, Arcs and HourGlass
drawOval(int top,int left,int width, int height)
fillOval(int top,int left,int width, int height)
drawArc(int top,int left,int width, int height,int startAngle, int sweepAngle)
fillArc(int top,int left,int width, int height,int startAngle, int sweepAngle)
drawPolygon(int x[], int y[], int numPoints)
fillPolygon(int x[], int y[], int numPoints)
Example: Ellipses, Arcs and HourGlass

36. Sizing Graphics
If we want to size a graphics object to fit the current size of the window, we have to obtain the current dimensions of the window
Dimension getSize()
Then we can scale our graphical output accordingly
Example: Resize

37. Working with color We can create our own color by:
Color(int red,int green,int blue) //values are 0 to 255
Color(int rgbValue) // values are like 0xffc00034
Color(float red, int green, float blue) //values are 0 to1.0
setForeground() and setBackground() is used to set any color as foreground or background
setColor() is used to set current graphics color
getColor() is used to know current color
Example: ColorDemo

38. Working with Fonts
AWT supports multiple type font-manipulation operations and dynamic selection
Each font has a family name (the general name like Courier), a logical name (the category like Monospaced) and face name (specific font like Courier Italic)
String[] getAvailableFontFamilyNames() is used to get available fonts
Font[] getAllFonts() return an array of Font objects for all available fonts

39. Working with Fonts (contd.)
To select a new font, first construct a Font object by:
Font (string fontName,int fontStyle,int pointSize)
Then set the font by setFont(Font fontObj)
Font style may be Font.PLAIN, Font.BOLD etc.
Example: ShowFonts and SampleFonts