Chapter 13 - Java AWT – Part II (Optional). In this chapter we will: discuss the use of Java in animating World Wide Web applications, show how to write Java applets, introduce Java graphical components (e.g. Textfields, Buttons, and Labels), show how Java programs can be made to react to user actions on Java graphical components, show how Java applications can be run from inside web browsers.
Trang 2Chapter Preview
In this chapter we will:
discuss the use of Java in animating World Wide Web applications
show how to write Java applets
introduce Java graphical components (e.g Texttfields, Buttons, and Labels)
show how Java programs can be made to react to
user actions on Java graphical components
Trang 3Frames
e A frame is a window with a title bar and a border
e The Frame class Is a subclass of Container class
* Container class objects may have other
components (e.g Buttons) added to them using the
add method
¢ Atypical Java GUI will create and display one or
more frames
¢ To make a frame visible the message
Trang 4Layout Managers
Governs how components are arranged Inside a Container object
The Container method setLayout allows the programmer to specify which layout manager
(e.g FLowLayout) Is desired for a particular container object
The size of aContainer object should be set explicitly by the programmer as well
Trang 5Frame Example public class TwoButtons { Frame f; Button redButton, blueButton; public TwoButttons() {
f = new Frame(“Two Buttons Frame”);
Trang 7UML Notation
The filled diamond represents composition
This shows that the class TwoButtons
contains a class Frame which contains a class Buttons
A good UML diagram only shows the classes and associations that are important to
understanding the architecture
Trang 8Using Inheritance
¢ It is possible to use inheritance to allow
TwoButtons to become a frame tn Its own right
¢ Note that the UML diagram will show
TwoButtons as a subclass of Frame
Trang 10Frame Inheritance Example
public class TwoButtons extends Frame {
Button redButton, blueButton;
public TwoButttons() {
super(“Two Buttons Frame”);
Trang 11Other Simple Java Components
¢ Label
— contains some text that can be set using a
constructor or the method setLabel ¢ TextField
— a box into which the user can type text
— text can be edited (backspace, delete, etc.)
— text can be retrieved using getText( )
Trang 12Java AWT Event Model
Including reactive program components involves: 1 Having the class header declare itself as
Implementing the ActionListener interface
Typically in the class constructor the class instance registers itself as being interested in listening for
events from a newly created component
One method (e.g actionPer formed) of the
Trang 13Frame Inheritance Example
public class classname extends Frame implements ActionListener { Button buttonname; public classname() { buttonname = new Button(“button label”); add(buttonname); but tonname ActionListener (this): }
public void actionPerformed(ActionEvent e) { what to do when button is pushed
Trang 15UML Notation for Button Program
¢ Abstract class indicated by italicizing the class name
¢ Implements association (concrete class) indicated by line with a closed arrowhead
¢ UML stereotype (constructors, accessor,
interface implementation methods) indicated by enclosing a descriptive word inside
Trang 16Closing the Window
¢ You must implement the WindowListener and its seven required methods
windowActivated, windowClosed,
windowClosing, windowDeactivated,
windowlconified, windowDeiconified,
w1ndowOpened
¢ This is usually done in a programmer defined class like ClosableFrame and all classes needing these services would extend
Trang 18Using Conditionals with Reactive Components
public void actionPerformed(ActionEvent e) {
double fahr, cent;
Trang 19Checkboxes
Used to allow user to select one or more items
Always has a label
Program often needs to react to the user selection(s)
Declaration example:
Checkbox powerBrakes = new Checkbox(“Power Brakes”);
Checkbox powerSteering = new Checkbox(“Power Steering”);
Checkbox ac = new Checkbox(“Air Conditioning” );
Trang 20Programming Checkboxes Program Is declared with implements ItemListener Checkbox Is registered by calling addItemListener
Event is handled using i1temStateChanged argument type ItemEvent
The ItemEvent argument Is used to tell which item triggered the event by calling
Trang 21Radio Buttons
A group of Checkboxes in which only one item can be selected at a time
Trang 22Processing Radio Buttons
public void compute() {
boolean female =
(gender getSelectedCheckbox() == femaleCheck);
if ((bodyMassIndex > 27.8) ||
(female && (bodyMassIndex > 27.3))
lHigh.setText(“This is considered high”); else
Trang 23Drawing Ina Frame
¢ To draw in a Frame you need to the override the frame’s paint method:
public void paint(Graphics g)
¢ Graphics objects are defined by the Java runtime system and are used for drawing operations
¢ The Frame should be considered to be a grid with upper left coordinates (0,0) and positive
Trang 24Typical Drawing Code
Trang 25Repaint and Update
¢ The paint method requires an argument and actionPerformed does not know what to supply when called
¢ The method repaint will clear the Frame and then call paint along with supplying the needed missing argument
Trang 27Panel and Canvas Classes
¢ The Panel class Is container subclass that Is used to reserve a rectangular portion of a
Frame to place other components
¢ The Canvas class Is not a Container Subclass, but does allow you to reserve a
Trang 28Comparing Layout Managers
FlowLayout
— Default frame layout
— Components are placed on new line only when needed GridLayout — Frame is declared as a grid of fixed size (e.g two rows and three columns) — Components are placed in the grid left to right and top to bottom BorderLayout
— Frame is divided into north, south, east, west, and center — Components are placed by the programmer in the desired
Trang 29Sonne hati
Trang 30
Typical GUI Frame
Trang 31Scrollbars
¢ The leftmost position of a horizontal scrollbar corresponds to some integer value and the rightmost position corresponds to a larger integer value
¢ Scrollbars can be displayed vertically ¢ User movement options
— Clicking either end button causes bubble to move in unit Increments
— Clicking the are between bubble and end button
causes movement in 10 unit increments
Trang 33HTML Text Displays ¢ Displays a centered heading in browser window <CENTER> <H1>Naploean I</H1> </CENTER> ¢ To identify text paragraphs to be displayed <P>
In my spare time I like to do origami
and spend quiet evenings at home with my wife Josephine
Trang 34HTML Using Pictures and Links ¢ Displays a centered picture <CENTER> <IMG SRC=“jojo.jpg”> </CENTER> ¢ To display a text link to another web page <A HREF="“jojo.html”>Josephine</A>
¢ To display a picture link to another web page
Trang 35Applet Tag and HTML
¢ To include a Java applet as part of an HTML document <APPLET CODE=“BodyMass.class” WIDTH=0 HEIGHT=0> </APPLET> ¢ Java applet shell for BodyMass.class import Java.applet ”;
public class BodyMass extends Applet {
public_html void start() {
new BodyMass();
Trang 36Transforming a Java Program Into an Applet 1 Set the size you want the applet to be in the HTML file 2 Add the import statement to the Java code jJava.applet.”; 3 Change header from extends Frame to extends Applet
4 Change constructor heading to
public void start();