Transcript Chapter 12 Java AWT – Part 1 (Optional)

Chapter 13
Java AWT – Part II
(Optional)
Lecture Slides to Accompany
An Introduction to Computer
Science Using Java (2nd Edition)
by
S.N. Kamin, D. Mickunas, E. Reingold
Chapter 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. 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
Frames
• A frame is a window with a title bar and a border
• 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
• A typical Java GUI will create and display one or
more frames
• To make a frame visible the message
setVisbible(true) must be sent to the frame
Layout 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 a Container object should be set
explicitly by the programmer as well
• Can use pack() to set size to accommodate
preferred sizes of components)
Frame Example
public class TwoButtons {
Frame f;
Button redButton, blueButton;
public TwoButttons() {
f = new Frame(“Two Buttons Frame”);
redButton = new Button(“Red”);
blueButton = new Button(“Blue”);
f.setLayout(new Flowlayout());
f.add(redButton);
f.add(blueButton);
f.pack();
f.setVisible(true);
}
}
UML 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
• This UML diagram does not include super
classes like Component or Container
Using Inheritance
• It is possible to use inheritance to allow
TwoButtons to become a frame in its own
right
• Note that the UML diagram will show
TwoButtons as a subclass of Frame
• You will need to use super()to set the frame
title
Frame Inheritance Example
public class TwoButtons extends Frame {
Button redButton, blueButton;
public TwoButttons() {
super(“Two Buttons Frame”);
redButton = new Button(“Red”);
blueButton = new Button(“Blue”);
f.setLayout(new Flowlayout());
f.add(redButton);
f.add(blueButton);
f.pack();
f.setVisible(true);
}
}
Other 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()
text can be set using setText()
Java AWT Event Model
Including reactive program components involves:
1. Having the class header declare itself as
implementing the ActionListener interface
2.
3.
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. actionPerformed) of the
ActiveListener interface is defined
Frame Inheritance Example
public class classname extends Frame implements ActionListener {
…
Button buttonname;
…
public classname() {
…
buttonname = new Button(“button label”);
…
add(buttonname);
…
buttonname.ActionListener(this);
…
}
…
public void actionPerformed(ActionEvent e) {
… what to do when button is pushed …
}
}
UML 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
guillements << >>
Closing the Window
• You must implement the WindowListener
and its seven required methods
windowActivated, windowClosed,
windowClosing, windowDeactivated,
windowIconified, windowDeiconified,
windowOpened
• This is usually done in a programmer defined
class like ClosableFrame and all classes
needing these services would extend
ClosableFrame
Using Conditionals with
Reactive Components
public void actionPerformed(ActionEvent e) {
double fahr, cent;
// check for user input in tFahr TextField
if (e.getSource() == tFahr) {
fahr = new Double(tFahr.getText()),doubleValue();
cent = 5.0 * (fahr – 32.0) / 9.0;
tCent.setText(cent + “”); // update tCent TextField
}
else {
cent = new Double(tCent.getText()),doubleValue();
fahr = 9.0 * cent / 5.0 + 32.0;
tFahr.setText(fahr + “”);
}
Checkboxes
•
•
•
•
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”);
…
add(powerBrakes);
add(powerSteering);
add(ac);
Programming Checkboxes
• Program is declared with implements
ItemListener
• Checkbox is registered by calling
addItemListener
• Event is handled using itemStateChanged
argument type ItemEvent
• The ItemEvent argument is used to tell
which item triggered the event by calling
getSource
Radio Buttons
• A group of Checkboxes in which only one item can be
selected at a time
• Implemented using a Java CheckboxGroup
• Items are declared initially as selected (true) or
unselected (false)
• Example:
CheckboxGroup gender;
Checkbox maleCheck =
new Checkbox(“Male”, gender, true);
Checkbox femaleCheck =
new Checkbox(“Female”, gender, true);
Processing 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
1High.setText(“this is not considered high”);
}
Drawing in a 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
coordinates (x,y) for the lower right
Typical Drawing Code
public class MyDrawing extends ClosableFrame {
…
public MyDrawing() {
…
}
…
public void paint(Graphics g) {
…
g.drawLine(…);
…
g.drawImage(…);
…
}
}
Repaint 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
• You can prevent clearing the Frame when
using repaint by overriding the method
update before calling repaint
Panel 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
portion of a Frame to draw in
Comparing 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
location using the add method
Typical GUI Frame
Panel p1 = new Panel();
p1.setLayout(new GridLayout(2,1));
p1.add(component);
p1.add(component);
Panel p2 = new Panel();
p2.setLayout(…);
p2.add(component);
…
setLayout(new BorderLayout());
add(“North”, p1);
add(“Center”, p2);
Scrollbars
• 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
– Clicking and dragging the bubble in the desired
direction
Hypertext Markup Language
HTML – Skeleton
<HTML>
<HEAD>
<TITLE>Page Title</TITLE>
</HEAD>
<BODY>
Text and graphics to display in browser
</BODY>
</HTML>
HTML 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.
</P>
HTML 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
<A HREF=“jojo.html”><IMG SRC=“jojo.jpg”></A>
Applet 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();
}
}
Transforming a Java Program
into an Applet
1.
2.
3.
4.
5.
Set the size you want the applet to be in the
HTML file
Add the import statement to the Java code
java.applet.*;
Change header from extends Frame to
extends Applet
Change constructor heading to
public void start();
Eliminate any Java code that is meaningless
in applets