Lecture An introduction to computer science using java (2nd Edition): Chapter 13 - S.N. Kamin, D. Mickunas, E. Reingold

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.

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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 Texttfields, Buttons, and Labels)

show how Java programs can be made to react to

user actions on Java graphical components

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Frames

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

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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 aContainer object should be set explicitly by the programmer as well

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Frame Example

public class TwoButtons { Frame f; Button redButton, blueButton; public TwoButttons() {

f = new Frame(“Two Buttons Frame”);

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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

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Using 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

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Frame Inheritance Example

public class TwoButtons extends Frame {

Button redButton, blueButton;

public TwoButttons() {

super(“Two Buttons Frame”);

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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( )

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Java 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

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Frame 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

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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

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Closing 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

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Using Conditionals with Reactive Components

public void actionPerformed(ActionEvent e) {

double fahr, cent;

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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” );

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Programming 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

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Radio Buttons

A group of Checkboxes in which only one item can be selected at a time

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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

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Drawing 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

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Typical Drawing Code

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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

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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

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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

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Sonne hati

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Typical GUI Frame

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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

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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

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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

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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();

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Transforming 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();