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.

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

show how Java applications can be run from inside web browsers

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

setVisbible( true) must be sent to the frame

Can use pack( ) to set size to accommodate

preferred sizes of components)

f = new Frame(“Two Buttons Frame”);

redButton = new Button(“Red” );

blueButton = new Button(“Blue”);

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 tn 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”);

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

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

ActiveListener interface is defined

Frame Inheritance Example

public class classname extends Frame implements ActionListener {

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

ClosableFrame

generatespP

Using Conditionals with Reactive Components

public void actionPerformed(ActionEvent e) {

double fahr, cent;

// check for user input in tFahr TextField

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(powerSteer1ng);

add(ac);

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

ltems are declared initially as selected (true) or

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 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 coordinates (x,y) for the lower right

Typical Drawing Code

public class MyDrawing extends ClosableFrame {

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

«contains

«governs layout

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

— Frame is divided into north, south, east, west, and center

— Components are placed by the programmer in the desired location using the add method

Sonne hati

Typical GUI Frame

Panel pi = new Panel();

¢ 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

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

¢ 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

public class BodyMass extends Applet {

public_html void start() {

new BodyMass();

}

}

Transforming a Java Program

4 Change constructor heading to

public void start();

1 Eliminate any Java code that is meaningless

In applets