Abstract Windowing Toolkit 2

8/12/2019 Abstract Windowing Toolkit 2

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introduce Java graphical components (e.g. Textfields,

Buttons, and Labels)

show how Java programs can be made to react touser actions on Java graphical components

show how Java applications can be run from inside

web browsers (Java applets)

Abstract Window Toolkit (AWT)


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Frames

A frameis a window with a title bar and a border

The Frameclass is a subclass of Containerclass

Containerclass objects may have othercomponents (e.g. Buttons) added to them using theaddmethod

A typical Java GUI will create and display one or

more frames

To make a frame visible the messagesetVisbible(true)must be sent to the frame


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Layout Managers

Governs how components are arranged inside aContainerobject

The Containermethod setLayoutallows the

programmer to specify which layout manager(e.g. FlowLayout) is desired for a particularcontainer object

The size of a Containerobject should be set

explicitly by the programmer as well Can use pack()to set size to accommodate

preferred sizes of components)


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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);

}

}


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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);

}

}


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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()

text can be set using setText()


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Java AWT Event Model

Including reactive program components involves:

1. Having the class header declare itself asimplementing the ActionListenerinterface

2. Typically in the class constructor the class instance

registers itself as being interested in listening for

events from a newly created component

3. One method (e.g. actionPerformed) of the

ActiveListenerinterface is defined


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Frame Inheritance Example

public class classnameextends 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

}

}


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Closing the Window

You must implement the WindowListenerand its seven required methodswindowActivated, windowClosed,

windowClosing, windowDeactivated,

windowIconified, windowDeiconified,

windowOpened

This is usually done in a programmer defined

class like ClosableFrameand all classesneeding these services would extendClosableFrame


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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);

add(powerBrakes);

add(powerSteering);

add(ac);


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Programming Checkboxes

Program is declared with implementsItemListener

Checkbox is registered by callingaddItemListener

Event is handled using itemStateChangedargument type ItemEvent

The ItemEventargument is used to tellwhich item triggered the event by callinggetSource


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Radio Buttons

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

Implemented using a Java CheckboxGroup

Items are declared initially as selected (true) orunselected (false)

Example:CheckboxGroup gender;

Checkbox maleCheck =new Checkbox(Male, gender, true);

Checkbox femaleCheck =

new Checkbox(Female, gender, true);


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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

1High.setText(this is not considered high);

}


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Drawing in a Frame

To draw in a Frameyou need to the overridethe frames paintmethod:public void paint(Graphics g)

Graphics objects are defined by the Javaruntime system and are used for drawingoperations

The Frameshould be considered to be a gridwith upper left coordinates (0,0) and positivecoordinates (x,y) for the lower right


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Typical Drawing Codepublic class MyDrawing extends ClosableFrame {

public MyDrawing() {

}

public void paint(Graphics g) {

g.drawLine();

g.drawImage();

}

}


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Repaint and Update

The paintmethod requires an argument and

actionPerformeddoes not know what to

supply when called The method repaintwill clear the Frame

and then call paintalong with supplying the

needed missing argument

You can prevent clearing the Frame whenusing repaintby overriding the method

updatebefore calling repaint


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Panel and Canvas Classes

The Panelclass is container subclass that is

used to reserve a rectangular portion of a

Frameto place other components The Canvasclass is not a Container

subclass, but does allow you to reserve aportion of a Frameto draw in


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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 desiredlocation using the addmethod


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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);


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Scrollbars

The leftmost position of a horizontal scrollbarcorresponds to some integer value and therightmost position corresponds to a largerinteger 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 buttoncauses movement in 10 unit increments

Clicking and dragging the bubble in the desireddirection


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Transforming a Java Program

into an Applet1. Set the size you want the applet to be in the

HTML file

2. Add the import statement to the Java codejava.applet.*;

3. Change header from extends Frametoextends Applet

4. Change constructor heading to

public void start();

5. Eliminate any Java code that is meaninglessin applets

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Abstract Windowing Toolkit 2