09 Apple Ts and Applications

7/29/2019 09 Apple Ts and Applications

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Appletsand

Applications

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Application

Applet

HTML for applets class Applet

Inevitable HelloWorld example

More simple applet examples

JApplet

(Practical alternative to Applets Java Web Start)

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Applets and Applications Application

independent program

full access to host machine normal file access (standard security constraints on file ownership)

ability to create sockets (arbitrary network connections)

(slightly restricted) access to environment variables

GUI create Frame (or JFrame) object as principal window

may have additional windows for dialogs, alerts etc

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Applets and Applications

Applet

program embedded in web page and run by browser

restricted access to machine on which it is run(restrictions can be varied by sophisticated browser)

no access to local files

can open network connection only to site from where appletwas itself loaded

can access some details of enclosing web page

GUI

use part of browsers page window for main window

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Applet

Applets do have many restrictions

but they were the feature that popularised Java

downloadable executable content

security policy easily enforced by browser environment

lots more pretty GUI features than you could readilyobtain with just HTML+forms

real computations, not just checks on data entry &trivial calculations as in J avascript

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Internet

Client machine Web servermachine

Web browserprogram

(J ava byteinterpreter,HTML, applet bytecodes)

httpdweb serverprogram

Screen with browser windowcontaining Applet subwindow

J ava

HTML

Bytes

Server fi lesystem

/http_docs


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Application

Applet




JApplet


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Applet -HTML

HTML code on web page contains tag specifyingapplet (


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HTML for Applets

Applet demo

See my applet

Your browser is Java challenged, modernise!

Text between and displayedif browser doesnt support applets.

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HTML for Appletsoptions in


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Parameters for applet

you have a java challenged browser, modernise

paramsplaced betweenandtags

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Application

Applet




JApplet


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class Applet

class MyApplet extends Applet/J Applet { ... }

AnApplet is a

Panel is a

Container is a

Component is an

Object

Or extends JAppletif you want the modern swing graphics

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Application

class MyApplication extends Frame{ ... }

An application (with a GUI) is something

that uses (or, sometimes, is) aFramewhich is a

Windowis aContainer is a

Component is an

Object

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Application

More typicallypubl i c cl ass MyGui ext ends Fr ame/ J Fr ame

{}

publ i c cl ass MyAppl i cat i on {

st ati c MyGui t heGUI ;

publ i c stati c voi d mai n(Str i ng[] args) {

theGUI = new MyGui () ;

}

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class Applet

Because an Applet is a Component

it is a basic GUI element in its own right

it has a paint() method, a repaint(), an update(),...

Because it is also a Container

it can have subwindows, so can holdCanvases, Checkboxes, Buttons, ...

And, because it is a Panel, it has adefined way of arranging subwindows


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class Applet

An Applet has its own unique functionality init() first method called when applet

being started; typically does thingslike read parameters, load images, ...

start() called after init() and each time userreturns to HTML page with applet;typical use -starting threads, ...

stop() called each time user leaves HTMLpage; typical use - suspend threads

destroy() free resources (doesnt Java do this? well,sometimes applet does have stuff to clean up)and kill threads

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class Applet

An Applet has its own unique functionality getAppletContext() returns an object which allows some

communication with browser & HTML page

getParameter(Stringname) returns value of parameter with name(or null)

getCodeBase(), getDocumentBase()return URLs of page, etc

showStatus(Stringmsg) puts message in status windowofbrowser

Applet - access environment

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class Applet

An Applet has its own unique functionality getImage(URL src) arranges to load an image (eg a .gif file) that

is to be displayed in (a subwindowof) Applet

getAudioClip(URL src) arranges to load a sound file

play(URL src) plays sound file

Image not loaded until try to display, then may get delays. Loading an Audio allows finer control, can start sound clip, stop

it etc; play() simply plays it, or does nothing if cannot load.

Multimedia features figure prominently in toy applets.

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Application

Applet

HTML for applets

class Applet



JApplet


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The inevitable

HelloWorldapplet

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HelloWorldapplet

All it does is

pick a color using argument

draw in its subwindowwithin browser

So,

a specialised subclass of Applet

has an init() method that reads parameter

has a paint(Graphics g) method that draws

accepts defaults (do nothing) for start(), stop(), ...

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HTML file for Hello Applet

Hello Applet

See my Applet

You are Java challenged!

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class helloextends Applet

import java.awt.*;

import java.applet.*;

public class hello extends Applet{

Color c =Color.black;

public void init() { ... }public void paint(Graphics g) { ... }

}

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public void init() {

String colorval =getParameter("color");

if(colorval ==null) return;

if(colorval.equals("red")) c=Color.red;

if(colorval.equals("blue")) c=Color.blue;

}

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public void paint(Graphics g) {

g.setColor(c);

g.drawOval(50, 60, 100, 150);

g.fillOval(70, 110, 20, 20);

g.fillOval(110, 110, 20, 20);

g.drawArc(70, 170, 50, 15, 0, -180);

}

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create hello.html and hello.java

javachello.java

appletviewer hello.html (NetBeansdoes this totest applet)

or

communicator, open hello.html

or

mozilla, open hello.html

Communicator (default browser onUnix) is quite old and has a numberof l imitations.Mozillacauses unpleasant colour fl ickerson Sun Blade workstations


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Problems testing applets

Remember supposed to be opening files acrosshttp style network.

This sometimes causes problems when trying totest an applet and just using local files.

Some operations dont work (browser appliessecurity restrictions so cannot get files).

Tests with local files usually work better if useAppletViewer.

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Applet constructor?

Typical class constructor of class does all work to initialize it

(so would expect a GUI class to do setup andinitialization in constructor)

Typical Applet class (as in Suns examples) work is done in init() rather than constructor

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Applet constructor has limitations

Reason is that some GUI elements cannot beinitialized in constructor (most can)

Some odd features about setting up Applet environment

Environment not set right until constructor completed

Some GUI elements require interrogation of

environment and cannot be created properly from inconstructor

(Only those needing an image to be loaded??)

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NetBeansversion

First chance to meet the GUI builder

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As usual, new Projectnew Package

New File/ Applet Form

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Pick Applet Form


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Form-editing view of applet class

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Code-editing view of Applet class

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GUI editing : form-page/code-page

(This style is common, if you get to work withMicrosofts Visual Studio you will see verysimilar operation.Actually, Microsoft really lead the development ofvisual editors starting way back in the late 1980s.)

Two views swap back and forth Point and click editor, adding visual components and

setting their properties

Code editor see the resulting code

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Hello Applet NetBeansstyle

Actually have now finished the GUI editing!

(It isnt usually that simple.)

This Applet has no subordinate GUI elements (no buttons,no text input fields etc etc)

Still have to define the code to pick up the color parameterand to draw the little picture!

Switch to text view.

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Add some lines to init() suppliedby NetBeans

NetBeanswill have generated some code in the init() function,add own code to read applet-paramsand set colour; add colourdata member

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Add the paint function

That little blue arrow?It is a tag that NetBeansadds; it means this functionoverrides the definition of the function in the base class


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Generated HTML file

Run file caused NetBeansto generate aHTML file that has request for embeddedapplet,then ran this with AppletViewer

Generated file is in buildfolder,copy and edit it if need parameters

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HTML

Applet HTML Page

Generated by NetBeansIDE

Need to add the paramtag specifying a colour

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Edit the copy add params


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and run again

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NetBeansapplet outside of NetBeans

Copy the generated .jar file

dist/AppletDemo.jar

Compose a HTML page

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HTML

My NetBeansHelloApplet HTML Page

My Hello applet will appear below:

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HTML pages

Usually, HTML page displaying an appletwill have lots of markup, content text, andmaybe pictures as well as the applet.

AppletViewer shows only the applet.

You have to view page in browser if youwant to see all the content.

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Application

Applet

HTML for applets

class Applet



JApplet



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More Appletexamples

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Horstmannsexample applets

Following examples from Horstmann, Core Java

chart applet

bookmark applet

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HorstmannsChart Applet

Illustration of use of

Applet

draw bar chart

named columns

relative heights defined by double values

tags allow same applet todisplay different data on different pages

could use in generated page with tags as output from some program

Minor mods to use standard i/o rather than Horstmanns library

Really nothing much more than the HelloWorld example

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...

...

Chart.html

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Chart.javaimport java.awt.*;

import java.applet.*;

import java.io.*;

public class Chart extends Applet

{

public void init() { ... }

public void paint(Graphics g) { ... }

private double[ ] values;

private String[ ] names;

private String title;

}


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public void init()

{ intn = Integer.parseInt(getParameter("values").trim());

values =new double[n];

names =new String[n];title =getParameter("title");

int i;

for (i =0; i


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Horstmannsbookmark applet

Illustrates communication with AppletContext

request that AppletContextdisplay a different page(display is in frame, could have used a separatewindow)

Multiple HTML files

frameset using two frames

contents for frames

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Bookmark Applet

Bookmark.java (HTML file that defines frameset with

contents Left.html and Right.html)

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A Bookmark Applet

Double-click on one of the names in the list box. The corresponding webpage

will be displayed in the frame on the right.

...

Left.html (HTML file that contains and

associated s)

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Web pages will be displayed here.

Double-click on one of the names in the list box to the left. The web page

will be displayed here.

Right.html (HTML file that contains initial

filler for right hand frame)

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import java.awt.*;

import java.awt.event.*;import java.applet.*;

import java.net.*;

import java.io.*;

public class Bookmark extends Applet implements ActionListener{

public void init() { ... }

public void actionPerformed(ActionEventevt) { ... }

private List links =new List(10, false);

}

A List GUI element can respond to a double click byreporting anActionEvent

something has to handle that event

might as well be the Applet, no need for any other object

Bookmark.java


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public void init(){

setLayout(new BorderLayout());

add("Center", links);

links.addActionListener(this);inti =1;

String s;

while ((s =getParameter("link_" +i)) !=null) { links.add(s); i++; }

}

public voidactionPerformed(ActionEventevt){

String arg=evt.getActionCommand();

try

{ AppletContextcontext =getAppletContext();

URL u =new URL((String)arg);

context.showDocument(u, "right");

} catch(Exception e) { showStatus("Error " +e); }

}

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Horstmansbookmark applet

Defined only

init This created a java.awt.List with data from Applet parmaters

actionPerformed This lets applet talk to applet-context (i.e. browser) asking for

display of another page

How did it get to paint the list? No paint() methoddefined!

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Applet.paint()

Applet is a Panel which is a Container

Container has a default paint method

For each component that I contain, tellcontained component to paint itself

This Applet contains a java.awt.Listpr i vate Li s t l i nks = new L is t ( 10, fa l se) ;

add(" Center", l i nks);

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java.awt.List.paint()

The java.awt.Listclass defines a paint method

Use a scrollable pane, draw scrollbar if necessary

Draw each entry (String) on a line in scrollable pane

So didnt need to redefine paint in the applet itself,

the default inherited methods do whatever isnecessary.

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Chooser Applet

Applet with interactive controls

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Chooser Applet

Most GUI interfaces, and many Applets,use standard controls that let user selectprocessing operations.

Following simple applet illustrates some ofthese controls and also shows off more ofNetBeansGUI Builder


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Chooser Applet

Applet is to show Rectangle or Oval

Top/left, width/height adjustable

Applet is to have drawing area where shape displayed

Control area Choice of Rectangle/Oval

Labels and text fields for dimensions

Action button

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Rough view

Drawing area

showing a rectangle

Controls in panel at bottom

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GUI components

This time Applet isnt simply a single panel usedto display data (as it has been in all previousexamples)

As a Panel (a kind of Container) it can hold otherGUI components.

Here, want it to hold

A Canvas a region reserved for drawing data object A collection of text input fields (with associated labels)

Fields for top, left, width, and height

A button which will cause applet to change data anddisplay

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GUI classes

Need to use Standard GUI components

Button

Label

TextField

Choice (pop-up list offering Rectangle/Oval)

Panel Customized GUI components

MyCanvasextends Canvas

Layout manager (arranges how components displayedin Applet)

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GUI components

Topic of next main lecture segment!

For now Label

Create Label object with String

Place in GUI

TextField Editable text automatically looks after all keyboard and

mouse editing action

Can ask a TextField for its current text when needed

Button Generates action events

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GUI components

GUI components: Label, TextField, Button,and Choice

Choice Pop-up list, lets you pick one out of a set of

String values

Create choice object

Place in GUI

Add option, add option, add option


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Customized GUI component

Canvas

Reserve an area of GUI for display of application data

paint() uhm, paint what?

Have to create a subclass of Canvas to haveanything useful!

Class MyCanvasextendsCanvas

Has link to data object

paint() function can arrange for painting of data object

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BorderLayout

Will be dealing with layout managersshortly.

BorderLayout is convenient for simpleapplications Divides Applet (or other container) into upto

five regions (dont have to use them all) Center, North, East, South, West

Each region can hold one other component Canvas, Button, Label

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BorderLayout

NORTH

SOUTH

WestEast

Center

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Extra panels

BorderLayoutallows for one component in each of itsregions

We need several Label and text-field for top

Label and text-field for left

Choice Action button

Have to use extra Panel objects Place Panel in south region

Add labels and text-fields etc to this panel

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BorderLayoutas used

NORTH holds a title (Label)

SOUTH panel holding labels, textfields, button

Center MyCanvasdisplaying rectangle oroval

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Event listening

Button will generate ActionEvents

Applet will listen for the ActionEvent Handle it by changing data and arranging for

update of display

Applet creates the button in its init()method, will add itself as the action-listeneronce button created.


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Event listening the usual

GUI interface

Component that generates events (Button)

Defined class that will implement appropriatelistener so that it can handle the event (Appletclass will implement ActionListener)

Initialization code will link listener to event source

You should be getting the idea of event listeningby now

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Classes

Data

Owns some Shape

Does Gets told to draw its shape or change its shape

MyCanvas

GUI component occupies space in GUI where drawdata

Simply owns link to data and passes on paint requests

ChooserApplet

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MyCanvas

public class MyCanvasextends Canvas {

private Data myData;

public MyCanvas(Datad) {

myData=d;

}

public void paint(Graphicsg) { myData.paint(g); }

}

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Datapublic class Data {private Rectangle myShape;private booleanoval;public Data() {

myShape=new Rectangle(40,80,100,50);oval =false;

}public void makeRectangle(int top, int left, int width, intheight) {

myShape=new Rectangle(top, left, width, height);oval =false;

}public void makeOval(int top, int left, intwidth, int height) {

myShape=new Rectangle(top, left, width, height);

oval =true;}public voidpaint(Graphicsg) {

if(oval)g.fillOval(myShape.y, myShape.x, myShape.width, myShape.height);

elseg.fillRect(myShape.y, myShape.x, myShape.width, myShape.height);

}

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NetBeans

New Project

New Package

New Java class MyCanvas

New Java class Data

New Applet Form ChooserApplet

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Applet

Create the applet class

In form view, click the outline and selectsetLayout,choose BorderLayout


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Add parts

Label at top

Palette/AWT

Drag Label into top of outlined applet panel

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Fix up properties

Select label

Go to properties subwindow Change Font (larger size, )

Change Text (Chooser Applet)

Change Alignment (center)

Also change Name of variable

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South

From AWT add Panel to South

Then add to the panel:

Pair (Label, TextField) for each of inputs

Fix variable names to make them meaningful

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Adding code

Properties box of component also has a codetab; Can specify extra code to be inserted before code that

adds component Can specify different specialized code for adding a

component Can specify extra code to be inserted after code that

adds component

For Choicecomponent Have to add the Choice object Then in Post creation codeneed to add the Strings


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Added Choice to PanelChanged name to shapeChoiceUsed properties panel to

set font etc;Then switched to Code

section of properties

Added code

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ActionButton

Similarly add Button

But now want something special a listenerwill be attached to this button

It is going to be the Applet

Need extra code something other thanroutine; addActionListener(this)

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Adding code

Here need extra code after

Add actionListener

Specify this (i.e. the applet) as the action listener

Note will result in code that is erroneous we

havent yet specified that the applet is toimplement ActionListener interface; will fixshortly

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MyCanvas

Main central region of Applet is to show aMyCanvaswhere will draw the data.

AWT palette doesnt have MyCanvas

AWT palette does have Canvas! Add a Canvas

Use code properties to customize creation code

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Switch to code view

Generated code includes an initComponentsfunction

Shown collapsed, with a do not Edit warning

Open collapsed view

See errors

addActionListener(this) when ChooserApplet is notan ActionListener

new MyCanvas(theData) when theDatanot defined

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Fix-ups

Add implements ActionListener to class

Fix imports

Then tell NetBeansto implement all declaredmethods

Get an empty actionPerformed() function added Declare a private data member of class Data

and create instance in init() function (beforecall to initComponents())

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Implement actionPerformed

When Button pressed

Check that one of items in choice has been chosen If none chosen, do nothing

Read data values from each of TextFieldsand convertto integers

If any input conversion errors, do nothing

Invoke operation on data object to set new shape anddimensions

Request update of My Canvaspart of display

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Action performed


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Run the Applet

Run file ChooserApplet

Will need to resize appletviewer window

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Event listening again

So, typical event listening application

Button in GUI is source of ActionEventevents

ChooserApplet is a listener for action events

implements ActionListener

Listener object registered with event sourcedoItButton.addActionListener(this);

actionPerformed() function does some workwhen event occurs

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Other events?

Choice

Potentially source of ItemEvents Each time you change a selection you get Deselected then

Selected events

TextField

Potentially source of TextEvents Each change to one of number fields will result in event

NOT listening for either TextEventsor ItemEvents

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Usually only monitor action buttons

Dont need to know that user is changing the Itemselection

Dont need to know details of each change madeto text input fields

Leave these controls to look after themselveswhile user is entering data

Wait for user to complete input; user will thenactivate action button.

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NetBeansauto-generation ofevent handling code


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Event handling

Normal

Make one of your classes implementappropriate listener

Add instance of class as listener to event source

NetBeanshas an option that automates partof code but uses a different approach

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Chooser2 applet

Simplified version of Chooser

Ive just added a Choice and an Button to panel

Ive selected Events from the properties panelassociated with the Button

Picked Action events

Dialog (not very clear) is asking me to enter a name fora function that will be added to enclosing applet thatwill handle the action-event

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Generated code 1

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How does handleButton() get called?Magic!

Well actually, look inside theinitComponentsand all will be revealed(sort of)

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Anonymous inner class code --- yuk

doIt.setLabel("Do it");

doIt.addActionListener(newjava.awt.event.ActionListener() {

public void actionPerformed(

java.awt.event.ActionEventevt) {

handleButton(evt);

}

});

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Inner class code

That stuff

Defines a new class with an actionPerformedmethod

This actionPerformedmethod calls the handleButtonfunction in the enclosing Applet class

Creates an instance of this class and attaches it to thebutton

Depends on obscure properties of inner classes inJava

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Auto-code

Personally, I dont like such code

I find inner classes and their conventions anunnecessary and confusing addition to Java

This approach forces the applet to be thelistener for the button

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Model-View-Control

These applications have three aspects Model (Data)

The data being manipulated

View The GUI views of application data, input fields for use by

user

Control Monitor the inputs (listen for those events that really need to

handle)

Read and validate data inputs

Invoke processing operation

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Model-View-Control : 3 separateaspects

These are different aspects of application

Best handled by three separate classes (or, usually,clusters of classes) Model main class owning all application data and

performing application actions View main GUI class Control best placed in another class

Main program creates model, GUI, and controlobject and links them up

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NetBeansauto-coder: View=Control

Auto-code approach forces Applet to beboth view and control in application

Maybe OK in simple cases

Not a good idea in more advanced applications


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NetBeansGUI editing

Is it worth it?

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NetBeansGUI editing

Is it worth it?

For simple examples like this

No It is easier to write the code for the GUI than fiddle

with all those dialogs especially the dialogs foradding custom code like Post create etc

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NetBeansGUI editing : for complexGUIs

With more complex GUI interfaces, the GUIeditor approach is definitely advantageous.

GUI editor may not help much (may even hinder)development of simple applications but use itanyway to see how it works

GUI editor very useful later with complex GUIs

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Applet restrictions

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Sandbox environment

Java programs run with security managerobjects

all calls to functions are interpreted, so haveopportunity to hook in extra checking at time of call;

classes get tagged with information (class loader &security manger) when loaded; if there is a securitymanager, then this will be used to vet function calls;

security manager loaded for applets is by default veryrestrictive

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No peeking by applets

Applets cannot

access directories or files on machine that hostsbrowser

cannot read pixels from the screen

cannot open socket connections to machineother than one from which applet downloaded


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Easing the restrictions

Restrictions prevent things like a simplespreadsheet applet (you couldnt store your data)

So more elaborate security system introduced.

Applets get security certificates (mechanism thatallows browser to verify that applet really is fromsource specified)

Applets that have such security certificates cannegotiate with user (via dialogs) can applet have permission to access files?

can applet open socket to xxx.yyy.zzz.aaa

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Permissions for applets

Allows applets to be used for more thansimple demos and games

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JApplet

If you want to use swing classes (fancythings like JTable) then you must use

JApplet rather than Applet

One problem! threads and swing!

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Swing thread issue & JApplet

Swing library is unchecked single threadedcode unchecked means no locks

Swing code should only be executed byevent handling thread

init() function of Applet (JApplet) is notexecuted by event handling thread

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invokeLater etc

Strictly, you cannot build a JAppletinterface in an init()method that adds JTextFields, JButtonsetc

Instead you have to use more elaborate code that createstasks that will be invoked later by the event handlingthread

Most simple JAppletexamples dont seem to do this they work

almost always

(if you create the display in the constructor they might alwayswork provided you dont use images etc)

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JApplet you have been warned

Cannot rely on simple JApplet interfacebuilding to always work

See Sun JAppletexamples for complexcode that does interface building correctly


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NetBeansand JApplets

Naturally, NetBeanscan generate the messyJAppletset up code

So OK, to build swing style applets(JApplets) if using NetBeans

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Applets?WebStart?

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Applets

Not used much in practice

Permission system improved scope

But still rather impractical hassles with JVMsin browsers, slow download times etc

Actually, Ive noticed usage increasing in last couple of years.Numerous sites (most of newspapers for example) appear to loadan Applet that tracks your usage and measures download times, sendingthe data back to web site this Applet doesnt have any GUI.SOLS, WebCT Vista applet dependent.

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WebStart

Alternative technology for downloadableexecutables

Code (.class files, .jar archives) cached on clientmachine in directory managed by WebStartsystem

Link in HTML page to WebStartprogram

start a JVM on client machine first task check that cached version of code is up to date, if

necessary download revised classes

run Java program separately from browser

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Webstart -2

Certification etc can verify that really runningthe correct code, not something interposed byhacker

Restrictions can define access rights for awebstartprogram

Full GUI, unrestricted by browser

Combines applet advantage of downloadable secure code

application advantages

09 Apple Ts and Applications

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