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Fachbereich Informatik und Elektrotechnik

Programming in Java, Helmut Dispert

Java Applets

Programming in Java

Java Applets

applet = app =

application snippet =

Anwendungsschnipsel

An applet is a small program that is intended

not to be run on its own, but rather to be

embedded inside another application.

Ref.: Sun Microsystems

CGI - Common Gateway Interface

Internet

Operating System

Browser

HTMLCGI

Application Program

Application

Client WebServer

Applications

Java Applications and Applets

Internet

Operating System

Browser

Applet

Application Program

Application

Client WebServer

Applications

Application

Server

Java Applications and Applets

Applet:

� Java application that runs within a WWW-browser.

Java AWT: Abstract Windowing Toolkit

Advantages:� Defined surface exists (window, graphic environment,

event handling)

Disadvantages (security):� no file access; no communication with other

computers; programs cannot be executed; native code

cannot be loaded.

AWT vs. Swing APIs

Java AWT (Abstract Windowing Toolkit)

� Package: java.awt

� GUI functionality (graphical user interface)

� Component libraries:

labels, buttons, textfields, etc. (platform dependent)

� Helper classes:

event handling, layout managers (window layouts), etc.

The Swing APIs

� Package: javax.swing

� Greater platform independence

- portable "look and feel" (buttons, etc.)

Easy upgrading using "J": Applet →→→→ JApplet

Button →→→→ JButton

AWT and Swing are part of the JFC (Java Foundation Classes)

Viewing Applets

Viewing Applets:

a. using a (Java enabled) Browser:

load HTML-file (*.html) that will call the applet-file (*.class)

from local directory.

b. Using the appletviewer (part of the java development kit):

appletviewer name.html

batch file:

appletviewer %1.html

Classes, Packages

Classes for Applet Programming

� java.awt.Graphics

� java.awt.Image

� java.awt.Font

� java.awt.Color

� java.awt.Event

� java.net.URL

Importing Packages / Subpackagesimport java.awt.Color;

import java.awt.*;

import java.awt.Graphics;

import java.awt.Font;

Classes, Packages

Component

Container

Applet

java.lang.Object

java.awt.Component

java.awt.Container

java.awt.Panel

java.applet.Applet

Class JApplet

Component

Container

Applet

java.lang.Object

java.awt.Component

java.awt.Container

java.awt.Panel

java.applet.Applet

javax.swing.JApplet

JApplet

Class JApplet:An extended version of java.applet.Applet

that adds support for the JFC/Swing

component architecture.

Classes, Packages

public class Name extends java.applet.Applet

Syntax:

imported package

Methods

Java Applets

Methods that are automatically called

(implicit call)

cleanup (before exiting)void destroy()

stop the appletvoid stop()

repaint screen: call paint() for updatevoid repaint()

paint screen: display text, graphicsvoid paint(Graphics g)

start the applet when loading HTML page

(e.g. start animation)

void start()

Initialization:

Set up colors, fonts, etc. when first loaded

void init()

Hello World Applet

public class HelloWorldApp extends java.applet.Applet

public void paint (Graphics g)

g.drawString("Hello World!", 40, 20);

Example Source Program "Hello World"

stored in:HelloWorldApp.java

compiled with:javac HelloWorldApp.java

The java.awt.Graphics class

Coordinate System:x

origin

Unit: pixel

Hello World Applet

<HTML>

<HEAD>

<TITLE>Hallo World Applet</TITLE>

</HEAD>

<BODY>

<APPLET CODE= "HelloWorldApp.class"

WIDTH = "210" HEIGHT = "50">

</APPLET>

</BODY>

</HTML>

Example Source Program "Hello World"

Hello World Applet

Object Tag:To address these issues, HTML 4 introduces the OBJECT element, which

offers an all-purpose solution to generic object inclusion.

The new OBJECT element thus subsumes some of the tasks carried out by

existing elements:

OBJECTIFRAMEAnother HTML document

OBJECTAPPLET (deprecated)Applet

OBJECTIMGImage

Generic elementSpecific elementType of inclusion

The chart indicates that each type of inclusion has a specific and a general

solution. The generic OBJECT element will serve as the solution for

implementing future media types.

width="200" height="250">

</object>

http://www.w3.org/TR/REC-html40/struct/objects.html

Hello World Applet Example

import java.awt.Graphics;import java.awt.Font;import java.awt.Color;

public class HelloWorldApp2 extends java.applet.Applet{

String str = "Hello World 2";int w = 300;int h = 80;Font f = new Font ("Arial", Font.BOLD + Font.ITALIC, 48);

public void init(){resize(w,h);}

public void paint (Graphics g){g.setFont(f);g.drawRect(0,0,w-1,h-1);g.setColor(Color.red);g.drawString(str, 10, 50);}

continued

Hello World Applet Example

<HTML>

<HEAD>

</HEAD>

<BODY>

<APPLET CODE= "HelloWorldApp2.class"

WIDTH = "350" HEIGHT = "100">

</APPLET>

</BODY>

</HTML>

Object

Hello World Applet Example using Swing

import javax.swing.*;

import java.awt.*;

public class JHWApplet extends JApplet

String msg;

public void init()

msg = "Hello J-World";

public void paint(Graphics g)

g.drawString(msg, 20, 30);

Methods

Further MethodsDetermines if this applet is active.boolean

isActive()

Returns information about this applet.String

getAppletInfo()

Plays the audio clip at the specified

absolute URL.

play(URL url)

Returns the AudioClip object specified by

the URL argument.

AudioClip

getAudioClip(URL url)

Returns an Image object that can then be

painted on the screen.

getImage(URL url)

Gets the base URL.URL

getCodeBase()

Returns the value of the named parameter

in the HTML tag.

public String

getParameter(String name)

Requests that the argument string be

displayed in the "status window (bar)".

showStatus(String msg)

Ref.: Sun Microsystems

Hello World Applet using Parameters

</APPLET>

Transfering Parameters from HTML to Applet

Check for Null Reference:

if (name == null)

Applet using Parameters

import java.awt.Font;

import java.awt.Color;

public class HelloWorldApp3 extends java.applet.Applet

String text, fontSize;

Font f;

int w = 300;

int h = 80;

int theFontSize;

public void init()

resize(w,h);

this.text = getParameter("text");

if(this.text == null) this.text = "Hello World - Error!";

this.fontSize = getParameter("fontSize");

if (this.fontSize == null) this.theFontSize = 18;

else this.theFontSize = Integer.parseInt(fontSize);

} continued

Font f = new Font("Arial",

Font.BOLD + Font.ITALIC, this.theFontSize);

g.setFont(f);

g.drawRect(0,0,w-1,h-1);

g.setColor(Color.red);

g.drawString(text, 10, 50);

continued

<HTML>

<HEAD><TITLE>Hallo World Applet</TITLE></HEAD>

<BODY>

</APPLET>

</BODY>

</HTML>

continued

With parameters

Without parameters

with Parameter

without Parameter

DescriptionMethod

x/y-translationcopyArea(6*int)

rect. in background colorclearRect(int,int,int,int)

filled ArcfillArc(6*int)

ArcdrawArc(6*int)

Oval (including circle)drawOval(int,int,int,int)

x,y-coordinates, numberdrawPolygon(int[],int[],int)

3D Rect. with shadow (y/n)draw3DRect(4*int, boolean)

last 2 int for roundingdrawRoundRect(6*int)

filled rectanglefillRect(int,int,int,int)

x/y-upper left to lower rightdrawRect(int,int,int,int)

x/y-start to x/y-enddrawLine(int,int,int,int)

Documentation

AWT - Example: Curve Plotting

import java.awt.*;

public class CurveApp extends java.applet.Applet

int f(double x)

return (int) ((Math.cos(x/5) + Math.sin(x/7) + 2) * 50);

for (int x = 0; x < 400; x++)

g.drawLine(x,f(x),x+1,f(x+1));

continued

AWT - Example: Curve Plotting

<HTML>

<HEAD>

</HEAD>

<BODY>

</APPLET>

</BODY>

</HTML>

Class: java.awt.Color

Standard colors are class variables

RGB-Color: RED, GREEN, BLUE (16.7 million colors)

Create New Colors:

(Integer)

Color name = new Color (int red, int green, int blue)

(Float)

Color name = new Color (float red, float green, float blue)

The class Color provides a series of static Color-objects that can be used directly:

public static Color green

public static Color yellow

public static Color magenta

public static Color cyan

public static Color orange

public static Color pink

public static Color white

public static Color lightGray

public static Color gray

public static Color darkGray

public static Color black

public static Color red

public static Color blue

Methods to determine the RGB-values of a color-object:

public int getRed()

public int getGreen()

public int getBlue()

Color Methods

Set the foreground colorsetForeground (Color)

Set the background colorsetBackground (Color)

Set the current colorsetcolor(Color)

DescriptionMethods

setColor (new Color(1.0f, 0.0f, 0.0f));

Color c = new Color (0,255,0);

setBackground (c); // RGB green

setBackground(Color.green); // Standard color green

Graphics g (lamp demo)

import java.awt.*;

public class Lamp extends java.applet.Applet

public void init()

resize(300,300);

public void paint(Graphics g)

// the moon

g.drawArc (20,20,100,100,90,180);

g.drawArc (40,20,60,100,90,180);

// the table

g.setColor (Color.orange);

g.fillOval (0,220,300,100);

g.setColor (Color.black);continued

Graphics g (lamp demo)

// lamp cable

g.drawRect (148,0,5,89);

// upper arc of lamp

g.drawArc (85,87,130,50,62,58);

// two lines of the lamp

g.drawLine (215,177,181,90);

g.drawLine (85,177,119,90);

// the lower oval

g.setColor (Color.yellow);

g.fillOval (85,157,130,50);

// lamp pattern

g.setColor (Color.red);

g.fillOval (120,100,20,20);

g.copyArea (120,100,20,20,40,-7);

g.copyArea (120,100,20,20,30,30);

g.copyArea (120,100,20,20,-15,35);

g.copyArea (120,100,20,20,60,40);

System Colors

public final class SystemColor:

A class to encapsulate symbolic colors representing the color of native

GUI objects on a system.

For systems which support the dynamic update of the system colors

(when the user changes the colors) the actual RGB values of these

symbolic colors will also change dynamically.

In order to compare the "current" RGB value of a SystemColor object with

a non-symbolic Color object, getRGB should be used rather than equals.

The color rendered for the text of control panels and

control objects, such as pushbuttons. SystemColor.controlText

The color rendered for the background of scrollbars. SystemColor.scrollbar

The color rendered for the background of control panels

and control objects, such as pushbuttons. SystemColor.control

The color rendered for the background of interior regions

inside windows. SystemColor.window

The color rendered for the background of the desktop. SystemColor.desktop

Examples:

AWT Structure

Types of classes and interfaces in package AWT

�Graphics

�Components (Windows, Menus)

�Layout Manager

�Event Handler

� Image Manipulation

java.awt

java.awt.Component

CLASSCLASS ABSTRACT CLASSABSTRACT CLASS INTERFACEINTERFACE

extends implements

ComponentComponent

WindowWindow

AWT Structure

ContainerContainer

PanelPanel ScrollPaneScrollPane

ButtonButton LabelLabelCanvasCanvas CheckboxCheckbox

partial listings

ColorColor FontFont ImageImageGraphicsGraphics FontMetricsFontMetrics

Class: java.awt.Font

Font types: Courier;

TimesRoman;

Helvetica,

Arial, etc.

Font Styles: Font.PLAIN // = 0

Font.BOLD // = 1

Font.ITALIC // = 2

Size: in pixel

Font styles are constants that can be added:

Example:Font.BOLD + Font.ITALIC // bold italic

Creating Fonts, Examples:Font f = new Font("TimesRoman", Font.BOLD, 48)

Returns “true” if italicisItalic()

Returns “true” if boldisBold()

Returns “true” if plainisPlain()

Return current styles (0-3)getStyle()

Return current font size (int)getSize()

Return name of font as stringgetName()

DescriptionMethod f.*

Examples:

Font f = new Font("TimesRoman", Font.PLAIN, 72);

g.setFont(f);

g.drawString("This is size 72",10 ,100);

Return current font objectgetFont()

Set font to be usedsetFont()

Set color to be usedsetColor()

Draw a stringdrawString()

DescriptionMethod g.*

FontMetrics

FontMetrics (abstract class)

Quality of text and font

Return the descent of the fontgetDescent()

Returns the leading of the fontgetLeading()

Returns the total height of fontgetHeight()

Return the ascent of the fontgetAscent()

Width of char ccharWidth(c)

Return width of string strstringWidth(str)

DescriptionMethod fm.*

2D Text Tutorial

FontMetrics

import java.awt.*;

public class Center extends java.applet.Applet

Font f = new Font ("TimesRoman", Font.PLAIN, 24);

FontMetrics fm;

g.setFont(f);

fm = getFontMetrics(f);

String str = "This text will be centered";

int x = (this.size().width - fm.stringWidth(str)) / 2;

int y = (this.size().height - fm.getHeight()) / 2;

g.drawString(str,x,y);

FontMetrics Demo

import java.awt.*;

import java.applet.*;

public class DrawText extends Applet

// output strings

byte byte_text[] = {'H','E','L','L','O'};

char char_text[] = {'h','e','l','l','o'};

String String_text = "\u00c4g";

// fonts

Font Arial24 = new Font ("Arial", Font.PLAIN,24);

Font Tmsrm72 = new Font ("TimesRoman", Font.PLAIN,72);

// font metrics

FontMetrics fm;

int ascent, descent;

int string_width;

int char_width;

int leading;continued

FontMetrics Demo

// simple output methods

g.drawBytes (byte_text,0,5,20,20);

g.drawChars (char_text,0,5,20,40);

g.drawString ("A complete string",20,60);

g.drawString (Arial24.toString(),20,80);

// selecting a font

g.setFont (Arial24);

g.drawString ("Now using Arial 24",20,120);

// translate origin

g.translate (100,250);

// select a large font

g.setFont(Tmsrm72);

continued

FontMetrics Demo

// font metrics

fm = getFontMetrics (Tmsrm72);

string_width = fm.stringWidth (String_text);

char_width = fm.charWidth ('\u00c4');

ascent = fm.getAscent ();

descent = fm.getDescent ();

leading = fm.getLeading ();

// draw vertical lines

g.drawLine (0, - ascent -10, 0, descent + 10);

g.drawLine (char_width, -ascent, char_width, descent + 10);

g.drawLine (string_width, - ascent -10, string_width, descent +10);

// draw horzontal lines

g.drawLine (-10, 0, string_width + 10, 0);

g.drawLine (-10, -ascent, string_width +10, -ascent);

g.drawLine (-10, descent, string_width + 10, descent);

g.drawLine (-10, descent + leading,

string_width + 10, descent + leading);

}continued

FontMetrics Demo

Applet Demo

Karnaugh-Veitch Diagramm

Model-View-Controller

Model-View-Controller-Architecture:

� Methodology / design pattern widely used in object-

oriented programming.

� Relates the the user interface (UI) to the underlying data

models.

� Used in program development with Java, Smalltalk, C,

and C++.

The MVC pattern proposes three main components or

objects to be used in software development:

� A Model:

represents the underlying, logical structure of data in a

software application and the high-level class associated

with it.

Does not contain information about the user interface.

� A View:

collection of classes representing the elements in the user

interface (visual display, possible user responses - buttons,

display boxes, etc.)

� A Controller:

represents the classes connecting the model and the view.

Controller

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