applet presentation

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JAVA Applets


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Applets and applications

An applet is a Java program that runs on a web page

Applets can be run within any modern browser

To run modern Java applets, old browsers need an up-to-date

Java plugin appletviewer is a program that can run java applets.


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Packages and classes

Java supplies a huge library of pre-written code,

ready for you to use in your programs

Code is organized into classes

Classes are grouped into packages

One way to use this code is to import it

You can import a single class, or all the classes in a

package


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The Applet class

To create an applet, you must import the Applet class

This class is in the java.applet package

TheAppletclass contains code that works with a

browser to create a display window Capitalization matters!

applet and Applet are different names


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Importing the Applet class

Here is the directive that you need:

import java.applet.Applet;


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Thejava.awtpackage

awt stands for Abstract Window Toolkit

The java.awt package includes classes for:

Drawing lines and shapes

Drawing letters

Setting colors

Choosing fonts

If its drawn on the screen, then java.awt isprobably involved!


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Importing the java.awt package

Since you may want to use many classes from the

java.awt package, simply import them all:

import java.awt.*;

The asterisk, or star (*), means all classes

The import directives can go in any order, but must be

the first lines in your program


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The applet so far


import java.awt.*;


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Your applet class

public class Drawing extends Applet { }

Drawing is the name of your class

Class names should always be capitalized extends Applet says that our Drawing is a kind ofApplet, but with added capabilities Javas Applet just makes an empty window

We are going to draw in that window The only way to make an applet is to extend Applet


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The applet so far


import java.awt.*;

public class Drawing extends Applet {

we still need to put some code in here...

}


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The paint method

Our applet is going to have a method to paint some

colored rectangles on the screen

This method must be named paint

paint needs to be told where on the screen it can draw This will be the only parameter it needs

paintdoesnt return any result


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The paint method, part 2

public void paint(Graphics g) { } public says that anyone can use this method

void says that it does not return a result

A Graphics(short for Graphics context) is an

object that holds information about a painting

It remembers what color you are using

It remembers what font you are using

You can paint on it (but it doesnt remember what youhave painted)


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The applet so far


import java.awt.*;

// CIT 591 example


public void paint(Graphics g) {

we still need to put some code in here

}

}


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Colors

The java.awtpackage defines a class named Color

There are 13 predefined colorshere are their fully-qualified names:

For compatibility with older programs (before the naming

conventions were established), Java also allows color

names in lowercase: Color.black, Color.darkGray, etc.

Color.BLACK Color.PINK Color.GREENColor.DARK_GRAY Color.RED Color.CYAN

Color.GRAY Color.ORANGE Color.BLUE

Color.LIGHT_GRAY Color.YELLOW

Color.WHITE Color.MAGENTA


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New colors

Every color is a mix of red, green, and blue

You can make your own colors:

new Color(red,green,blue)

Amounts range from 0 to 255 Black is (0, 0, 0), white is (255, 255, 255)

Yellow is red + green, or (255, 255, 0)


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Using Colors

static int HSBtoRGB(float h, float s, float b)

static float[] RGBtoHSB(int r, int g, int b, float hsb[])

Color brighter()

Color darker()

static Color decode(String str) throws NumberFormatException

boolean equals(Object obj)

int getBlue()

int getGreen()

int getRGB()

int getRed()

http://java.sun.com/javase/6/docs/api/java/awt/Color.html

import java.applet.Applet;import java.awt.Color;import java.awt.Graphics;

/*

*/

public class BlueString extends Applet {

public void paint(Graphics g) {g.setColor(Color.blue);g.drawString("Blue String", 100, 50);

}}

Color(int red, int green, int blue)

Color(int rgb)

Color(float r, float g, float b)

Color Constructors


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Setting a color

To use a color, we tell our Graphicsgwhat color we

want:

g.setColor(Color.RED);

g will remember this color and use it for everythinguntil we tell it some different color


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The paint method so far


g.setColor(Color.BLUE);

draw a rectangle

g.setColor(Color.RED);draw another rectangle

}

}


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Pixels

A pixel is a picture (pix) element

one pixel is one dot on your screen

there are typically 72 to 90 pixels per inch

java.awt measures everything in pixels


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Javas coordinate system

Java uses an (x, y) coordinate system

(0, 0) is the top left corner

(50, 0) is 50 pixels to the right of (0, 0) (0, 20) is 20 pixels down from (0, 0)

(w - 1, h - 1) is just inside the bottom right corner, where w

is the width of the window and h is its height

(0, 0)

(0, 20)

(50, 0)

(50, 20)

(w-1, h-1)


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Drawing rectangles

There are two ways to draw rectangles:

g.drawRect(left,top,width,height);

g.fillRect(left,top,width,height);


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The complete applet


import java.awt.*;

// CIT 591 example



g.setColor(Color.BLUE);

g.fillRect(20, 20, 50, 30);g.setColor(Color.RED);

g.fillRect(50, 30, 50, 30);

}

}


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Some more java.awt methods

g.drawLine(x1 ,y1,x2 ,y2 );

g.drawOval( left,top,width,height );

g.fillOval( left,top,width,height);

g.drawRoundRect( left,top,width,height );

g.fillRoundRect( left,top ,width,height);

g.drawArc( left,top,width,height,

startAngle,arcAngle ); g.drawString(string,x,y);


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Displaying Text

FontMetrics(Font font)

http://java.sun.com/javase/6/docs/api/java/awt/Font.html

Font(String name, int style, int ps) Font Constructor

abstract void setFont(Font font)

setFont() Method

FontMetrics Constructor


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Using Applet Dimensions

http://java.sun.com/javase/6/docs/api/java/awt/Dimension.html

Dimension getSize()getSize() Method

Dimension()

Dimension(Dimension d)

Dimension(int w, int h)

Dimension Constructors


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Using Applets in a Web Page

[]

[]

.

[]

Applet HTML Tag

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

/*

*/

public class AppletParameters extends Applet {

public void paint(Graphics g) {String background = getParameter("background");String foreground = getParameter("foreground");String message = getParameter("message");setBackground(Color.decode(background));setForeground(Color.decode(foreground));Font font = getFont();FontMetrics fm = getFontMetrics(font);Dimension d = getSize();int x = (d.width - fm.stringWidth(message)) / 2;int y = d.height / 2;g.drawString(message, x, y);

}}


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The Applet Class

java.lang.Object

java.awt.Component

java.awt.Container

java.awt.Panel

java.applet.Applet

Applet and itssuperclasses

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

/*

*/

public class BackgroundForeground extends Applet {

public void paint(Graphics g) {setBackground(Color.yellow);setForeground(Color.blue);g.drawLine(0, 0, 200, 200);g.fillRect(100, 40, 50, 50);

}

}


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The Life Cycle of an Applet


import java.awt.Graphics;/*

*/

public class AppletLifecycle extends Applet {String str = "";

public void init() {str += "init; ";}

public void start() {str += "start; ";

}

public void stop() {str += "stop; ";

}

public void destroy() {System.out.println("destroy");

}

public void paint(Graphics g) {g.drawString(str, 10, 25);

}}

init() : called only when the appletbegins execution.

start() : executed after init() method.Called by the applet viewer or Webbrowser.

stop() : when applet viewer is minimized. destroy() : called by the applet viewer or

Web browser before the applet isterminated.


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Action /Event Listeners

Now that you know how to display things on screen youneed a way to responds to the user actions. This is done

with the EventListeners. An Event is a user interaction.

Reacting on GUI cmponents is done by adding

ActionListeners:

MouseListener

MouseMotionListener

KeyboardListener (but it is not a common class you will

use.)

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http://www.realapplets.com/tutorial/MouseClickExample.htmlhttp://www.realapplets.com/tutorial/MouseMotionExample.htmlhttp://www.realapplets.com/tutorial/MouseMotionExample.htmlhttp://www.realapplets.com/tutorial/MouseClickExample.html


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Thank you!

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