03 Applets

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Applets and applications

An application is an ordinary program

Examples: Notepad, MS Word, Firefox, Halo, etc.

An applet is a Java program that runs within anotherprogram (usually a browser)

Applets can be run within any browser

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

appletviewer is a program that can run applets

When you download the Java SDK, appletviewer comes with it

appletviewer is always up-to-date with your Java system

Eclipse has an built-in applet viewer

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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 intopackages One way to use this code is to import it

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

For this applet, you will need to import two drawingpackages, awt and swing

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Importing some things we need

To create an applet, you will import the JApplet class

The JApplet class is in the javax.swingpackage

This is the only class we will need from the swing package

import javax.swing.JApplet;

Capitalization matters!

Japplet and JApplet are different names; be careful!

Since you will want to use many classes from thejava.awtpackage, we will just import them all:

import java.awt.*;

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Your first class, part 2

public class Drawing extends JApplet {

}

Drawing is the name of your class Class names should always be capitalized

extends JApplet says that ourDrawing is a kind of

JApplet, but with added capabilities

Javas JAppletjust makes an empty window

We are going to draw in that window

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Your first class, part 3


the code for your class goes in here

}

The braces, { }, mark the beginning and ending ofyour code

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The applet so far


import java.awt.*;

// CIT 591 examplepublic class Drawing extends JApplet {

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

}

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Methods

A method is a group of commands that tell the

computer to do something

C programmers: methods are similar to functions

A method takes information in, does something

with it, and returns a result

The input information is called the methodsparameters,

orarguments

The result is just called a result

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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 mustbe named paint

In applets, this is a special method name

paint needs to be told where on the screen it can draw

This will be the only parameter it needs

paint doesnt return any result

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The paint method, part 2

public void paint(Graphicsg) { }

public says that anyone can use this method

void says that it does not return a result

paint is the name of the method Method names should begin with a lowercase letter

The argument, or parameter (theres only one) is

inside parentheses

The methods commands are inside braces

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By the waynames

( ) areparentheses

{ } arebraces

[ ] arebrackets

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The paint method, part 3

public void paint(Graphicsg) { }

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 you

have painted)

In this program, The type of the parameter is Graphics

The name of the parameter is g

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The applet so far


import java.awt.*;

// CIT 590 examplepublic class Drawing extends JApplet {

public void paint(Graphics g) {

w

e 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)

We are mixing lights, not pigments

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

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Setting a color

To use a color, we tell our Graphics g what color we

want:

g.setColor(Color.RED);

g will remember this color and use it for everything

until 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

Apixel 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)

(50, 0)(0, 0)

(0, 20) (50, 20)

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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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Drawing strings

A String is a sequence of characters enclosed in doublequote marks

"Hello, World!"

A double quote mark in a String must be preceded by abackslash (\ ) "He said, \"Please don't go!\""

To draw a string, you need to specify not only whatyou wantto say, but where to say it

g.drawString( string, left, top ); For example,

g.drawString("Example JApplet", 20, 80);

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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);

g.setColor(Color.BLACK);

g.drawString("Example JApplet", 20, 80);

}

}

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More java.awt.Graphics 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,

arcWidth, arcHeight); arcWidth, arcHeightdefine the roundedness of corners

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

g.drawArc( left, top, width, height, startAngle, arcAngle);

Angles are in degrees

0 degrees is the 3 oclock position

Positive angles are to the right

g.FillArc( left, top, width, height, startAngle, arcAngle);

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Still more Graphics methods

g.drawPolygon(xPoints,yPoints, n);

g.fillPolygon(xPoints,yPoints, n); xPoints and yPoints are int arrays of size n

One way to write an int array is:new int[] { value1, value2, ..., valueN}

Example: g.drawPolygon(new int[] { 250, 290, 210 },new int[] { 210, 290, 290 }, 3);

draws a triangle using the 3 points (250, 210), (290, 290), and(210, 290).

g.drawPolyline(xPoints,yPoints, n); A polyline is like a polygon, except the first and last points are not

automatically connected

Hence, there is no fillPolyline method

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The HTML page

You can only run an applet from an HTML page

The HTML looks something like this:

Drawing Applet

Eclipse will create this HTML for you

You dont even need to think about the HTML just yet

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The End

Documents

03 Applets