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A Simple Applet

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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” another program (usually a browser) Applets can be run within any browser To run Java applets, browsers need an up-to-date Java plugin 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 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 For this applet, you will need to import two drawing

packages, 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.swing package 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 the java.awt package, 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 our Drawing is a kind of JApplet, but with added capabilities Java’s JApplet just makes an empty “window” We are going to draw in that window

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Your first class, part 3

public class Drawing extends JApplet { …the code for your class goes in here…}

The braces, { }, mark the beginning and ending of your code

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The applet so far

import javax.swing.JApplet;import java.awt.*;

// CIT 591 example

public 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 method’s parameters,

or arguments 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 must be 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 doesn’t 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 paint is the name of the method

Method names should begin with a lowercase letter

The argument, or parameter (there’s only one) is inside parentheses

The method’s commands are inside braces

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By the way…names

( ) are parentheses

{ } are braces

[ ] are brackets

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The paint method, part 3

public void paint(Graphics g) { … } 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 doesn’t “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 javax.swing.JApplet;import java.awt.*;

// CIT 590 example

public class Drawing extends JApplet {

public void paint(Graphics g) { …we still need to put some code in here… }}

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Colors

The java.awt package defines a class named Color There are 13 predefined colors—here 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.CYANColor.GRAY Color.ORANGEColor.BLUEColor.LIGHT_GRAY Color.YELLOWColor.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

public void paint(Graphics g) { 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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Java’s 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 double quote marks "Hello, World!"

A double quote mark in a String must be preceded by a backslash ( \ ) "He said, \"Please don't go!\" " To draw a string, you need to specify not only what you want

to 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 javax.swing.JApplet;import java.awt.*;

// CIT 591 example

public class Drawing extends JApplet {

public void paint(Graphics g) {

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, arcHeight define 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 o’clock 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:

<html> <body> <h1>Drawing Applet</h1> <applet code="Drawing.class" width="100" height="150"> </applet> </body></html>

Eclipse will create this HTML for you You don’t even need to think about the HTML just yet

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The End