A Simple Applet

Applets and applications

• An applet is a Java program that runs on a web page– Applets can be run from:

• Internet Explorer

• Netscape Navigator (sometimes)

• appletviewer

• An application is a Java program that runs all by itself

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

The Applet class

• To create an applet, you must import the Applet class– This class is in the java.applet package

• The Applet class contains code that works with a browser to create a display window

• Capitalization matters! – applet and Applet are different names

Importing the Applet class

• Here is the directive that you need: import java.applet.Applet;

• import is a keyword

• java.applet is the name of the package

• A dot ( . ) separates the package from the class

• Applet is the name of the class

• There is a semicolon ( ; ) at the end

The java.awt package

• “awt” stands for “Abstract Window Toolkit”• The java.awt package includes classes for:

– Drawing lines and shapes– Drawing letters– Setting colors– Choosing fonts

• If it’s drawn on the screen, then java.awt is probably involved!

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

C and C++ programmers only

• C and C++ have an #include directive that copies a library function into your program

• This makes your program bigger

• Java’s import gives you access to the library

• It does not make your program bigger

• It’s OK to use lots of include directives!

The applet so far

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

Comments

• A comment adds information for the reader

• Java ignores everything inside comments

• There are three kinds of comments:// starts a comment that goes to the end of the line/* starts a comment that can extend over many

lines, and ends at *//** is a “javadoc” comment that can be extracted

from your program and used in documentation */

Classes

• In Java, all code occurs in classes– Except for the package and import directives– We will talk about package some day

• The code that you import is in classes

• Your code will also be in classes

• For now, a class is a bundle of code– We will talk about what it really is very soon

Your first class

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

• public says your class is not hidden– This makes your class visible to BlueJ– We will talk later about why we hide code

• class says we are making a class (Duh!)

Your first class, part 2

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 of Applet, but with added capabilities– Java’s Applet just makes an empty window– We are going to draw in that window

Your first class, part 3

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

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

The applet so far

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

// CIT 591 example

public class Drawing extends Applet {

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

}

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

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

• paint doesn’t return any result

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• The argument (there’s only one) is inside

parentheses• The method’s commands are inside braces

By the way…names

• ( ) are parentheses

• { } are braces

• [ ] are brackets

The paint method, part 3

public void paint(Graphics g) { … }• A Graphics is something 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, and it remembers what

you have painted

Classes and objects

• A class is a description of some objects

• An object is a member of a class– The type of an object is the class it belongs to – Classes are more abstract than objects

• If I have a dog named Fido, I can’t pet “dog,” but I can pet “Fido”– Fido is an object of type Dog

The paint method, part 4

public void paint(Graphics g) { … }• g is the parameter, or argument

– we could use any name we wanted for it– but it should not begin with a capital letter

• The type of a name tells what kind of thing the name refers to

• Graphics g says g is an object of type Graphics• We can paint on g

The applet so far

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

// CIT 591 example

public class Drawing extends Applet {

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

Colors

• The java.awt package defines a class named Color• There are 13 predefined colors--here are their fully-

qualified names:

Color.black Color.pink Color.greenColor.darkGray Color.red Color.cyanColor.gray Color.orange Color.blueColor.lightGray Color.yellowColor.white Color.magenta

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)

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

The paint method so far

public void paint(Graphics g) { g.setColor(Color.blue); …draw a rectangle… g.setColor(Color.red) …draw another rectangle… }}

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

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, h) is the bottom right corner, where w is the width of the window and h is its height

Drawing rectangles

• There are two ways to draw rectangles:

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

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

The complete appletimport java.applet.Applet;import java.awt.*;

// CIT 591 example

public class Drawing extends Applet {

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); }}

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 );

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.\""

• g.drawString( string , x , y );

The End