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A Simple Applet

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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 applets

When you download the Java SDK, appletviewer comes with it appletviewer is always up-to-date with your Java system

An application is a Java program that runs all by itself

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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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AWT and Swing

There are two ways to build Graphical User Interfaces (GUIs): AWT, the “Abstract Window Toolkit,” is the original way Swing is the new way (since Java 1.2)

Swing is built “on top of” the AWT When you build an AWT GUI, you use only AWT

methods When you build a Swing GUI, you typically use

both AWT methods and Swing methods However, there are some incompatibilities you will

need to learn about (later)

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The Applet and JApplet classes

To create an applet, you must import either the Applet class (to write an AWT applet) or the JApplet class (to write a Swing applet) The Applet class in in the java.awt package The JApplet class is in the javax.swing package

Both Applet and JApplet contain code that works with a browser to create a display window

We will use Swing (hence, JApplet) in this example Capitalization matters!

Japplet and JApplet are different names

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Importing the JApplet class

Here is the directive that you need: import javax.swing.JApplet;

import is a keyword javax.swing is the name of the package A dot ( . ) separates the package from the class JApplet is the name of the class There is a semicolon ( ; ) at the end This directive should be before your class definitions

Only the package directive, if you have one, goes before the import directives

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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

Swing builds on the AWT, it doesn’t replace it If it’s drawn on the screen, then java.awt is

probably 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 ...unless you have a package directive, which must precede

your import directives

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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 import directives!

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The applet so far

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

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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 extractedfrom your program and used in documentation */

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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

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Your first class

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

public says your class is not hidden This makes your class visible to “the whole world” We will talk later about why we hide code

class says we are making a class (Duh!)

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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

The only way to make an applet is to extend Applet or JApplet

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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 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 591 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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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.drawPolygon( xPoints, yPoints, n ); g.fillPolygon( xPoints, yPoints, n );

xPoints and yPoints are int arrays of size n

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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>

BlueJ will create this HTML for you

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The End