Java Applet Basics

Introduction
Life Cycle,Graphics, etc.
User Interface Components
Layout Managers

This tutorial assumes that you know fundamentals of Java application programming. However, I'll offer some important notes for the beginners among you.

To compile and execute Java programs, you should install Java Development Kit (JDK) Version 1.2 or higher (recommended). You can use older versions of the JDK, but this tutorial is being prepared for use with JDK 1.2 or higher. You can download the JDK from Sun's Website free of cost. You can also get the JDK through some Java textBooks. Just double-click the setup file and proceed with the installation.

After installing the JDK, you have to set the correct path in order to work with Java programs. For setting the path, execute the MSCONFIG.exe (System Configuration utility) Program from the start-run menu. Press the autoexec.bat tab and click on the New button. Type in the following command: set path=c:\jdk1.2\bin;%path%. You have to substitute the correct version of the drive and the JDK version number. I'll assume that the JDK is in C:\ Drive and the version is 1.2.

Anyway, you can also use the above command everytime you practice Java programming, but it is recommended to follow the above procedure. Also supply the Doskey command (in autoexec.bat) so that you have to type in each command at the command prompt only once. Just press the up and down arrow keys to retrieve your earlier command (earlier in the sense of the current session).

You can write code using Notepad or MSDOS Editor (supply EDIT Command at the DOS prompt). However, Notepad is the most preferred editor among most programmers. Save your files with .java extensions. Compile the program using javac .java. Execute your applet using the appletviewer utitlity included with the JDK or open the corresponding htm or htm file in a browser.

Overview

• Java programs consists of applications and applets.
• Applications are executed from MSDOS prompt by using the interpreter
• Applets are executed with the help of the appletviewer utility or the Net browser.
• Compilation stages are same in both the cases.

What are Applets

• Applets are dynamic (animations and graphics).
• Applets are interactive programs (via GUI components).

What you need to execute an Applet

• Browser (IE 4.0 or Netscape Navigator 4.0 or higher)
• Appletviewer utility in the JDK.

Features of an Applet

• Provides facility for frames
• Event handling facility and userInteraction
• GUI user interface
• Graphics and multimedia

Your First Applet

Type in the following code using any text editor or in DOS editor.

import java.awt.* 
import java.applet.*;
public classhello extends Applet { 
public void paint(Graphics g) { 
g.drawString("Welcome to Java Applets",20,20); } } 

Save the file as hello.java and compile it by using javac. Now, type in the following HTML code in your editor and save the file as hello.html

<applet code =
"classhello.class" width = 200 height =
150></applet>

Execute the HTML file by giving appletviewer hello.html. Another way to run the applet is to give the above HTML coding as a comment after the two import statements. Execute the applet as appletviewer hello.java

Concepts and Explanation

• All applets are the subclasses of Applet class. All applets must import the java.applet package
• The Paint() method is defined by AWT Component class. It takes one parameter of type Graphics.
• The drawString is the member of the Graphics class of the AWT package.
• This method is responsible for printing the String "welcome to java applets".

Facts

• Applets do not begin execution at main() method. However, it is possible to execute the applets by using the Java interpreter (by using the main() method) if you extend your class with Frame.
• I/O Streams do not provide much scope for applets.
• It is not possible for an applet to access the files on the users hard disk.
• It is not possible to access the source code of the applets. It can be accessed only from the original server.

Part II

Life Cycle, Graphics, Fonts, Colors

Life Cycle of an Applet

The repaint() method - If the applet wants to be repainted again, then this method is called. Useful for animation purposes.

Parameter Passing

• Use a special parameter tag <param> in the HTML file. This tag takes two attributes, namely Name and Value.
• Use a method getParameter() inside the init() method, which takes one argument (i.e., the string representing the name of the parameter being looked for). Give this name a value in the HTML coding.

Graphics Class

• Graphics class in the java.awt package contains methods for drawing strings, lines and rectangles, ovals, polygon, fill rect, fill oval, etc.

Methods

• To draw a string use the drawString(String str,int X,int Y) method, where str is the name of the string and X and Y are the coordinates for where the string is to be printed.
• To draw a line use drawLine(int x1,int y1,int x2,int y2), where x1 and y1 are the starting point coordinates and x2 and y2 are the ending point coordinates.
• To draw a rectangle use drawRect(int x1,int y1,int width,int height), where x1 and y1 are the starting point coordinates and width and height are the measurements for rectangle
• To draw a RoundRect use drawRoundRect(int x1,int y1,int width,int height,width1,height1), where x1 and y1 are the starting point coordinates and width and height are the measurements for rectangle and width1 and height1 are the angles of the corners.
• To draw an oval use drawOval(int x1,int y1,int width,int height), where x1 and y1 are the coordinates of the top corners and width and height are the respective measurements of the oval.
• To draw an arc use drawArc(int x1,int y1,int width,int height,angle1 ,angle2), where x1 and y1 are the coordinates of the top corners and width and height are the respective measurements of the arc and angle1 and angle2 are the starting arc and ending arcs (in degrees).

Font Class

• The Font class in the java.awt package contains methods for displaying fonts.

1. Declare the Font Name, style and the size using the Font Constructor.
2. Finally, pass the font's object by using the setFont method in the java.awt package.

Sample

Font f = new Font("Courier",Font.BOLD+Font.Italic,
16); ||||| g.setFont(f);

Color Class

• The Color class in the java.awt package contains methods for dealing with colors.

Methods

• setColor(Color.gray) sets the string color to gray .
• setBackground(Color.red) sets the background color to red.

Part III

User Interface Components