October 30, 2014
Hot Topics:
RSS RSS feed Download our iPhone app

Java Applet Basics

  • June 25, 2001
  • By Anand Narayanaswamy
  • Send Email »
  • More Articles »

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

Part I

Introduction

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

Method Class Description
init() Applet First method to be called, initialize variables in this method
start() Applet Called when restarted after being stopped.occurs after init()
stop() Applet Called when the applet leaves the webpage
destroy() Applet Called when the applet wants to be removed out of memory
paint() Component Called when the applet needs to be drawn

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

The Class Hierachy

Usage of Label

  1. Label() - Creates an empty label
  2. Label(String) - Creates a label with the given string
  3. Label(String, align) - Creates a string label with the specified alignment (RIGHT,LEFT,CENTER)

Usage of Button

  1. Button() - Creates a button without a label
  2. Button(String str) - Creates a button with the string

Usage of Checkbox()

  1. Checkbox() - Creates a checkbox without a label
  2. Checkbox(String str) - Creates a checkbox with a string label in it

Usage of CheckboxGroup

  1. Create a Checkbox Group and add the group's object to the individual checkboxes to get a radio-button style of interface. Only one box can be selected at a time.

Usage of TextField

  1. TextField() - Creates a empty Text Field
  2. TextField(String str) - Creates a Text Field with the specified String
  3. TextField(String str,align) - Creates a Text Field with the specified String with the alignment
  4. The setText() method is used in connection with the Text Fields. For instance, to set a text in Choice to the text field, the method setText() method is used

Usage of Text Area

  1. TextArea() - Creates empty Text Area
  2. TextArea(rows,charcters) - Creates empty Text Area, with the specified rows and charcters.
  3. TextArea(String str,rows,charcters) - Creates a default String Text Area, with the specified rows and charcters

Usage of Choice

  1. Create a Choice object (Choice c = new Choice() )
  2. Add the individual items to the Choice by add method and connecting the Choice object
  3. Finally, add the choice object to the container.
  • Choice ch = new Choice(); ch.add("Java"), ch.add("XML"); add(ch)

Usage of Lists

  1. Create a List Object (List l = new List() )
  2. Add the individual items to the List by add method and connecting the List object
  3. Finally, add the List object to the Container.
  • You can select multiple items from the list box. But only one from a Choice.
  • The setEditable(Boolean) method is used to edit the text inside a choice component.

All these above components together form a GUI Interface. You can create any type of user-friendly applications you want by making use of the above components. In the next section, we will take a look at Layout Mangers in Java, with which you can dynamically place the above components at your desired location.

Part IV

Layout Managers

The Concept

You can place the components according to your taste and position by using Layout Managers. The basic Layout Managers include Flow Layout, Border Layout, and Grid Layout.

Basic Steps to be Followed

  1. Find the Layout Manager and Instantiate it by using their Constructors

  2. Associate the manager with the components in the Container.

  3. The Layout Manager is set by setLayout() method.

  4. If no layout manager is specified, then the default manager is taken.

Flow Layout

  • Default Layout Manager works like a typical word processor from Left to right.

  • Flow Layout() creates a default layout centered and leaves a position of 5 pixels of space between each Component.

  • Flow Layout( int how) creates a layout with the specified alignment (LEFT,RIGHT,CENTER).

  • Flow Layout(int how,int h,int v) creates a layout with the specified spaces.

Border Layout

  1. Each Component can be placed on the border of a container.

  2. With Border Layout the placement of the component is specified as North, South, East, and West.

  • Border Layout() creates a default Border Layout.

  • Border Layout(int horz,int vert) leaves the specified spaces between components.

Grid Layout

  1. Lays out component in a two-dimensional grid

  • Grid Layout() - creates single column grid layout.

  • Grid Layout(num rows ,num cols) creates a Grid containing the specified rows and columns. For example, Grid Layout(3,3) creates a 3 x 3 Grid.

  • Grid Layout(num rows,num cols,int horz,int vert) creates a Grid containing the specified spaces between the Grid.

Card Layout

  1. Unique among other Layout managers

  2. Each layout is a separate index card in a deck that can be shuffled so that any card is on the top at a given time.

Panels

  1. The Panel is also a container.

  2. The Panel can contain UI Components and other Containers.

  3. The general Constructor is Panel p1 = new Panel().

  4. The add() method of the Container Class can be used to add a Component to the Panel.

Glossary

GUI: A graphical user interface. It is an interface to the Windows operating system. It includes user friendly controls, like Buttons, textfields for entering text, message boxes, etc.

AWT: The Abstract Windowing Toolkit. It is a package in JavaAPI and consists of Graphics, Font, color, Image, etc. classes. You have to call this package by using the import keyword (import java.awt.*). if You are using methods and interfaces from this package.

LABEL: It denotes uneditable text.

BUTTON: It is a standard control found in Windows.

CHECKBOX: It consist of a set of items. Users can select one or more items at a time.

CHECKBOXGROUP: It denotes a radio (small, black circle shape) button. Users can select only one item at a time.

TEXTFIELD: Users can enter information in the the boxes. You can enter in one single line.

TEXTAREA: By using this control, users can enter multiple lines of text.

CHOICE: This component is the same as combo box. When a user clicks on the dropdown arrow, she will get a list of items, only one item is selectable at a time.

LIST: This is a variation of the above component. Users can select multiple items at a time.

About the Author

Anand Narayanaswamy is a graduate of the University of Kerala. He is currently working as an instructor teaching Java, Visual Basic, and technologies such as ASP and XML. He enjoys learning new programming languages like C#. Currently, Anand lives in Thiruvananthapuram, Kerala State, India. He can be contacted via his Website.






Comment and Contribute

 


(Maximum characters: 1200). You have characters left.

 

 


Sitemap | Contact Us

Rocket Fuel