java applet

applet

An applet is a Java program that runs in a Web

browser.

An applet is typically embedded inside a web

page and runs in the context of a browser.

The Applet class provides the standard interface

between the applet and the browser environment.

Difference between applets and java

application.

There are some important differences between an

applet and a standalone Java application,

including the following:

An applet is a Java class that extends the java.applet.Applet class.

A main() method is not invoked on an applet, and an applet class will

not define main().

Applets are designed to be embedded within an HTML page.

When a user views an HTML page that contains an applet, the code for

the applet is downloaded to the user's machine.

A JVM is required to view an applet. The JVM can be either a plug-in of

the Web browser or a separate runtime environment.

Java applet scenario.

Applets cont

An applet program is a written as a inheritance of thejava.Appletclass

There is no main()method in an Applet.

The JVM on the user's machine creates an instance of the applet class and

invokes various methods during the applet's lifetime.

Because Java applets run inside the Java browser, they have access to the

same capabilities that the browser has: sophisticated graphics, drawing,

and image processing packages; user interface elements; networking; and

event handling.

Life Cycle of an Applet

Four methods in the Applet class give you the framework on which you build any serious applet:

init: This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed.

start: This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages.

stop: This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet.

destroy: This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet.

paint: Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt

Every Applet needs to implement one or more of

the init(), the start( ) and the paint( ) methods.

At the end of the execution, the stop( ) method is

invoked, followed by the destroy( ) method to

deallocate the applets resources.

Applet life cycle

browser visits page containing an applet

browser calls init on that applet, once

browser calls start on that applet

browser goes away from that page

browser calls stop on that applet

browser comes back to that page

browser calls start again on that applet

browser shuts down

browser calls destroy on the applet, once

The Applet CLASS:

//for graphics

View applet (Java Virtual Machine)

In order to view the output of applet we use either

Appletviewer

browser

The coordinate system

The applet is drawn in a rectangle, which consists of

pixels.

Example.

import java.applet.Applet;

import java.awt.Color;

import java.awt.Font;

import java.awt.Graphics;

public class qw extends Applet {

Font f = new Font("TimesRoman",Font.BOLD,36);

public void paint(Graphics g) {

g.setFont(f);

g.setColor(Color.red);

g.drawString("Hello again!", 5, 50);

}

}

Including an Applet on a Web Page

After you create a class or classes that contain your applet

and compile them into class files as you would any other Java

program, you have to create a Web page that will hold that

applet by using the HTML language.

There is a special HTML tag for including applets in Web

pages; Java-capable browsers use the information contained

in that tag to locate the compiled class files and execute the

applet itself.

Applet tags

The Hello, World Applet

If your browser was Java-enabled, a "Hello, World"

message would appear here.

Getting Applet Parameters:

Parameters are passed to applets in NAME=VALUE pairs

in tags between the opening and

closing APPLET tags.

Inside the applet, you read the values passed through

the PARAM tags with the getParameter() method of

the java.applet.Applet class.

So, you can pass the parameters from your html page to the

applet embedded in your page. Theparam tag() is used to pass the parameters

to an apple

Passing Parameter in Java Applet

This is the applet:

APPLET

import java.applet.*;

import java.awt.*;public class appletParameter extends Applet {private String strDefault = "Hello! Java Applet.";public void paint(Graphics g) {String strParameter = this.getParameter("Message");if (strParameter == null)strParameter = strDefault;g.drawString(strParameter, 50, 25);}

}

EXAMPLE:


import java.awt.Font;

import java.awt.Color;

public class MoreHelloApplet extends java.applet.Applet {

Font f = new Font(TimesRoman,Font.BOLD,36);

String name;

public void init() {

this.name = getParameter(name);

if (this.name == null)

this.name = Laura;

this.name = Hello + this.name + !;

}


g.setFont(f);

g.setColor(Color.red);

g.drawString(this.name, 5, 50); }}

The HTML file

Hello!

Hello to whoever you are!

JAVA PROGRAMMING

Graphics, Fonts, and Color

The Graphics Class

With Javas graphics capabilities, you can draw lines, shapes, characters, and images to the screen inside your applet. Most of the graphics operations in Java are methods defined in the Graphics class. You dont have to create an instance of Graphics in order to draw something in your applet; in your applets paint() method , you are given a Graphics object. By drawing on that object, you draw onto your applet and the results appear on screen

The Graphics class is part of the java.awt package, so if your applet does any painting (as it usually will), make sure you import that class at the beginning of your Java file:


public class MyClass extended java.applet.Applet {

... }

Drawing and Filling

Lines

package q;





g.drawLine(25,25,75,75);

}

}

Drawing and Filling Cont

Plain rectangles.

package q;





g.drawRect(20,20,60,60);

g.fillRect(120,20,60,60);

}

}

To draw a plain rectangle, use either the drawRect or fillRectmethods. Both take four arguments: the x and y coordinates of the top left corner of the rectangle, and the width and height of the rectangle to draw.

Drawing and Filling Cont

Rounded rectangles, which are rectangles with rounded corners





g.drawRoundRect(20,20,60,60,10,10);

g.fillRoundRect(120,20,60,60,20,20);

}}

Heres a paint method that draws two rounded rectangles: one as an outline with a rounded corner 10 pixels square; the other, filled, with a rounded corner 20 pixels square

Polygons

Polygons are shapes with an unlimited number of sides. To

draw a polygon, you need a set of x and y coordinates, and

the drawing method then starts at one, draws a line to the

second, then a line to the third, and so on.

Using the first method, the drawPolygon and fillPolygon

methods take three arguments:

An array of integers representing x coordinates

An array of integers representing y coordinates

An integer for the total number of points

Polygon drawing.





int exes[] = { 39,94,97,142,53,58,26 };

int whys[] = { 33,74,36,70,108,80,106 };

int pts = exes.length;

g.drawPolygon(exes,whys,pts);

}}

Ovals

Use ovals to draw ellipses or circles. Ovals are just like rectangles with

overly rounded corners.

In fact, you draw them using the same four arguments: the x and y of the

top corner, and the width and height of the oval itself. Note that, because

youre drawing an oval, the starting point is some distance to the left and

up from the actual outline of the oval itself.

As with the other drawing operations, the drawOval method draws an

outline of an oval, and the fillOval method draws a filled oval.

Heres an example of two ovals, a circle and an ellipse

Example.





g.drawOval(20,20,60,60);

g.fillOval(120,20,100,60);

}}

Arc

An arc is a part of a oval; in fact, the easiest way to think of an arc is as a

section of a complete oval.

The drawArc method takes six arguments: the starting corner, the width and

height, the angle at which to start the arc, and the degrees to draw it

before stopping

Once again, there is a drawArc method to draw the arcs outline and the

fillArc to fill the arc. Filled arcs are drawn as if they were sections of a pie;

instead of joining the two endpoints, both endpoints are joined to the center

of the circle.

Arc example.





g.drawArc(100,20,60,60,45,180);

g.fillArc(120,20,60,60,90,180);

}}

Arc example.





g.drawArc(10,20,150,50,25,-130);

g.fillArc(10,80,150,50,25,-130);

}}

Copying and Clearing

The copyArea method copies a rectangular area of the screen to

another area of the screen.

copyArea takes six arguments: the x and y of the top corner of the

rectangle to copy, the width and the height of that rectangle, and

the distance in the x and y directions to which to copy it.

For example, this line copies a square area 100 pixels on a side

100 pixels directly to its right:

g.copyArea(0,0,100,100,100,0);

Example.


import java.awt.*;

public class as extends Applet {

public void paint(Graphics g){

g.drawRect(20,20,60,60);

g.copyArea(0, 0, 100, 100, 200, 0);

}

}

Clearing

To clear a rectangular area, use the clearRect method.

clearRect, which takes the same four arguments as the

drawRect and fillRect methods, fills the given rectangle with the

current background color of the applet

To clear the entire applet, you can use the size() method, which

returns a Dimension object representing the width and height

of the applet. You can then get to the actual values for width

and height by using the width and height instance variables:

Example.


import java.awt.*;



g.drawRect(20,20,60,60);

g.copyArea(0, 0, 100, 100, 200, 0);

g.clearRect(0, 0, 50, 100);

}

}

Drawing Characters and Strings

With a font object in hand, you can draw text on the screen using the methods

drawChars and drawString. First, though, you need to set the current font to

your font object using the setFont method.


Font f = new Font(TimesRoman, Font.PLAIN,72);

g.setFont(f);

g.drawString(This is a big font.,10,100);

}

Example.


import java.awt.*;



Font f = new Font("TimesRoman", Font.PLAIN, 18);

Font fb = new Font("TimesRoman", Font.BOLD, 18);

Font fi = new Font("TimesRoman", Font.ITALIC, 18);

Font fbi = new Font("TimesRoman", Font.BOLD + Font.ITALIC, 18);

g.setFont(f);

g.drawString("This is a plain font", 10, 25);

g.setFont(fb);

g.drawString("This is a bold font ", 10, 50);

g.setFont(fi);

g.drawString("This is an italic font ", 10, 75);

g.setFont(fbi);

g.drawString("This is a bold italic font ", 10, 100);

}

}

Color

Drawing black lines and tests on a gray background is all very

nice, but being able to use different colors is much nicer. Java

provides methods and behaviors for dealing with color in

general through the Color class, and also provides methods for

setting the current foreground and background colors so that

you can draw with the colors you created

Using Color Objects

Using Color Objects

To draw an object in a particular color, you must create an instance of the

Color class to represent that color. The Color class defines a set of standard

color objects, stored in class variables, that enable you quickly to get a

color object for some of the more popular colors.

Testing and Setting the Current Colors

To draw an object or text using a color object, you have to set the current color to be that color object, just as you have to set the current font to the font in which you want to draw.

Use the setColor method (a method for Graphics objects) to do this:

g.setColor(Color.green);

After setting the current color, all drawing operations will occur in that color

The setBackground method sets the background color of the applet, which is usually a dark grey. It takes a single argument, a color object:

setBackground(Color.white);

Example.


import java.awt.*;



g.setColor(Color.GREEN);

g.fillRect(100,200,250,250);

}

}

AWT.

The Java Abstract Windowing Toolkit

AWT

The AWT provides the following:

A full set of UI widgets and other components, including windows, menus,

buttons, checkboxes, text fields, scrollbars, and scrolling lists

Support for UI containers, which can contain other embedded

containers or UI widgets

An event system for managing system and user events between and

among parts of the AWT

Mechanisms for laying out components in a way that enables platform-

independent UI design

AWT Component

These are the major components you can work with in the AWT

Containers. Containers are generic AWT components that can contain

other components, including other containers. The most common form of

container is the panel, which represents a container that can be

displayed on screen

Canvases. A canvas is a simple drawing surface. Although you can draw

on panels (as youve been doing all along), canvases are good for

painting images or other graphics operations.

UI components. These can include buttons, lists, simple popup menus,

checkboxes, test fields, and other typical elements of a user interface.

Window construction components. These include windows, frames, menu

bars, and dialogs

java applet

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