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Java Applet Basics - GeeksforGeeks

Java Applets was once a very popular feature of web applications. Java Applets were small programs written in Java that ran inside a web browser. Learning about Applet helps us understand how Java has evolved and how it handles graphics.

Note: java.applet package has been deprecated in Java 9 and later versions, as applets are no longer widely used on the web.

A Java Applet is a Java program that runs inside a web browser. An Applet is embedded in an HTML file using <applet> or <objects> tags. Applets are used to make the website more dynamic and entertaining. Applets are executed in a sandbox for security, restricting access to local system resources.

Key Points:

• Applet Basics: Every applet is a child/subclass of the java.applet.Applet class.
• Not Standalone: Applets don’t run on their own like regular Java programs. They need a web browser or a special tool called the applet viewer (which comes with Java).
• No main() Method: Applets don't start with main() method.
• Display Output: Applets don't use System.out.prinln() for displaying the output, instead they use graphics methods like drawString() from the AWT (Abstract Window ToolKit).

The below diagram demonstrates the life cycle of Java Applet:

It is important to understand the order in which the various methods shown in the above image are called.

• When an applet begins, the following methods are called, in this sequence: 
  • init( )
  • start( )
  • paint( )
• When an applet is terminated, the following sequence of method calls takes place: 
  • stop( )
  • destroy( )

Let’s look more closely at these methods. 

1. init( ): The init( ) method is the first method to be called. This is where you should initialize variables. This method is called only once during the run time of your applet.

2. start( ): The start( ) method is called after init( ). It is also called to restart an applet after it has been stopped.

Note: init( ) is called once i.e. when the first time an applet is loaded whereas start( ) is called each time an applet’s HTML document is displayed onscreen. So, if a user leaves a web page and comes back, the applet resumes execution at start( )

3. paint( ): The paint( ) method is called each time an AWT-based applet’s output must be redrawn. This situation can occur for several reasons. For example, the window in which the applet is running may be overwritten by another window and then uncovered. Or the applet window may be minimized and then restored. 

• paint( ) is also called when the applet begins execution. Whatever the cause, whenever the applet must redraw its output, paint( ) is called.
• The paint( ) method has one parameter of type Graphics. This parameter will contain the graphics context, which describes the graphics environment in which the applet is running. This context is used whenever output to the applet is required.
• paint() is the only method among all the methods mention above (which is parameterized).

This method is crucial for updating or redrawing the visual content of the applet.

Example:

public void paint(Graphics g)

{

// Drawing a string on the applet window

// g is an object reference of class Graphic. 

g.drawString("Hello, Applet!", 50, 50);

}

Now the below Question Arises:

In the prototype of paint() method, we have created an object reference without creating its object. But how is it possible to create object reference without creating its object?

Ans. Whenever we pass object reference in arguments then the object will be provided by its caller itself. In this case the caller of paint() method is browser, so it will provide an object. The same thing happens when we create a very basic program in normal Java programs. For Example:

public static void main(String []args) {

}

Here we have created an object reference without creating its object but it still runs because it’s caller, i.e. JVM will provide it with an object.

4. stop( ): The stop( ) method is called when a web browser leaves the HTML document containing the applet, when it goes to another page.

For example: When stop( ) is called, the applet is probably running. You should use stop( ) to suspend threads that don’t need to run when the applet is not visible. You can restart them when start( ) is called if the user returns to the page.

5. destroy( ): The destroy( ) method is called when the environment determines that your applet needs to be removed completely from memory. At this point, you should free up any resources the applet may be using. The stop( ) method is always called before destroy( ).

• java.applet.Applet: Base class for applets.
• java.awt.Graphics: Used for drawing on the applet screen.
• java.awt: Provides GUI components and event-handling mechanisms.

Let’s begin with the HelloWorld applet :

import java.applet.Applet;
import java.awt.Graphics;

// HelloWorld class extends Applet
public class HelloWorld extends Applet {
    
    // Overriding paint() method
    @Override public void paint(Graphics g)
    {
        g.drawString("Hello World", 20, 20);
    }
}

Explanation: 

1. The above java program begins with two import statements. The first import statement imports the Applet class from applet package. Every AWT-based(Abstract Window Toolkit) applet that you create must be a subclass (either directly or indirectly) of Applet class. The second statement import the Graphics class from AWT package.
2. The next line in the program declares the class HelloWorld. This class must be declared as public because it will be accessed by code that is outside the program. Inside HelloWorld, paint( ) is declared. This method is defined by the AWT and must be overridden by the applet.
3. Inside paint( ) is a call to drawString( ), which is a member of the Graphics class. This method outputs a string beginning at the specified X,Y location. It has the following general form:

void drawString(String message, int x, int y)

Here, message is the string to be output beginning at x,y. In a Java window, the upper-left corner is location 0,0. The call to drawString( ) in the applet causes the message "Hello World" to be displayed beginning at location 20,20.

Notice that the applet does not have a main( ) method. Unlike Java programs, applets do not begin execution at main( ). In fact, most applets don’t even have a main( ) method. Instead, an applet begins execution when the name of its class is passed to an applet viewer or to a network browser.

After you enter the source code for HelloWorld.java, compile in the same way that you have been compiling java programs (using javac command). However, running HelloWorld with the java command will generate an error because it is not an application. 

java HelloWorld

Error: Main method not found in class HelloWorld, please define the main method as:

public static void main(String[] args)

There are two standard ways in which you can run an applet:

1. Executing the applet within a Java-compatible web browser.
2. Using an applet viewer, such as the standard tool, applet-viewer. An applet viewer executes your applet in a window. This is generally the fastest and easiest way to test your applet.

Each of these methods is described next.

• To execute an applet in a web browser we have to write a short HTML text file that contains a tag that loads the applet.
• We can use APPLET or OBJECT tag for this purpose
• Using APPLET, here is the HTML file that executes HelloWorld

<applet code="HelloWorld" width=200 height=60>

</applet>

The width and height statements specify the dimensions of the display area used by the applet. The APPLET tag contains several other options. After you create this html file, you can use it to execute the applet.

Note: Chrome and Firefox no longer supports NPAPI (technology required for Java applets).

• This is the easiest way to run an applet.
• To execute HelloWorld with an applet viewer, you may also execute the HTML file shown earlier.
• For example, if the preceding HTML file is saved with RunHelloWorld.html, then the following command line will run HelloWorld.

appletviewer RunHelloWorld.html

If you include a comment at the head of your Java source code file that contains the APPLET tag then your code is documented with a prototype of the necessary HTML statements, and you can run your compiled applet merely by starting the applet viewer with your Java source code file. If you use this method, the HelloWorld source file looks like this: 

// A Hello World Applet
// Save file as HelloWorld.java
import java.applet.Applet;
import java.awt.Graphics;

/*
<applet code="HelloWorld" width=200 height=60>
</applet>
*/

// HelloWorld class extends Applet
public class HelloWorld extends Applet 
{
    // Overriding paint() method
    @Override
    public void paint(Graphics g) 
    {
        g.drawString("Hello World", 20, 20);
    }
    
}

With this approach, first compile HelloWorld.java file and then simply run the below command to run applet : 

appletviewer HelloWorld

To prove above mentioned point,i.e paint is called again and again.

To prove this, let’s first study what is “Status Bar” in Applet?

• Status Bar”is available in the left bottom window of an applet. To use the status bar and write something in it, we use method showStatus() whose prototype is public void showStatus(String)
• By default status bar shows “Applet Started”
• By default background color is white.

To prove paint() method is called again and again, here is the code:

Note: This code is with respect to Netbeans IDE.

Example:

//Code to illustrate paint  
//method gets called again 
//and again 
import java.applet.*;

//to access showStatus() 
import java.awt.*;//Graphic 

//class is available in this package 
import java.util.Date;

//to access Date object 
public class GFG extends Applet 
{ 
public void paint(Graphics g) 
{ 
    
Date dt = new Date(); 
super.showStatus("Today is" + dt);

//in this line, super keyword is 
// avoidable too. 
} 
} 

Note: Here, we can see that if the screen is maximized or minimized we will get an updated time. This shows that paint() is called again and again.

• Displaying dynamic web pages of a web application.
• Playing sound files.
• Displaying documents
• Playing animations

Due to security reasons, the following restrictions are imposed on Java applets:

• An applet cannot load libraries or define native methods.
• An applet cannot ordinarily read or write files on the execution host.
• An applet cannot read certain system properties.
• An applet cannot make network connections except to the host that it came from.
• An applet cannot start any program on the host that’s executing it.

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