Java Applet

by

A Java applet is a program written in the Java programming language that is designed to run within a web browser. Java applets are typically used to provide interactive features on a website, such as games or animations. They were popular in the late 1990s and early 2000s, but have since been largely replaced by other web technologies such as JavaScript and HTML5.

To run a Java applet, the user must have a Java Runtime Environment (JRE) installed on their computer. The JRE provides the necessary software components to run Java applications, including applets. However, many modern web browsers no longer support Java applets due to security concerns and the availability of alternative technologies.

Java applets are created using the Java programming language and typically use the AWT (Abstract Window Toolkit) or Swing GUI (Graphical User Interface) libraries to create a graphical user interface. They can interact with the web page that contains them using JavaScript, and can also communicate with a web server to retrieve data or perform other tasks.

Overall, while Java applets were once a popular way to add interactive features to websites, they have since fallen out of favor in favor of newer web technologies.

Advantage of Applet:

There are several advantages of using Java applets:

  1. Platform independence: Java applets can run on any platform that has a Java Virtual Machine (JVM) installed. This means that the same applet can be run on Windows, Mac, Linux, and other platforms without the need for modification.
  2. Security: Java applets are executed within a sandboxed environment that limits their access to the user’s system. This reduces the risk of malicious code running on the user’s computer.
  3. Rich user interface: Java applets can create a rich and interactive user interface using the AWT or Swing libraries. This allows developers to create complex user interfaces with features such as animation, sound, and video.
  4. Easy deployment: Java applets can be easily deployed on a website, as they are downloaded from the server and run within the web browser. This makes it easy to distribute and update the applet without the need for the user to install any software.
  5. Network communication: Java applets can communicate with a web server to retrieve data or perform other tasks. This makes them suitable for creating web-based applications that require interaction with a server.

Overall, Java applets offer several advantages for creating interactive web applications, including platform independence, security, rich user interfaces, easy deployment, and network communication. However, due to security concerns and the availability of alternative web technologies, applets have become less popular in recent years.

Drawback of Applet:

While Java applets offer several advantages, they also have some drawbacks:

  1. Security concerns: Java applets have been a frequent target for hackers due to security vulnerabilities in the Java runtime environment. As a result, many web browsers no longer support Java applets or require the user to explicitly enable them.
  2. Performance: Java applets can be slower to load and run than other web technologies such as HTML5 and JavaScript. This can result in a poor user experience, particularly on older computers or slower internet connections.
  3. Limited functionality: Java applets are limited in the types of applications they can create, particularly in comparison to more modern web technologies such as HTML5 and JavaScript. This can make it difficult to create complex web applications using only Java applets.
  4. Browser compatibility: Java applets require a web browser with a Java Virtual Machine (JVM) installed, which may not be available on all devices or browsers. This can limit the reach of Java applets to a specific subset of users.

Overall, while Java applets offer some advantages, they also have several drawbacks that have contributed to their decline in popularity in recent years.

Hierarchy of Applet:

In Java, the Applet class is a subclass of the Panel class, which is a subclass of the Container class, which is a subclass of the Component class. This means that an Applet inherits all of the properties and methods of these classes.

Here is the hierarchy of the Applet class:

java.lang.Object
   java.awt.Component
      java.awt.Container
         java.awt.Panel
            java.applet.Applet

This hierarchy means that an Applet is a type of Panel, which is a type of Container, which is a type of Component. This hierarchy also means that an Applet can be treated as a component and can be added to other containers such as Frame, Dialog, or another Panel.

It’s important to note that while Applet extends Panel, it also adds additional functionality specific to applets, such as the init() and start() methods for initializing and starting the applet.

Lifecycle of Java Applet:

The lifecycle of a Java applet can be divided into four stages:

  1. Initialization: In this stage, the applet is loaded by the browser and its init() method is called. The init() method is used to initialize the applet and is called only once during the applet’s lifecycle. The init() method is typically used to set up the user interface and any data structures that the applet will use.
  2. Starting: In this stage, the applet’s start() method is called. The start() method is used to start any threads or animations that the applet may use. This method is called each time the applet is displayed on the screen.
  3. Stopping: In this stage, the applet’s stop() method is called. The stop() method is used to stop any threads or animations that the applet may be running. This method is called each time the applet is no longer visible on the screen.
  4. Destruction: In this stage, the applet’s destroy() method is called. The destroy() method is used to clean up any resources that the applet may have allocated, such as memory or file handles. This method is called only once during the applet’s lifecycle, just before the applet is unloaded from memory.

It’s important to note that the applet lifecycle is controlled by the browser, which can pause, stop, or reload the applet at any time. Applet developers must be aware of this behavior and design their applets to gracefully handle these events.

Lifecycle methods for Applet:

There are three lifecycle methods for a Java Applet:

  1. init(): This method is called when the applet is initialized, typically when the web page containing the applet is loaded. It is used to set up the applet’s initial state, such as initializing variables, loading resources, or creating the user interface. The init() method is called only once during the applet’s lifecycle.
  2. start(): This method is called when the applet is started or displayed on the screen. It is used to start any threads or animations that the applet may use. The start() method is called every time the applet is displayed on the screen.
  3. stop(): This method is called when the applet is stopped or no longer visible on the screen. It is used to stop any threads or animations that the applet may be running. The stop() method is called every time the applet is no longer visible on the screen.

In addition to these three lifecycle methods, there is also a destroy() method that is called when the applet is being unloaded from memory. The destroy() method is used to clean up any resources that the applet may have allocated, such as memory or file handles.

These lifecycle methods allow the applet to respond to changes in the environment in which it is running, such as changes in the user interface or changes in network connectivity. By implementing these methods, applet developers can create applets that are more responsive and better integrated with the web page and the user’s browsing experience.

java.applet.Applet class:

java.applet.Applet is a class in the Java API that provides the basic framework for creating an applet. It is a subclass of the java.awt.Panel class and is designed to be embedded in a web page and run within a web browser.

The Applet class provides a set of methods that are used to manage the applet’s lifecycle and handle events. These methods include:

  • init(): This method is called when the applet is initialized and is used to set up the applet’s initial state.
  • start(): This method is called when the applet is started and is used to start any threads or animations that the applet may use.
  • stop(): This method is called when the applet is stopped and is used to stop any threads or animations that the applet may be running.
  • destroy(): This method is called when the applet is being unloaded from memory and is used to clean up any resources that the applet may have allocated.
  • getParameter(): This method is used to retrieve a parameter passed to the applet from the web page.
  • getAppletContext(): This method is used to get the context of the applet, which is used to access other applets or resources in the same web page.

The Applet class also provides methods for handling events such as mouse clicks, keyboard input, and focus changes. These methods include:

  • mouseClicked(): Called when the mouse is clicked inside the applet’s area.
  • keyPressed(): Called when a key is pressed while the applet has the keyboard focus.
  • focusGained(): Called when the applet gains the keyboard focus.

By extending the Applet class and overriding these methods, applet developers can create custom applets that respond to user input and events.

Note that Java applets are no longer widely used due to security concerns and the availability of alternative web technologies.

java.awt.Component class:

java.awt.Component is a class in the Java API that provides the basic functionality for creating graphical user interface (GUI) components in a Java application. It is a subclass of java.lang.Object and is the superclass of all AWT (Abstract Window Toolkit) components.

The Component class provides methods for handling user input, drawing graphics, and managing the component’s state. Some of the methods provided by the Component class include:

  • setVisible(): This method is used to show or hide the component.
  • setEnabled(): This method is used to enable or disable user input to the component.
  • setBounds(): This method is used to set the position and size of the component on the screen.
  • paint(): This method is called to paint the component on the screen.
  • addMouseListener(): This method is used to register a listener for mouse events on the component.
  • addActionListener(): This method is used to register a listener for action events on the component.

By extending the Component class and overriding these methods, developers can create custom components and handle user input and events.

The Component class is used extensively in creating graphical user interfaces in Java, including the creation of windows, buttons, text fields, and other UI components.

Simple example of Applet by html file:

Sure, here’s an example of an HTML file that embeds a simple Java applet:

<!DOCTYPE html>
<html>
  <head>
    <title>Simple Applet Example</title>
  </head>
  <body>
    <h1>Simple Applet Example</h1>
    <hr />
    <applet code="MyApplet.class" width="200" height="200">
      <param name="message" value="Hello World!" />
    </applet>
    <hr />
  </body>
</html>

This HTML file embeds a Java applet using the <applet> tag. The code attribute specifies the name of the applet class file (in this case, MyApplet.class). The width and height attributes specify the dimensions of the applet’s display area.

The <param> tag inside the <applet> tag allows passing parameters to the applet. In this case, it passes a parameter named “message” with the value “Hello World!”.

Here’s an example of a simple applet class that displays the message passed as a parameter:

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

public class MyApplet extends Applet {
  private String message;

  public void init() {
    message = getParameter("message");
  }

  public void paint(Graphics g) {
    g.drawString(message, 20, 20);
  }
}

This applet extends the Applet class and overrides the init() and paint() methods. The init() method retrieves the “message” parameter passed by the HTML file using the getParameter() method. The paint() method displays the message on the applet’s display area using the drawString() method of the Graphics class.

When the HTML file is loaded in a web browser that supports Java applets, it will display the applet and the message “Hello World!” inside the applet’s display area.

Simple example of Applet by appletviewer tool:

Sure! Here’s a simple example of an Applet that you can view using the appletviewer tool:

import java.applet.Applet;
import java.awt.*;

public class SimpleApplet extends Applet {
   public void paint(Graphics g) {
      g.drawString("Hello, world!", 50, 25);
   }
}

Save this code in a file called SimpleApplet.java. Then, compile it using the following command:

javac SimpleApplet.java

Once the code is compiled successfully, create an HTML file to display the Applet using the appletviewer tool. Here’s the code for the HTML file:

<html>
<head>
   <title>Simple Applet Example</title>
</head>
<body>
   <applet code="SimpleApplet.class" width="200" height="200">
      If you see this text, your browser does not support Java.
   </applet>
</body>
</html>

Save this code in a file called index.html. Finally, run the appletviewer tool and pass the HTML file as an argument:

appletviewer index.html

This should open a window displaying the Applet, which will simply show the message “Hello, world!” in the top left corner of the window.