Applet
Java Applets were created when the Internet was becoming popular with educational institutions, businesses and consumers. Java Applets were originally meant to be part of products from the cable company, but developers of Java Applets could not convince the cable industry to use their invention. Java Applets were created by programmers at Sun in 1995. They were James Gosig, Patrick Naughton and Mike Sheridan who called themselves the Green Team.
Java Applets share some similarities with the java programming language. Java is text and characters that is written as instructions. A programmer is the person who writes the instructions on the computer. The computer will not perform an action without getting instructions from the person who writes the instructions. The java programming language allows a programmer to use the same code on both a personal computer and a Macintosh.
The same rule applies to a Web browser. The programmer who creates web pages is called the web master. The web master uses hypertext markup language to create a webpage. In order for the webpage images to move an applet is added. An applet is a small application that is placed inside the hypertext markup language.
The most popular web browsers in the 1990s were Netscape and Mosaic. Both browsers displayed still images and text. Java applets allowed the programmer to write applications for the web. For the first time, using Java Applets, images could move. Animation allowed webpages to come alive. Java Applets could also be written to display pop-up windows, create forms, and protect passwords.
The Java Applets are placed with the hypertext markup language. Since the Java Applets are a set of instructions, in order for them to perform a function the Java Applets are read by a java virtual machine. The java virtual machine is a program that debugs or check for errors in the Java Applets. The java virtual machine is part of the Web browser. Programmers can reuse the applet code on any computer platform without changing the software instructions. Using Java Applets, programmers have more control of their web browser.
To view a Java Applet go to the Web browsers, View | Page Source pull down menu. You will notice that the hypertext begins and ends with an applet tag. For example, the tags have to be present in all Web browsers that run applets. This lets the java virtual machine know that an applet is present within the browser. To view the applet animation the end user web browser has to be enabled.
Java Applets are widely used in industry; they can run in the web browser or independently. Java applets have become part of many electronic devices. Java Applets are used in remote control devices. The applets direct the movement of the electronic devices.
In smart homes java applets are used to program light switches, preprogram the T.V., and the house alarm. Java applets are also used in smart phones like the iPhone and Blackberry. They can be used from a cell phone to keep track of daily appointments, read current stock trend, play games, or make a reservation at your favorite restaurant.
A Java applet is an applet delivered to users in the form of Java bytecode. Java applets can be part of a web page and executed by the Java Virtual Machine (JVM) in a process separate from the web browser, or run in Sun's AppletViewer, a stand-alone tool for testing applets. Java applets were introduced in the first version of the Java language in 1995, and are written in programming languages that compile to Java bytecode, usually in Java, but also in other languages such as Jython, JRuby, or Eiffel (via SmartEiffel).
Java applets run at very fast speeds comparable to, but generally slower than, other compiled languages such as C++, but until approximately 2011, it had been many times faster than JavaScript. In addition, they can use 3D hardware acceleration that is available from Java. This makes applets well suited for non-trivial, computation intensive visualizations. As browsers have gained support for hardware accelerated graphics thanks to the canvastechnology (or specifically WebGL in the case of 3D graphics), as well as just in time compiled JavaScript, the speed difference has become less noticeable.
Since Java's bytecode is cross-platform or platform independent, Java applets can be executed by browsers for many platforms, including Microsoft Windows, Unix, OS X and Linux. It is also trivial to run a Java applet as an application software with very little extra code so that it can be run directly from the integrated development environment (IDE).
Java applets are executed in a sandbox by most web browsers, preventing them from accessing local data like clipboard or file system. The code of the applet is downloaded from a web server and the browser either embeds the applet into a web page or opens a new window showing the applet's user interface.
A Java applet extends the class java.applet.Applet, or in the case of a Swing applet, javax.swing.JApplet. The class must override methods from the applet class to set up a user interface inside itself (Applet) is a descendant of Panel which is a descendant of Container. As applet inherits from container, it has largely the same user interface possibilities as an ordinary Java application, including regions with user specific visualization.
The first implementations involved downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation as slow loading components. However, since jars were introduced, an applet is usually delivered as a single file that has a size similar to a large image file (hundreds of kilobytes to several megabytes).
The domain from where the applet executable has been downloaded is the only domain to which the usual (unsigned) applet is allowed to communicate. This domain can be different from the domain where the surrounding HTML document is hosted.
Java system libraries and runtimes are backwards compatible, allowing one to write code that runs both on current and on future versions of the Java virtual machine.
Applets are used to provide interactive features to web applications that cannot be provided by HTML alone. They can capture mouse input and also have controls likebuttons or check boxes. In response to the user action an applet can change the provided graphic content. This makes applets well suitable for demonstration, visualization and teaching. There are online applet collections for studying various subjects, from physics to heart physiology. Applets are also used to create onlinegame collections that allow players to compete against live opponents in real-time.
An applet can also be a text area only, providing, for instance, a cross platform command-line interface to some remote system. If needed, an applet can leave the dedicated area and run as a separate window. However, applets have very little control over web page content outside the applet dedicated area, so they are less useful for improving the site appearance in general (while applets like news tickers or WYSIWYG editors[15] are also known). Applets can also play media in formats that are not natively supported by the browser
HTML pages may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on the parameters that were passed.
As applets have been available before CSS, they were also widely used for trivial effects like navigation buttons. This use is criticized and declining.
-------------------------------------------------
Example
The following example is made simple enough to illustrate the essential use of Java applets through its java.applet package. It also uses classes from the Java Abstract Window Toolkit (AWT) for producing actual output (in this case, the "Hello, world!" message). import java.applet.Applet; import java.awt.*; // Applet code for the "Hello, world!" example. // This should be saved in a file named as "HelloWorld.java". public class HelloWorld extends Applet { // This method is mandatory, but can be empty (i.e., have no actual code). public void init() { } // This method is mandatory, but can be empty.(i.e., have no actual code). public void stop() { } // Print a message on the screen (x=20, y=10). public void paint(Graphics g) { g.drawString("Hello, world!", 20, 10); // Draws a circle on the screen (x=40, y=30). g.drawArc(40, 30, 20, 20, 0, 360); } }

Additional simple applets are available at Wikiversity.
For compiling, this code is saved in a plain-ASCII file named after the class, with the suffix .java, e.g. HelloWorld.java. The resultingHelloWorld.class applet should be placed on the web server and is invoked within an HTML page by using an <applet> or an <object> tag. For example: <!DOCTYPE html> <html> <head> <title>HelloWorld_example.html</title> </head> <body> <h1>A Java applet example</h1> <p>Here it is: <applet code="HelloWorld.class" height="40" width="200"> This is where HelloWorld.class runs. </applet></p> </body> </html>
Displaying the HelloWorld_example.html page from a Web server, the result should look as this:
A Java applet example
Here it is: Hello, world!
To minimize download time, applets are usually delivered in a form of compressed zip archive (having jar extension). If all needed classes (only one in our case) are placed in compressed archiveexample.jar, the embedding code would look different: <p>Here it is: <applet archive="example.jar" code="HelloWorld" height="40" width="200"> This is where HelloWorld.class runs. </applet></p>

-------------------------------------------------
Advantages
A Java applet can have any or all of the following advantages: * It is simple to make it work on Linux, Microsoft Windows and OS X i.e. to make it cross platform. Applets are supported by most web browsers. * The same applet can work on "all" installed versions of Java at the same time, rather than just the latest plug-in version only. However, if an applet requires a later version of the Java Runtime Environment (JRE) the client will be forced to wait during the large download. * Most web browsers cache applets so will be quick to load when returning to a web page. Applets also improve with use: after a first applet is run, the JVM is already running and starts quickly (the JVM will need to restart each time the browser starts afresh). It should be noted that JRE versions 1.5 and greater stop the JVM and restart it when the browser navigates from one HTML page containing an applet to another containing an applet. * It can move the work from the server to the client, making a web solution more scalable with the number of users/clients. * If a standalone program (like Google Earth) talks to a web server, that server normally needs to support all prior versions for users which have not kept their client software updated. In contrast, a properly configured browser loads (and caches) the latest applet version, so there is no need to support legacy versions. * The applet naturally supports the changing user state, such as figure positions on the chessboard. * Developers can develop and debug an applet direct simply by creating a main routine (either in the applet's class or in a separate class) and calling init() and start() on the applet, thus allowing for development in their favorite Java SE development environment. All one has to do after that is re-test the applet in the AppletViewer program or a web browser to ensure it conforms to security restrictions. * An untrusted applet has no access to the local machine and can only access the server it came from. This makes such an applet much safer to run than a standalone executable that it could replace. However, a signed applet can have full access to the machine it is running on if the user agrees. * Java applets are fast - and can even have similar performance to native installed software.

-------------------------------------------------
Disadvantages
A Java applet may have any of the following disadvantages: * It requires the Java plug-in. * Some browsers, notably mobile browsers running Apple iOS or Android do not run Java applets at all. * Some organizations only allow software installed by the administrators. As a result, some users can only view applets that are important enough to justify contacting the administrator to request installation of the Java plug-in. * As with any client-side scripting, security restrictions may make it difficult or even impossible for an untrusted applet to achieve the desired goals. However, simply editing the java.policy file in the JAVA JRE installation, one can grant access to the local filesystem or system clipboard for example, or to other network sources other than the network source that served the applet to the browser. * Some applets require a specific JRE. This is discouraged. * If an applet requires a newer JRE than available on the system, or a specific JRE, the user running it the first time will need to wait for the large JRE download to complete. * Java automatic installation or update may fail if a proxy server is used to access the web. This makes applets with specific requirements impossible to run unless Java is manually updated. The Java automatic updater that is part of a Java installation also may be complex to configure if it must work through a proxy. * Unlike the older applet tag, the object tag needs workarounds to write a cross-browser HTML document. * There is no standard to make the content of applets available to screen readers. Therefore, applets can harm the accessibility of a web site to users with special needs.