A Java applet is an applet delivered to the users in the form of Java bytecode. Java applets can run in a Web browser using a Java Virtual Machine (JVM), or 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. Java applets are usually written in the Java programming language but they can also be written in other languages that compile to Java bytecode such as Jython[7], Ruby[8] or Eiffel[9].
Applets are used to provide interactive features to web applications that cannot be provided by HTML alone. They can capture mouse input (like rotating 3D object) and also have controls like buttons 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 differential equations[10] till heart physiology[3]. Applets are also used to create online game collections that allow to play against live opponents in real-time,
An applet can also be text area only, providing, for instance, cross platform command-line interface to some remote system[11]. If needed, applet can leave the dedicated area and run as separate window. However applets have very little control on 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[12] or WYSIWYG editors[13] are also known). Applet can also play media in formats that are not natively supported by the browser[14].
Java applets run at a speed that is comparable to (but generally slower than) other compiled languages such as C++, but many times faster than JavaScript[15]. In addition they can use 3D hardware acceleration that is available from Java. This makes applets well suitable for non trivial, computation intensive visualizations.
HTML page may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on that parameters were passed. First implementations were downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation of slow loading components. However since jars were introduced an applet is usually delivered as a single file that has a size of the bigger image (hundreds of kilobytes to several megabytes).
Since Java's bytecode is platform independent, Java applets can be executed by browsers for many platforms, including Windows, Unix, Mac OS and Linux. It is also trivial to run Java applet as an application with very little extra code. This has the advantage of running a Java applet in offline mode without the need for internet browser software and also directly from the development IDE.
Many Java developers, blogs and magazines are recommending that the Java Web Start technology be used in place of Applets [16][17].
A Java Servlet is sometimes informally compared to be "like" a server-side applet, but it is different in its language, functions, and in each of the characteristics described here about applets.
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