What Is an Applet?

A Java Applet is a small Java program designed to run inside another application - historically a web browser - rather than as its own independent process. Instead of starting from a public static void main(String[] args) method the way a normal Java program does, an applet is a class that extends java.applet.Applet (or, later, javax.swing.JApplet) and hands control of its own startup, drawing, and shutdown over to whatever container is hosting it. You don't call its methods yourself; the browser or a testing tool called appletviewer calls them for you, in a fixed order, at the right moments. That calling order is the applet lifecycle, covered in full in the next two lessons.

A Bit of History

Applets were introduced by Sun Microsystems in the mid-1990s, around the same time as the Java language itself and the early Netscape Navigator browser. At that point, HTML pages were almost entirely static, and JavaScript - itself brand new - could do very little beyond simple form validation and DOM tricks. Applets were Sun's answer to "how do we get real, general-purpose, compiled programs running inside a web page?" A browser with the right plugin installed could download a compiled .class file referenced by an HTML tag, load it into a sandboxed Java Virtual Machine, and run it right there on the page - complete with graphics, animation, and user interaction that plain HTML of the era simply couldn't offer.

For a while this was genuinely useful: interactive charts, simple games, chat widgets, and educational simulations were all common applet use cases in the late 1990s and early 2000s. Over time, though, JavaScript matured enormously, Flash (and later HTML5 canvas, video, and web APIs) took over rich media on the web, and the security problems inherent in running downloaded native-ish code inside a browser became harder to justify. Browser vendors eventually removed the plugin architecture (NPAPI) that applets depended on, and the technology quietly died out well before most developers noticed it was gone.

Applet vs. Standalone Application

The clearest way to understand an applet is to contrast it with the standalone Java application you already know - the kind you run with java ClassName from a main method. A standalone application owns its own process from start to finish: your code decides when it starts, what it does, and when it exits. An applet owns none of that. It is a passive class whose lifecycle methods are invoked by a host container, it draws into a rectangle the host gives it, and by default it runs inside a security "sandbox" that restricts what it's allowed to touch on the user's machine (no arbitrary file access, no arbitrary network connections outside its own originating server, and so on) unless it is specially signed and the user trusts it.

Why It's Still Worth Studying

Even with the browser story dead, the applet API is a compact, self-contained way to learn several ideas that matter well beyond applets themselves: a component-based lifecycle driven by a framework instead of your own code, drawing with the AWT Graphics object, and handling mouse and keyboard events through listener interfaces. These are the same foundational ideas Swing desktop applications use, and they echo in event-driven UI frameworks generally. Treat this track as a guided tour of that foundation using a genuinely simple, self-contained API, not as a technology you'll deploy.