Hi,

Pouh, that question is not very specific! And it may be a very complex
thing to do.

In short (and easy):

Define an interface (or abstract class) that declares all methods you
will need:

interface Plugin {
  void doSomething();
}

Now, the spcific plugin is an implementation of that interface:

class MyPlugin implements Plugin {
  void doSomthing() {
    System.out.println("Hello World!");
  }
}

Put all plugins/classes in a certain directory (or another resource you
can "list") and which is included in your classpath.

Let your application scan the specific plugin-directory and instantiate
the classes as follows:

String[] files=pluginDir.list();
for(String file:files) {
  Plugin p=(Plugin)Class.forName(file).newInstance();
  // do whatever you want with the plugin, e.g.:
  p.doSomething();
}

That's the basics. The difficult thing is the definition of the
interface and how to interact between the application and the plugin.
(keywords: event listener, callback, visitor-pattern, ...)

Hth,
Ingo