Inherit/modify class at runtime

[Matti Poro]

    
URLClassLoader loader = new URLClassLoader(new URL[] { file.toURL() });
Class clazz = loader.loadClass("Main");
       
Applet applet = (Applet) clazz.newInstance();

If I load a class that way, is it possible to replace (override) methods in the created object?

[Ernest Friedman-Hill]

No. It is possible to do it before you load the class using a byte-code manipulation library like BCEL

[Ernest Friedman-Hill]

On the other hand, I think I was answering the question you asked, rather than solving the problem you actually have. You can create a proxy object for the object you load, which implements the same interface and/or extends the same base class as your "target" object. In this proxy class, you can implement most of the methods to simply "forward" the call to the target, while others might add code; i.e.,

class MyAppletProxy {
    private Applet applet;
    MyAppletProxy(Applet a) {
        applet = a;
    }
    // Just forward this call...
    public void init() {
        applet.init();
    }
 
     // ... but do extra work here
    public void start() { 
        doSomethingExtraHere(); 
        applet.start();
    }
}
 
// Then do this:
URLClassLoader loader = new URLClassLoader(new URL[] { file.toURL() });
Class clazz = loader.loadClass("Main");
       
Applet applet = new MyAppletProxy((Applet) clazz.newInstance());