Errors modifying an existing class using JavaAssist

Question

I'm working on the following CodeWars challenge :

https://www.codewars.com/kata/hack-22/train/java

Here's what I've written :

public static Yossarian loophole() throws Throwable {
    ClassPool pool = ClassPool.getDefault();
    //Loader cl = new Loader(pool);
    CtClass yossarianClass = pool.get("Yossarian");
    int modifiers = yossarianClass.getDeclaredMethod("isCrazy").getModifiers();
    if(Modifier.isFinal(modifiers)) {
        System.out.println("Removing Final");
        int notFinalModifier = Modifier.clear(modifiers, Modifier.FINAL);
        yossarianClass.getDeclaredMethod("isCrazy").setModifiers(notFinalModifier);
        yossarianClass.rebuildClassFile();
    }
    final CtClass saneYossarianClass = ClassPool.getDefault().makeClass("SaneYossarian");
    saneYossarianClass.setSuperclass(yossarianClass);
    final CtMethod overrideMethod = CtNewMethod.make("public boolean isCrazy() { return true; }", saneYossarianClass);
    saneYossarianClass.addMethod(overrideMethod);
    final Class<?> aClass = saneYossarianClass.toClass(Yossarian.class.getClassLoader(), Yossarian.class.getProtectionDomain());
    return (Yossarian) aClass.newInstance();
}

and I get the following error :

Exception in thread "main" javassist.CannotCompileException: by java.lang.ClassFormatError: class SaneYossarian overrides final method isCrazy.()Z

Which makes no sense! I used JavaAssist precisely to modify the original class. I'm not looking for a solution to the challenge, just an understanding of what I did wrong in the step where I modified the class. Any help is appreciated.

UPDATE : I also tried directly modifying the method in the base class to return true,

Answer 1

Instead of

final Class<?> aClass = saneYossarianClass.toClass(Yossarian.class.getClassLoader(), Yossarian.class.getProtectionDomain());

use

 final Class<?> aClass = saneYossarianClass.toClass();

For some reasons this works for me:

public final class SomeClassHavingFinals {

    public final void sayHelloBoy() {
        System.out.println("I am saying hello original");
    }
}

And:

public static void main(String[] args)
            throws NotFoundException, CannotCompileException, InstantiationException, IllegalAccessException,
            NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, IOException {
        ClassPool pool = ClassPool.getDefault();
        CtClass clazz = pool.get("testapp.SomeClassHavingFinals");
        clazz.defrost();
        clazz.setModifiers(Modifier.PUBLIC);
        clazz.getDeclaredMethod("sayHelloBoy").setModifiers(Modifier.PUBLIC);
        clazz.toClass();

        CtClass extension = pool.makeClass("SomeExtension");
        extension.setSuperclass(clazz);
        final CtMethod overrideMethod = CtNewMethod.make("public void sayHelloBoy() { System.out.println(\"Im overriden\"); }",
                extension);
        extension.addMethod(overrideMethod);
        Object ei = extension.toClass().newInstance();
        ei.getClass().getDeclaredMethod("sayHelloBoy").invoke(ei);
    }

outputs

Im overriden

so it looks like it is working. IDK internals, but it seems that toClass() causes the file definition to be rebuilt.