Class not found exception using reflection on class saved in device on Android

I'm playing with reflection on Android. I've saved JAR file on device and I'm trying to get class from this JAR but it isn't working and I'm getting exception:

java.lang.ClassNotFoundException: Main.class

My code:

File classFile = new File(getExternalFilesDir(null) + File.separator);
URLClassLoader classLoader = new URLClassLoader(new URL[] { classFile.toURI().toURL() });
Class classToInvestigate = classLoader.loadClass("Main.class");

Also I tried saved there Main.class file, but I'm still getting errors. Is there something I'm missing? Thank you.