AbstractClass - more flexible grading

[jy95]

Hello @AlexandreDubray,

If you want that exercise AbstractClass (and others in the future with this kind) to be more "tolerant" to students errors, here are a few tips on how to do it with an example :

1. Replace all .class invocations by Class.forName() (that is what causes students to see message "You modified ..." )
2. Add multiple Exception handlers (mainly ClassNotFoundException and NoSuchMethodError )

Example :

try {
   String classNameWithPackage = "templates.Shape";
   Class shapeClass = Class.forName(classNameWithPackage);
   Method my_method = shapeClass.getDeclaredMethod("getArea", double.class);
   // ... Some piece of code later ...
   throw new CustomGradingResult(TestStatus.SUCCESS,1, "You defined a class Shape with a method called getArea")
} catch(ClassNotFoundException err) {
   throw new CustomGradingResult(TestStatus.FAILED, 0, "Shape class not found");
} catch(NoSuchMethodError err) {
   throw new CustomGradingResult(TestStatus.FAILED, 0, "Shape method getArea cannot be found");
} catch(Exception err) {
   if (err instanceof CustomGradingResult) {
       throw err;
   }
   throw new CustomGradingResult(TestStatus.FAILED, 0, "Unexpected error");
}

PS: As I'm busy with work, I can't make any PR for a long time.