code: private boolean inSamePackage(Class<?> c1, Class<?> c2) {
          String nameC1 = c1.getName();
          String nameC2 = c2.getName();
          int indexDotC1 = nameC1.lastIndexOf('.');
          int indexDotC2 = nameC2.lastIndexOf('.');
          if (indexDotC1 != indexDotC2) {
              return false; // cannot be in the same package if indices are not the same
          }
          if (indexDotC1 == -1) {
              return true; // both of them are in default package
          }
          return nameC1.regionMatches(0, nameC2, 0, indexDotC1);
      }
docs: Checks if two classes belong to the same package.
  
  @param c1
  one of the classes to test.
  @param c2
  the other class to test.
  @return {@code true} if the two classes belong to the same package,
  {@code false} otherwise.