code: public ClassInfoList getClassesImplementing(final String interfaceName) {
          if (closed.get()) {
              throw new IllegalArgumentException("Cannot use a ScanResult after it has been closed");
          }
          if (!scanSpec.enableClassInfo) {
              throw new IllegalArgumentException("Please call ClassGraph#enableClassInfo() before #scan()");
          }
          final ClassInfo classInfo = classNameToClassInfo.get(interfaceName);
          return classInfo == null ? ClassInfoList.EMPTY_LIST : classInfo.getClassesImplementing();
      }
docs: Get all classes that implement (or have superclasses that implement) the named interface (or one of its
  subinterfaces).
  
  @param interfaceName
  The interface name.
  @return A list of all classes that implement the named interface, or the empty list if none.