code: public final SgClass get(final String className) {
          if (className == null) {
              throw new IllegalArgumentException("The argument 'className' cannot be null!");
          }
          return cache.get(className);
      }
docs: Returns a class from the internal cache.
  
  @param className
  Class to find - Cannot be null.
  
  @return Class or null if it's not found.