code: public static synchronized Class<?> getClass(String name, Configuration conf
                                              ) throws IOException {
      Class<?> writableClass = NAME_TO_CLASS.get(name);
      if (writableClass != null)
        return writableClass.asSubclass(Writable.class);
      try {
        return conf.getClassByName(name);
      } catch (ClassNotFoundException e) {
        IOException newE = new IOException("WritableName can't load class: " + name);
        newE.initCause(e);
        throw newE;
      }
    }
docs: Return the class for a name.  Default is {@link Class#forName(String)}.