code: @Override
      public List<String> getList(final String key) {
          return retrieve(new Callable<List<String>>() {
              @Override
              public List<String> call() throws Exception {
                  try {
                      return configuration.getStringList(key);
                  } catch (ConfigException.WrongType e) {
                      // Not a list.
                      String s = get(key);
                      if (s != null) {
                          return ImmutableList.of(s);
                      } else {
                          throw new IllegalArgumentException("Cannot create a list for the key '" + key + "'", e);
                      }
                  }
              }
          }, Collections.<String>emptyList());
      }
docs: Retrieves the values as a list of String, the format is: key=[myval1,myval2].
  
  @param key the key the key used in the configuration file.
  @return an list containing the values of that key or empty if not found.