code: public String get(final String key, final Function<String, String> mapper) {
          try {
              return cache.get(key, () -> mapper.apply(key));
          } catch (final ExecutionException ex) {
              LOGGER.warn("Error fetching {} from cache: {}", key, ex.getMessage());
              return null;
          }
      }
docs: Lazily get a value from the cache.
  @param key the cache key
  @param mapper the function for deriving the value
  @return the value