code: public T get(String key) {
          purge();
          final Value val = map.get(key);
          final long time = System.currentTimeMillis();
          return val != null && time < val.expire ? val.token : null;
      }
docs: Get a token from the cache.
  
  @param key The key to look for.
  @return The token, if present and not yet expired, or null otherwise.