code: private void sweepExpiredNonces() {
  
          // Do not sweep until enough time has elapsed since the last sweep
          long currentTime = System.currentTimeMillis();
          if (currentTime - lastSweep < SWEEP_INTERVAL)
              return;
  
          // Record time of sweep
          lastSweep = currentTime;
  
          // For each stored nonce
          Iterator<Map.Entry<String, Long>> entries = nonces.entrySet().iterator();
          while (entries.hasNext()) {
  
              // Remove all entries which have expired
              Map.Entry<String, Long> current = entries.next();
              if (current.getValue() <= System.currentTimeMillis())
                  entries.remove();
  
          }
  
      }
docs: Iterates through the entire Map of generated nonces, removing any nonce
  that has exceeded its expiration timestamp. If insufficient time has
  elapsed since the last sweep, as dictated by SWEEP_INTERVAL, this
  function has no effect.