code: protected void rolling() {
          synchronized (this) {
              if (unflushed != null) {
                  buckets.put(currBucketTime, unflushed);
                  unflushed = null;
              }
  
              currBucketTime = getCurrBucketTime();
  
              return;
          }
      }
docs: /*
  Move the "current bucket time" index and clean the expired buckets