code: public double getMeanValue() {
          Lock lock = this.lock.readLock();
          lock.lock();
          try {
              return this.meanValue;
          } finally {
              lock.unlock();
          }
      }
docs: Return the mean (average) value.
  
  @return the mean (average), or 0.0 if the {@link #getCount() count} is 0
  @see #getMean()