code: public void setWarningThreshold(double warningThreshold)
      {
          if(warningThreshold <= 0 || Double.isNaN(warningThreshold) || Double.isInfinite(warningThreshold))
              throw new IllegalArgumentException("warning threshold must be positive, not " + warningThreshold);
          this.warningThreshold = warningThreshold;
      }
docs: Sets the multiplier on the standard deviation that must be exceeded to
  initiate a warning state. Once in the warning state, DDM will begin to
  collect a history of the inputs <br>
  Increasing the warning threshold makes it take longer to start detecting
  a change, but reduces false positives. <br>
  If the warning threshold is set above the
  {@link #setDriftThreshold(double) }, the drift state will not occur until
  the warning state is reached, and the warning state will be skipped.
  @param warningThreshold the positive multiplier threshold for starting a
  warning state