code: public void setMaximumGap(Integer maximumGap) {
          if (maximumGap != null && maximumGap.compareTo(ZERO) < 0) {
              throw new IllegalArgumentException(
                      "maximumGap must be null or >= 0");
          }
          this.maximumGap = maximumGap;
          initRelation();
      }
docs: Sets the the gap function's maximum gap value. A value of
  <code>null</code> means any gap.
  
  @param maximumGap the maximum gap {@link Integer}. Must be non-negative.
  If set to zero, the gap function always will return <code>false</code>.