code: private FieldPosition[] getNegativePrefixFieldPositions() {
          if (negativePrefixFieldPositions == null) {
              if (negPrefixPattern != null) {
                  negativePrefixFieldPositions = expandAffix(negPrefixPattern);
              } else {
                  negativePrefixFieldPositions = EmptyFieldPositionArray;
              }
          }
          return negativePrefixFieldPositions;
      }
docs: Returns the FieldPositions of the fields in the prefix used for
  negative numbers. This is not used if the user has explicitly set
  a negative prefix via <code>setNegativePrefix</code>. This is
  lazily created.
  
  @return FieldPositions in positive prefix