code: public long nClass() {
      if( _nclass!=-1 ) return _nclass;
      if(_isClassification==true){
        long cnt=0;
        cnt = _fr.vec(_response).domain().length;
        /*for(int i=0;i<_cols.length;++i) {
        if(_cols[i]._response==true) cnt = _cols[i]._v.domain().length;
      }*/
        return(_nclass=cnt);
      }else{
        return(_nclass=0);
      }
    }
docs: number of classes if classification problem