请求帮助java.lang.NullPointerException

我建了一个nClass*（num+1）行 7列的表格，
当col =0的时候，运行成功，当col＝1或者其它数的时候，就出现java.lang.NullPointerException错误          int row=1;
           int col=0; //此处若不等于1就会出问题
           int i=0;
            for(row=1;row<num+1;row++){
                int p=0;
                for(i=row;i<(nClass-1)*(num+1);i+=(num+1)){
                    if(jTable1.getValueAt(i, col).equals(""))break; //有NullPointerException问题
                    p++;
                    for(int j=1;j<nClass-p+1;j++){
                       if(jTable1.getValueAt(i, col).equals(jTable1.getValueAt(i+(num+1)*j, col))){
                                                         
                       }/// if
                    }//for j
                }//for i
            }//for row