code: public void swapColumns(int i, int j) {
          if (i != j) {
              Vector ii = getColumn(i);
              Vector jj = getColumn(j);
  
              setColumn(i, jj);
              setColumn(j, ii);
          }
      }
docs: Swaps the specified columns of this matrix.
  
  @param i the column index
  @param j the column index