code: public void swapRows(int i, int j) {
          if (i != j) {
              Vector ii = getRow(i);
              Vector jj = getRow(j);
  
              setRow(i, jj);
              setRow(j, ii);
          }
      }
docs: Swaps the specified rows of this matrix.
  
  @param i the row index
  @param j the row index