code: public double get(int i, int j) {
          if (i < 0 || i >= size() || j < 0 || j >= ncols()) {
              throw new IllegalArgumentException("Invalid index: i = " + i + " j = " + j);
          }
  
          for (SparseArray.Entry e : get(i).x) {
              if (e.i == j) {
                  return e.x;
              }
          }
  
          return 0.0;
      }
docs: Returns the value at entry (i, j).
  @param i the row index.
  @param j the column index.