code: public VectorIterator iteratorOfColumn(int j) {
           final int jj = j;
           return new VectorIterator(rows) {
               private int i = -1;
               @Override
               public int index() {
                   return i;
               }
  
               @Override
               public double get() {
                   return Matrix.this.get(i, jj);
               }
  
               @Override
               public void set(double value) {
                   Matrix.this.set(i, jj, value);
               }
  
               @Override
               public boolean hasNext() {
                   return i + 1 < rows;
               }
  
               @Override
               public Double next() {
                   if(!hasNext()) {
                       throw new NoSuchElementException();
                   }
                   i++;
                   return get();
               }
           };
       }
docs: Returns a vector iterator of the given column {code j}.
  
  @return a vector iterator