Dot product with arrays

Dot product with arrays

In class we had to write a small code using Dot Product to find the sum of
two arrays(array a and array b). I have written my code however when I run
it it does not it does not give me the answer. My professor said my loop
was wrong however I do not think it is. Is the part that says i<a.length
not allowed in a for loop parameter?
Here is my code:
> public class arrayExample {
> public static void main (String [] args) {
>
> int[] a = {1,2,2,1};
> int[] b = {1,2,2,1};
> int n = a.length;
>
> int sum = 0;
> for (int i = 0; i < a.length; i++) {
> sum += a[n] * b[n]; }
> System.out.println(sum);
> } }