Write a method named computeAverage that computes and returns the mean of all elements in an array of integers. For example, if an array named acontains [1, -2, 4, -4, 9, -6, 16, -8, 25, -10], then the call of computeAverage(a) should return 2.5.

Constraints: You may assume that the array contains at least one element. Your method should not modify the elements of the array.