Compute variance via functional programming
First off, watch your formating. Java standard is to have spaces around operators, and a space after commas. In the reduce lambda you can leave out the return and the braces, just like you did't for iterate.
It's a bit ugly to use a DoubleStream for the array indices, and number streams provide a sum() method, so using reduce is unnecessary.
An array can easily streamed using Arrays.stream(), which produces an IntStream and  with mapToDouble a DoubleStream can be created:
double summation = Arrays.stream(array)
                     .mapToDouble(x -&gt; (x - mean) * (x - mean))
                     .sum();