Integer.max(a, b) will return the greater value of the given a and b. If you use that result somehow as a comparator, a positive value returned will be regarded as meaning that a > b so a will be kept.
The first two elements are 3 and 4. Both are positive. Integer.max(3, 4) = 4 > 0. So you're effectively saying that 3 > 4 with such a comparator, so 3 is kept. Then, the same goes for the rest: Integer.max(3, 6) = 6 > 0, so 3 is considered the max, etc.
