The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed, information would be lost, as multiplication by 2 is equivalent to shifting. The advantage of using a prime is less clear, but it is traditional. A nice property of 31 is that the multiplication can be replaced by a shift and a subtraction for better performance: 31 * i == (i << 5) - i. Modern VMs do this sort of optimization automatically.
(from Chapter 3, Item 9: Always override hashcode when you override equals, page 48, Joshua Bloch’s Effective Java)
Talat Uyarer 27 Temmuz 2016