Why use 31 as multiplier in Java String hashCode() method

In Java, the hash code for a String object is computed as

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

using int arithmetic, where s[i] is the ith character of the string, n is the length 

of the string, and ^ indicates exponentiation.

According to Josua Bloch's Book Effective Java he has explianed given below

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. 

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.