Why does Java's hashCode() in String use 31 as a multiplier?

Question

Per the Java documentation, the hash code for a String object is computed as:

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

I'm not sure, but I would guess they tested some sample of prime numbers and found that 31 gave the best distribution over some sample of possible Strings.