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]
<         


        

Answer 1

In latest version of JDK, 31 is still used. https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/lang/String.html#hashCode()

The purpose of hash string is

• unique (Let see operator ^ in hashcode calculation document, it help unique)
• cheap cost for calculating

31 is max value can put in 8 bit (= 1 byte) register, is largest prime number can put in 1 byte register, is odd number.

Multiply 31 is <<5 then subtract itself, therefore need cheap resources.