Question

I recently have to override the equals and hashCode methods in Java. I hence looked for a fast and efficient way to compute hash codes.

Java developers seem to aggree on the following method :

    int hash = 23;
    hash = hash * 37 + paramOne;
    hash = hash * 37 + paramTwo;
    // And so on...

It might be simple arithmetic but I don’t really get it. What are the garantees ? What are the corner cases ? Is there a better (rather simple) way to do it ?

Thank you !

Answer 1

In words of Joshua Bloch (explaining the default implementation of hashCode() method in String class , that is : s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]):

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.

For further reading , refer this and this.