Why Are Prime Numbers Used in HashCode Implementations?

Utilizing Prime Numbers in HashCode Implementation

A HashCode is a compact mathematical representation of an object designed to efficiently identify it. To ensure optimal distribution among hash buckets, prime numbers are strategically employed in the hashCode() method.

The Rationale for Prime Numbers

Prime numbers, devoid of any factors except one and themselves, lend themselves well to data distribution. They minimize the possibility of hash collisions, where two distinct objects yield the same hash code. This issue arises when common patterns exist in the data input, such as memory alignment.

For instance, in the case of 32-bit integers aligned to addresses divisible by 4, using a prime number modulus (e.g., 7) yields a more uniform distribution than a non-prime modulus (e.g., 8):

Input	Modulo 8	Modulo 7
0	0	0
4	4	4
8	0	1
12	4	5
16	0	2
20	4	6
24	0	3
28	4	0

Conclusion

While the use of prime numbers is a common strategy to optimize data distribution in hash tables, it is essential to consider the expected input patterns to determine the most effective modulus choice.

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