Why Are Prime Numbers Used in the hashCode Method?

Why are primes, such as 31, commonly used in the hashCode method in Java?

public int hashCode() {
    final int prime = 31;
    // other code...
}

Prime numbers are used in the hashCode method to minimize collision rates and improve the distribution of hash codes, enhancing the performance of hash-based collections like HashMap.

public int hashCode() {
    int result = 17; // Start with a non-zero constant
    result = 31 * result + (field1 != null ? field1.hashCode() : 0);
    result = 31 * result + (field2 != null ? field2.hashCode() : 0);
    return result;
}

Causes

• Prime numbers help create a more uniform distribution of hash values.
• Using primes reduces the likelihood of input values resulting in the same hash output, which minimizes collision rates.
• Mathematically, primes contribute positively to the properties of multiplication in hash functions. When combined with other integers, they provide better randomness.

Solutions

• Adopt prime numbers such as 31, 37, or 53 when creating hash codes to improve efficiency.
• Utilize built-in hash functions provided by languages or libraries that optimize these calculations.

Mistake: Using non-prime numbers for hashCode calculations leading to high collision rates.

Solution: Use a prime number to enhance the distribution of hash values.

Mistake: Using constant values in a way that does not consider field significance.

Solution: Multiply the computed hash by a prime number and add the hash values of individual fields.