Best implementation for hashCode method for a collection

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The best implementation? That is a hard question because it depends on the usage pattern.

A for nearly all cases reasonable good implementation was proposed in Josh Bloch‘s Effective Java in Item 8 (second edition). The best thing is to look it up there because the author explains there why the approach is good.

A short version

  1. Create a int result and assign a non-zero value.

  2. For every field f tested in the equals() method, calculate a hash code c by:

    • If the field f is a boolean:
      calculate (f ? 0 : 1);
    • If the field f is a byte, char, short or int: calculate (int)f;
    • If the field f is a long: calculate (int)(f ^ (f >>> 32));
    • If the field f is a float: calculate Float.floatToIntBits(f);
    • If the field f is a double: calculate Double.doubleToLongBits(f) and handle the return value like every long value;
    • If the field f is an object: Use the result of the hashCode() method or 0 if f == null;
    • If the field f is an array: see every field as separate element and calculate the hash value in a recursive fashion and combine the values as described next.

  3. Combine the hash value c with result:

    result = 37 * result + c

  4. Return result

This should result in a proper distribution of hash values for most use situations.

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