What is a best practice of writing hash function in java?

Question

I'm wondering what is the best practice for writing #hashCode() method in java. Good description can be found here. Is it that good?

Answer 1

Here's a quote from Effective Java 2nd Edition, Item 9: "Always override hashCode when you override equals":

While the recipe in this item yields reasonably good hash functions, it does not yield state-of-the-art hash functions, nor do Java platform libraries provide such hash functions as of release 1.6. Writing such hash functions is a research topic, best left to mathematicians and computer scientists. [... Nonetheless,] the techniques described in this item should be adequate for most applications.

Josh Bloch's recipe

• Store some constant nonzero value, say 17, in an int variable called result
• Compute an int hashcode c for each field f that defines equals:
  • If the field is a boolean, compute (f ? 1 : 0)
  • If the field is a byte, char, short, int, compute (int) f
  • If the field is a long, compute (int) (f ^ (f >>> 32))
  • If the field is a float, compute Float.floatToIntBits(f)
  • If the field is a double, compute Double.doubleToLongBits(f), then hash the resulting long as in above
  • If the field is an object reference and this class's equals method compares the field by recursively invoking equals, recursively invoke hashCode on the field. If the value of the field is null, return 0
  • If the field is an array, treat it as if each element is a separate field. If every element in an array field is significant, you can use one of the Arrays.hashCode methods added in release 1.5
• Combine the hashcode c into result as follows: result = 31 * result + c;

Now, of course that recipe is rather complicated, but luckily, you don't have to reimplement it every time, thanks to java.util.Arrays.hashCode(Object[]).

@Override public int hashCode() {
    return Arrays.hashCode(new Object[] {
           myInt,    //auto-boxed
           myDouble, //auto-boxed
           myString,
    });
}

As of Java 7 there is a convenient varargs variant in java.util.Objects.hash(Object...).

Answer 2

A great reference for an implementation of hashCode() is described in the book Effective Java. After you understand the theory behind generating a good hash function, you may check HashCodeBuilder from Apache commons lang, which implements what's described in the book. From the docs:

This class enables a good hashCode method to be built for any class. It follows the rules laid out in the book Effective Java by Joshua Bloch. Writing a good hashCode method is actually quite difficult. This class aims to simplify the process.

Answer 3

It's good, as @leonbloy says, to understand it well. Even then, however, one "best" practice is to simply let your IDE write the function for you. It won't be optimal under some circumstances - and in some very rare circumstances it won't even be good - but for most situations, it's easy, repeatable, error-free, and as good (as a hash code) as it needs to be. Sure, read the docs and understand it well - but don't complicate it unnecessarily.