what is the preferred way of implementing hashCode()?

Question

Sometimes I need to implement an obj's hashCode() method by combining the hashCodes of its several instance members. For example, if the combinational obj has members a, b, and c, I often see ppl implement it as

int hashCode(){
   return 31 * 31 * a.hashCode() + 31 * b.hashCode() + c.hashCode();
}

Where does this magic number 31 come from? Is it the length of 4-bytes or just a prime number?

Is there any other preferred/standard way of implementing hashCode()?

Answer 1

See Effective Java's recipe. It's just the best source, hands down.

The use of a prime number is just to try to get a reasonably good distribution without knowing the domain. It will take a while to overflow to the same value. The value 31 is pretty arbitrary if I recall correctly.

According to Bloch (he uses 17 as an initial value and 37 as the constant multiplier):

A nonzero initial value is used (...) so the hash value will be affected by initial fields whose hash value (...) is zero. If zero was used as the initial value (...) the overall hash value would be unaffected by any such initial fields, which could increase collisions. The value 17 is arbitrary.
...
The multiplier 37 was chosen because it is an odd prime. If it was even and the multiplication overflowed, information would be lost because multiplication by two is equivalent to shifting. The advantages of using a prime number are less clear, but it is traditional to use primes for this purpose.

Answer 2

Use HashCodeBuilder from Commons Lang:

public int hashCode() {
    return HashCodeBuilder.reflectionHashCode(this);
}

See the API for ways to do it without using reflection. You can tell it which fields to include, or which to ignore.

See also EqualsBuilder, for overriding an equals method.

Answer 3

One good option is Guava's Objects.hashCode method. It takes any number of arguments and creates a hashcode based on them:

@Override public int hashCode() {
  return Objects.hashCode(a, b, c);
}

Answer 4

basically, your hash code should consist of the key parameter of your POJO. One Example is below.

public int hashCode() {
    int hash = 0;
    if (getRollId() != null) {
        hash += getRollId().hashCode();
    }
    if (getName() != null) {
        hash += getName().hashCode();
    }
    return hash == 0 ? System.identityHashCode(this) : hash;
}

In the above example, the roll id and name are the key parameter of that POJO.

It is a good practice if you add only those parameter into the hashCode method that you add in Eqauls method of the same POJO.

Answer 5

Generate it using your IDE.