what is the preferred way of implementing hashCode()?

Sometimes I need to implement an obj's hashCode() method by combining the hashCodes of its several instance members. For example, if th开发者_JAVA百科e 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()?

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.

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);
}

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.

Generate it using your IDE.

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.

I believe the following is a good practice for simple scenarios: If your class contain any readonly members, they would be good candidates for generating the object's hashcode. If your class, however, contains only the mutating members, you could create a readonly int field that gets the value based on the non-null values passed to the constructor.