Is the hashCode function generated by Eclipse any good?

user_438319 0
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Eclipse source menu has a "generate hashCode / equals method" which generates functions like the one below. 

String name; 
@Override
public int hashCode()
{
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
}

@Override
public boolean equals(Object obj)
{
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    CompanyRole other = (CompanyRole) obj;
    if (name == null)
    {
        if (other.name != null)
            return false;
    } else if (!name.equals(other.name))
        return false;
    return true;
}

If I select multiple fields when generating hashCode() and equals() Eclipse uses the same pattern shown above.

I am not an expert on hash functions and I would like to know how "good" the generated hash function is? What are situations where it will break down and cause too many collisions?

hashmap hash hashtable eclipse java
8 Answers

One potential drawback is that all objects with null fields will have a hash code of 31, thus there could be many potential collisions between objects that only contain null fields. This would make for slower lookups in Maps.

This can occur when you have a Map whose key type has multiple subclasses. For example, if you had a HashMap<Object, Object>, you could have many key values whose hash code was 31. Admittedly, this won't occur that often. If you like, you could randomly change the values of the prime to something besides 31, and lessen the probability of collisions.

user_3523190
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