An efficient hashcode function

Question

I have a Employee class with many attributes. One of the attributes is employeeId which is of type int.

Can I have hascode function for Employee as follows?

int hashCode(){
    return new Integer(employeeId).hashCode();
}

Is it efficient?

Answer 1

How about:

return employeeId;

Answer 2

As long as it's performing as intended, and not giving you any issues, then yes.

EDIT: Look at http://mindprod.com/jgloss/hashcode.html and http://www.javamex.com/tutorials/collections/hash_function_guidelines.shtml

Answer 3

The only requirement for hashCode() is that the values it returns are sufficiently unique and that two instances of Employee who are equal according to equals() have the same hash code. So returning employeeId is the better choice in this case.

To answer the original question, no it is not very efficient. The new object construction will give you a very small performance hit if you're doing a lot of calls, though the compiler (and implementations that use the hash code) might be smart enough to optimize some of that away. Of course you will only see this if you make very heavy use of it, which you typically do not.

Answer 4

If employeeId is really of type int, then this should work and it is considerably more efficient than your version ... which creates an Integer object for no good reason:

public int hashCode() {
    return employeeId;
}

If employeeId is an integer represented as a String, then the following might be better than your version.

public int hashCode() {
    return Integer.parseInt(employeeId);
}

All of these solutions (including yours) assume that employeeId is a unique key; i.e. that no two employees (and their corresponding Employee objects) have the same employeeId value.

Answer 5

No,it's not efficient.

An new object will be create when a client invoke this hashCode().

Your employeeId is a int type , it just returns it,does not need to new Integer Object.

Answer 6

I wouldn't think so, I know Java is supposed to have the fastest new object allocation in the west, but its an unnecessary allocation. Assuming that 2 integer objects with the same value would return the same hashcode, I don't see a reason it wouldn't, but reading the Javadoc, I don't see any language that would force them to have the same hashcode, I believe Findbugs would suggest using

return Integer.valueOf(employeeId).hashCode();

as that is supposed to be more efficient than new in some instances. Though I agree that returning the employee id would be the best solution. as Mercy and Steven C have suggested.

Answer 7

I would be a little wary of returning raw EmployeeIDs. Depending on the company, doing so could easily lead to clustering. One obvious pattern is that when things are going well, companies hire a lot of people. Then, when things aren't going so well, they lay people off, largely in reverse order of seniority. Then, when things pick back up again, they hire more people again.

This means you'll tend to have relatively densely populate runs alternating with relatively sparsely populated runs. Even though you've met the requirements for hashing to work and you've generated a hash code very quickly, performance of a hash table of these items may be adversely affected.