Does hashCode() have to return a prime number?

Question

The signature of the hashCode() method is

public int hashCode(){
    return x;
}

in this case x must be an int(primitive) but plz can anyone explain it to me that the number which the hashCode() returns must be a prime number, even number...etc or there is no specification ? the reason behind i am asking this question is i have seen it in different ids the auto generated code always returns a prime number, so i need to know why?

thanks in advance

Answer 1

Actually there is a pretty well-specified contract concerning hashCode():

Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable. The general contract of hashCode is:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
• It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)

—Documentation for the java.lang.Object class

What you return there is up to you, if you want to, you can just emit 0 for every object instance. It might be a bad idea but perfectly valid. It's by no means always a prime number. As stated in the documentation the default implementation just returns the object's internal address—this is consistent with the default implementation of equals checking for reference equality.

Often you can construct the hash from invoking the hashCode method on the fields that make up your object's state, such as for example:

class Person {
    private String firstName;
    private String lastName;

    @Override
    public int hashCode() {
        return firstName.hashCode() + lastName.hashCode();
    }
}

Keep in mind to update both hashCode and equals, though, whenever what you consider the representation of the object's state changes.

Answer 2

It's about efficiency.

Prime number is NOT a must to hashCode, even hashCode can return a constant int. But for efficiency, prime number leads good performance, while constant hashCode leads the worst performance (hash table deteriorates be a list).