what must be hashcode of null objects in Java?

Question

According to a comment from this post, hascode of null objects can throw NPE or a value of zero. This is implementation specific. but within the same implementation, why does Objects.hashcode and hascode(instance) return different values. for ex:

public class EqualsTesting {

    public static void main(String[] args){
        String p1 =null;
        String p2 = null;
        System.out.println(Objects.hashCode(p1));
        System.out.println(p2.hashCode());

    }
}

Output:

0
Exception in thread "main" java.lang.NullPointerException
    at BinaryTrees.EqualsTesting.main(EqualsTesting.java:14)

If this is the case, will this not affect the key look-up in HashMap where null Key-value pairs are allowed. (It might either hash to bucket 0 or throw a NPE)

Answer 1

How would you calculate hashCode of an object that doesn't even exists? When p2 is null, invoking any method on it will throw a NPE. That isn't giving you any particular value of a hashCode.

Objects.hashCode() is just a wrapper method, which performs a pre-check for null values, and for reference that is not null, it returns the same value as p2.hashCode() as in this case. Here's the source code of the method:

public static int hashCode(Object o) {
    return o != null ? o.hashCode() : 0;
}