Java中空对象的哈希码必须是什么？

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;
}

Answer 2

If you will search around, you'll notice that HashMap has a special handling for null keys. null values are fine as you don't compute hash code for them in a HashMap . This is the reason why null keys work fine in HashMap . As to why Objects.hashCode works fine, take a look at Rohit's answer.

Answer 3

As the javadoc says:

Objects.hashCode(Object o)

Returns the hash code of a non-null argument and 0 for a null argument.

p2.hashCode() throws a NullPointerException because you are trying to access a method of a null object.

Answer 4

According to a comment from this post, hascode of null objects can throw NPE or a value of zero.

That is not true. (And it is not what @Bohemian's comment is saying!)

What happens in HashMap and HashSet is that they treat null as a special case. Instead of calling hashcode() on the null object (which would NPE!!), they use zero in as a hard-coded alternative hashcode.

I stress ... this is special case behaviour of HashMap and HashSet ... not hashcode() .

As your example shows, if you do attempt to call the hashcode() method on null , you will get an NPE. The JLS says that that is what will happen ... and it happens whenever you try to invoke any instance method on null .

(On the other hand, the Objects.hashCode(obj) method does deal with the case where obj is null as a special case. And that's the whole point of the static method!)

Answer 5

The hash code of null is 0 (see Objects.hash() )

    public static int hashCode(Object a[]) {
        if (a == null)
            return 0;

        int result = 1;

        for (Object element : a)
            result = 31 * result + (element == null ? 0 : element.hashCode());

        return result;
    }

Answer 6

0 is hard coded value for null in Hashmap. You can see the following actual implementations done in HashMap and Objects java classes.

from HashMap.java

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

from Object.java

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