When to use System.identityhashcode() and hashcode() method?

Question

When to use System.identityhashcode() and hashcode() method?*

Answer 1

According to the javadoc, the System.identityHashCode(Object o):

Returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode(). The hash code for the null reference is zero.

So, at the first place, System.identityHashCode(nullReference) will always give you 0, instead of nullReference.hashCode() which will obviously give you a NullPointerException at Runtime.

Let's, however, consider the following class:

public class MysteriousOne {
    @Override
    public int hashCode() {
        return 0xAAAABBBB;
    }

    //override equals() and so on...
}

The class overrides hashCode(), which is perfectly fine, even though the hash code for every instance would be the same, which however is not fine, if you want to distinguish identities of several instances. Usually, you'd try the output of the .toString() method (which by default gives the classname, followed by a @ followed by the hashCode() output), for example, to find out the real identity of an object, but is in this case the output would be the same:

MysteriousOne first = new MysteriousOne();
MysteriousOne second = new MysteriousOne();
System.out.println("First: " + first);
System.out.println("Second: " + second);

The output would be:

First: MysteriousOne@aaaabbbb
Second: MysteriousOne@aaaabbbb

So, having such implementation of hashCode() is impossible to distinguish between identities of several instances. Which is where System.identityHashCode() is being handy.

If you do

System.out.println("First: " + System.identityHashCode(first));
System.out.println("Second: " + System.identityHashCode(second));

you'd get two different numbers for the different instances, even though the hashCode() of their class implementation returns a constant (actually here the overridden implementation of hashCode() will not be called at all, as per javadoc):

First: 366712642
Second: 1829164700

Further, you can even pass primitives to System.identityHashCode(Object o), as they will be boxed to their corresponding wrappers:

int i = 5;
System.out.println(System.identityHashCode(i));

More info:

• How do Object#hashCode() and System#identityHashCode() work at the back end?