Default hashCode for null object

Question

In Java, Objects.hash(null) return 0

but

Map<Integer, Integer> map = null;
Objects.hash(map)

will return 31

null is not an object with a .hashCode() method. (And on a programming note: what would hashing null even achieve?) — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Apr 6, 2020 at 18:00

Michael · Accepted Answer · 2020-04-06 18:11:27Z

2

It is related to how varargs are interpretted. Under the hood, varargs parameters are implemented by creating an array.

In the Objects.hash(null) case, you are passing obviously literally passing null. There is no array. When you do Objects.hash(map), this is converted to an array of length 1, with null as the first and only element.

Because of the way that hash code is calculated, null and an array with 1 null element get different hash codes.

answered Apr 6, 2020 at 18:11

Michael

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