The contract of equals with regards to null, is as follows:

For any non-null reference value x, x.equals(null) should return false.

This is rather peculiar, because if o1 != null and o2 == null, then we have:

o1.equals(o2) // returns false
o2.equals(o1) // throws NullPointerException

The fact that o2.equals(o1) throws NullPointerException is a good thing, because it alerts us of programmer error. And yet, that error would not be catched if for various reasons we just switched it around to o1.equals(o2), which would just "silently fail" instead.

So the questions are:

• Why is it a good idea that o1.equals(o2) should return false instead of throwing NullPointerException?
• Would it be a bad idea if wherever possible we rewrite the contract so that anyObject.equals(null) always throw NullPointerException instead?