Comparable and Comparator contract with regards to null

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Comparable doesn’t allow null simply because:

a.compareTo(b) == -b.compareTo(a)

for all objects a and b where !a.equals(b). More specifically:

a.equals(b) ? b.equals(a) && a.compareTo(b) == 0 &&
                  b.compareTo(a) == 0 && a.hashCode() == b.hashCode()
            : !b.equals(a) && a.compareTo(b) != 0 &&
                  a.compareTo(b) == -b.compareTo(a)

must evaluate to true to satisfy the relevant contracts.

So null isn’t allowed because you can’t do:

null.compareTo(a)

Comparator is more flexible so handling of null is an implementation-specific issue. Support it or not depending on what you want your Comparator to do.

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