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Which one of the following statements about what happens when a null key is used with a TreeMap that has no comparator is true?

  • A.NullPointerException.
  • B.The null key is accepted and sorted before every other key.
  • C.The null key is accepted and sorted after every other key.
  • D.The map silently ignores the put and leaves its contents unchanged.

Answer: A

A sorted map must order every key, and with no comparator supplied it calls compareTo on the key itself, which fails immediately on null. So unlike HashMap, a TreeMap has no room for a null key at all, though null values are fine.

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