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Which one of the following statements about what Collections.synchronizedMap returns and what it does not solve is true?

  • A.A wrapper whose methods are synchronized; iteration still needs an external lock.
  • B.A wrapper whose methods are synchronized, including safe iteration.
  • C.A concurrent map equivalent in behaviour to ConcurrentHashMap.
  • D.A copy of the map that no other thread can reach.

Answer: A

Each individual method call becomes atomic, which is not enough for a loop built from several of them, so the javadoc requires the user to hold the wrapper's lock while iterating. A check-then-act sequence needs the same treatment.

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