Mastering Java: The Ultimate Quiz for 'Thinking in Java'

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Which concurrent collection class does not permit null elements?

  1. ConcurrentHashMap

  2. CopyOnWriteArrayList

  3. DelayQueue

  4. All of the above

The correct answer is: DelayQueue

A ConcurrentHashMap does not allow null keys, but it can store null values. B: CopyOnWriteArrayList does not allow null elements to be added, but it can store null elements if they are already present. D: This option is incorrect because while ConcurrentHashMap and CopyOnWriteArrayList may have some restrictions on null elements, the question is specifically about a collection class that does not permit ANY null elements, making DelayQueue the only correct answer.

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