Discussion
Consider what Map.putIfAbsent does when the key is already mapped to a non-null value. Which statement is correct?
Answer: B
Correct answer: It leaves the existing value in place and returns it.
putIfAbsent inserts only when the key is absent or mapped to null; otherwise it makes no change and returns the value already there. That return value is the point of the method - the caller learns whether it won the race and, if not, what the existing value is. put is the method that overwrites and returns the previous value.
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