ConcurrentHashMap: avoid extra object creation with “putIfAbsent”?

Question

I am aggregating multiple values for keys in a multi-threaded environment. The keys are not known in advance. I thought I would do something like this:

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Answer 1

Waste of memory (also GC etc.) that Empty Array list creation problem is handled with Java 1.7.40. Don't worry about creating empty arraylist. Reference : http://javarevisited.blogspot.com.tr/2014/07/java-optimization-empty-arraylist-and-Hashmap-cost-less-memory-jdk-17040-update.html