Why there is no ConcurrentLinkedHashMap class in jdk?
This question follows directly from my previous question here in SO . I think the answer to my second question is no . So I would like understand why there is no ConcurrentLinkedHashMap in java.util.concurrent package ? I mean there is a ConcurrentHashMap but no ConcurrentLinkedHashMap . Does it not make any sense at all to have such a class in Concurrent environments ? I mean what is the main technical reason here for its non availabalility ? Is there something similar in Guava/ Apache Commons ?
Answer
Why there is no ConcurrentLinkedHashMap class in jdk?
You would need to ask the Oracle Java guys that, but I imagine that it is a combination of:
- a perception that not many people would need it, and
- the inherent difficulties in implementing data structures with good performance properties in highly concurrent use cases.
In this case, it seems to me that implementing the collection class so that iterating the key/value/entry sets is not a concurrency bottleneck would be ... um ... difficult. (And even if people have figured a way to do it, the fact remains that designing and implementing and proving the correctness of general purpose highly concurrent data structures and algorithms is hard.)