什么时候对TreeMap和ConcurrentSkipListMap进行排序？

Question

I have a Map for which I provide a key class for which I've implemented a comparitor. The comparitor is partly time-based, so the order may change as time marches on. The question is: when is the comparison between objects done, at insertion time into the map or at iterator creation time?

Answer 1

The tree is built on insert.

Also, be aware of the following warning in the javadoc of Map :

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of a [key] object is changed in a manner that affects equals comparisons while the object is a key in the map.

So, the compare method should be stable, ie should not change because of the progression of time.

Any time a key changes, it must be removed before the change, and re-added after the change.