why not use extends :
public TreeSet(Comparator<? extends E> comparator)
thank you !
search stackoverflow ,but not answer。
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1https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super – Eng.Fouad Mar 06 '20 at 06:50
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1the comparator needs to read elements (not *write*). Example, if you have a set of `Number`, a comparator for `Object` would work, but not a comparator for `Integer` (set could contain a `Double`) – user85421 Mar 06 '20 at 06:58