A cautionary tale: not all TreeSets are equal, but some are more equal than others.pic.twitter.com/mMC4bVJFwe
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What's going on here? Did you look under the hood?
I didn't need to look... what it's doing is as reasonable as what I'm doing is unreasonable: the Ordering I created considers there to be no difference in order between two Foos with the same id. The logic inside TreeSet uses that to equality-of-order to infer equality.
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In terms of efficiency, that's a reasonable thing to do because the Ordering and an equals method may both be expensive and should probably do the same thing.
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But in my real-world use-case I was exploiting this as a convenient way of guaranteeing I never had two elements with the same ID in the same TreeSet (and giving the TreeSet's append method "replace semantics" for free).
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Yeah, but it seems to me to make sense to compare the whole value in each position, not just that which determines the order
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For most other collections, it would. But people could justifiably complain about TreeSets doing twice as much work as necessary for the common cases, then.
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