[kccqzy]

It's a bit disingenuous to say java has given up and created a binary tree fallback mode. It doesn't actually switch the whole hash table into a binary tree, but rather it switches the linked list inside each bucket into a binary tree, and only when a certain threshold is passed.

[Gankro]

If you trigger this fallback (e.g. someone is colliding all your hashes), your entire collection is probably stuffed into one bucket.

Degrading only individual buckets is of course a nice middle ground that avoids harsh discontinuities in performance for slightly bad inputs.