Choosing HashSet vs TreeSet

I'd start with HashSet as the default because it's faster. HashSet gives you constant time operations: add, remove, and contains are all order one. That's hard to beat. You only switch to TreeSet when you actually need its ordering features. So when would you need TreeSet? If you're building something like a leaderboard where you display players sorted by score, TreeSet keeps them sorted automatically, and you can iterate in order. Or if you need range queries, like finding all orders between two timestamps, TreeSet has methods like subSet that make that efficient. TreeSet also gives you quick access to the smallest and largest elements with first and last. But if you're just tracking unique values, like collecting unique user ids or removing duplicates, HashSet is the better choice. A common mistake is using TreeSet because sorted output looks nice, even when the business logic never needs the sorting. That's paying a cost for nothing. The performance gap matters at scale. With a million elements, order one is instant, but order log n is about twenty comparisons per operation. So my rule is simple: use HashSet unless you have a real need for sorted order or range operations. Don't over-engineer it.