HashSet比TreeSet快得多(对于添加、删除和包含等大多数操作，HashSet是常量时间，而不是日志时间)，但不像TreeSet那样提供排序保证。

HashSet

该类为基本操作(添加、删除、包含和大小)提供恒定的时间性能。 它不能保证元素的顺序随时间保持不变 迭代性能取决于初始容量和HashSet的负载因子。 接受默认的负载因子是相当安全的，但您可能希望指定的初始容量大约是您期望该集增长的两倍。

TreeSet

保证基本操作(添加、删除和包含)的时间成本为log(n) 确保set的元素将被排序(升序，自然或由你通过它的构造函数指定)(实现SortedSet) 不为迭代性能提供任何调优参数 提供了一些方便的方法来处理有序集，如first()， last()， headSet()和tailSet()等

重要的几点:

Both guarantee duplicate-free collection of elements It is generally faster to add elements to the HashSet and then convert the collection to a TreeSet for a duplicate-free sorted traversal. None of these implementations are synchronized. That is if multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. LinkedHashSet is in some sense intermediate between HashSet and TreeSet. Implemented as a hash table with a linked list running through it, however,it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet.

因此，使用方法的选择完全取决于您的需要，但我认为，即使您需要一个有序的集合，那么您仍然应该使用HashSet来创建Set，然后将其转换为TreeSet。

例如:SortedSet<String> s = new TreeSet<String>(hashSet);

HashSet是O(1)来访问元素，所以这当然很重要。但是保持集合中对象的顺序是不可能的。

如果维护顺序(根据值而不是插入顺序)对您很重要，TreeSet是有用的。但是，正如您所注意到的，您正在以顺序换取访问元素的更慢时间:基本操作为O(log n)。

来自TreeSet的javadocs:

该实现为基本操作(添加、删除和包含)提供了log(n)的时间成本。

TreeSet是两个排序集合之一(另一个是 TreeMap)。它使用红黑树结构(但你知道)，并保证 元素会按照自然的顺序，按升序排列。可选地, 您可以使用构造函数构造TreeSet，该构造函数允许您为集合提供您的 自己制定顺序规则(而不是依赖于定义的顺序) 通过使用Comparable或Comparator)

LinkedHashSet是HashSet的有序版本 在所有元素之间维护一个双链接列表。使用这个类而不是HashSet 当你关心迭代顺序时。迭代HashSet时 顺序是不可预测的，而LinkedHashSet允许您迭代元素 按照它们被插入的顺序

明明可以吃橘子，为什么要吃苹果?

Seriously guys and gals - if your collection is large, read and written to gazillions of times, and you're paying for CPU cycles, then the choice of the collection is relevant ONLY if you NEED it to perform better. However, in most cases, this doesn't really matter - a few milliseconds here and there go unnoticed in human terms. If it really mattered that much, why aren't you writing code in assembler or C? [cue another discussion]. So the point is if you're happy using whatever collection you chose, and it solves your problem [even if it's not specifically the best type of collection for the task] knock yourself out. The software is malleable. Optimise your code where necessary. Uncle Bob says Premature Optimisation is the root of all evil. Uncle Bob says so

基于@shevchyk在地图上可爱的视觉回答，以下是我的看法:

╔══════════════╦═════════════════════╦═══════════════════╦═════════════════════╗
║   Property   ║       HashSet       ║      TreeSet      ║     LinkedHashSet   ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║              ║  no guarantee order ║ sorted according  ║                     ║
║   Order      ║ will remain constant║ to the natural    ║    insertion-order  ║
║              ║      over time      ║    ordering       ║                     ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║ Add/remove   ║        O(1)         ║     O(log(n))     ║        O(1)         ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║              ║                     ║   NavigableSet    ║                     ║
║  Interfaces  ║         Set         ║       Set         ║         Set         ║
║              ║                     ║    SortedSet      ║                     ║
╠══════════════╬═════════════════════╬═══════════════════╬═════════════════════╣
║              ║                     ║    not allowed    ║                     ║
║  Null values ║       allowed       ║ 1st element only  ║      allowed        ║
║              ║                     ║     in Java 7     ║                     ║
╠══════════════╬═════════════════════╩═══════════════════╩═════════════════════╣
║              ║   Fail-fast behavior of an iterator cannot be guaranteed      ║
║   Fail-fast  ║ impossible to make any hard guarantees in the presence of     ║
║   behavior   ║           unsynchronized concurrent modification              ║
╠══════════════╬═══════════════════════════════════════════════════════════════╣
║      Is      ║                                                               ║
║ synchronized ║              implementation is not synchronized               ║
╚══════════════╩═══════════════════════════════════════════════════════════════╝

A lot of answers have been given, based on technical considerations, especially around performance. According to me, choice between TreeSet and HashSet matters. But I would rather say the choice should be driven by conceptual considerations first. If, for the objects your need to manipulate, a natural ordering does not make sense, then do not use TreeSet. It is a sorted set, since it implements SortedSet. So it means you need to override function compareTo, which should be consistent with what returns function equals. For example if you have a set of objects of a class called Student, then I do not think a TreeSet would make sense, since there is no natural ordering between students. You can order them by their average grade, okay, but this is not a "natural ordering". Function compareTo would return 0 not only when two objects represent the same student, but also when two different students have the same grade. For the second case, equals would return false (unless you decide to make the latter return true when two different students have the same grade, which would make equals function have a misleading meaning, not to say a wrong meaning.) Please note this consistency between equals and compareTo is optional, but strongly recommended. Otherwise the contract of interface Set is broken, making your code misleading to other people, thus also possibly leading to unexpected behavior.

这个链接可能是关于这个问题的一个很好的信息来源。