TreeSet的一个尚未被提及的优点是它有更大的“局部性”，这是以下说法的简写:(1)如果两个条目在顺序上是相邻的，TreeSet将它们放在数据结构中彼此相邻的地方，因此在内存中也是如此;并且(2)这种布局利用了局部性原则，该原则说类似的数据通常被一个应用程序以相似的频率访问。

这与HashSet相反，HashSet将条目分布在内存中，而不管它们的键是什么。

当从硬盘读取的延迟成本是从缓存或RAM读取的延迟成本的数千倍，并且当数据确实是通过局部性访问时，TreeSet可能是更好的选择。

当然，HashSet实现要快得多——开销更少，因为没有排序。http://java.sun.com/docs/books/tutorial/collections/implementations/set.html提供了Java中各种Set实现的很好的分析。

这里的讨论还指出了一种有趣的“中间地带”方法来解决树与哈希的问题。Java提供了一个LinkedHashSet，它是一个HashSet，其中运行着一个“面向插入”的链表，也就是说，链表中的最后一个元素也是最近插入到哈希中的元素。这允许您避免无序散列的无序性，而不会增加TreeSet的成本。

消息编辑(完全重写)当顺序无关紧要时，就是这样。两者都应该给出Log(n) -看看其中一个是否比另一个快5%以上是有用的。HashSet可以在循环中给出O(1)测试，应该可以揭示它是否正确。

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;

public class HashTreeSetCompare {

    //It is generally faster to add elements to the HashSet and then
    //convert the collection to a TreeSet for a duplicate-free sorted
    //Traversal.

    //really? 
    O(Hash + tree set) > O(tree set) ??
    Really???? Why?



    public static void main(String args[]) {

        int size = 80000;
        useHashThenTreeSet(size);
        useTreeSetOnly(size);

    }

    private static void useTreeSetOnly(int size) {

        System.out.println("useTreeSetOnly: ");
        long start = System.currentTimeMillis();
        Set<String> sortedSet = new TreeSet<String>();

        for (int i = 0; i < size; i++) {
            sortedSet.add(i + "");
        }

        //System.out.println(sortedSet);
        long end = System.currentTimeMillis();

        System.out.println("useTreeSetOnly: " + (end - start));
    }

    private static void useHashThenTreeSet(int size) {

        System.out.println("useHashThenTreeSet: ");
        long start = System.currentTimeMillis();
        Set<String> set = new HashSet<String>();

        for (int i = 0; i < size; i++) {
            set.add(i + "");
        }

        Set<String> sortedSet = new TreeSet<String>(set);
        //System.out.println(sortedSet);
        long end = System.currentTimeMillis();

        System.out.println("useHashThenTreeSet: " + (end - start));
    }
}

TreeSet的一个尚未被提及的优点是它有更大的“局部性”，这是以下说法的简写:(1)如果两个条目在顺序上是相邻的，TreeSet将它们放在数据结构中彼此相邻的地方，因此在内存中也是如此;并且(2)这种布局利用了局部性原则，该原则说类似的数据通常被一个应用程序以相似的频率访问。

这与HashSet相反，HashSet将条目分布在内存中，而不管它们的键是什么。

当从硬盘读取的延迟成本是从缓存或RAM读取的延迟成本的数千倍，并且当数据确实是通过局部性访问时，TreeSet可能是更好的选择。

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。