| Operations | Time Complexity | Notes |
| add, remove, contains, size | O(1) | assuming the hash functions has dispersed the elements properly among the buckets |
| iteration | proportional to the number of the elememts and buckets | do not set the initial capacity too high(or the load factor too low) for the iteration-critical program |
- Implementing the java.util.Set interface, internally backed by a java.util.HashMap instance(permitting null values).
- Making no guarantees to the iteration order and the constantness of the order over time either.
- Not synchronized; must be synchronized externally, for example: Set s = Collections.synchronizedSet(new HashSet());
- Iterators returned by the iterator() method are fail-fast: it will throw a ConcurrentModificationException if the set is modified after the creation of the iterator and by any method except through the iterator‘s remove() method. Notice that the fail-fast behavior cannot be guaranteed because nothing can be guaranteed in the presence of unsynchronized concurrent modification.
- 实现了java.util.Set接口，内部由java.util.Map示例实现（允许null值）
- 不保证迭代顺序，也不保证迭代顺序一直不变
- 非同步；必须从外部同步，比如：Set s = Collections.synchronizedSet(new HashSet());
- iterator()返回的iterator是fail-fast的：如果这个iterator创建以后，这个set被修改了（不是这个iterator的remove()方法修改的），那么会抛出ConcurrentModificationException异常。（注意，这个fail-fast也是无法保证的，因为在非同步的并发修改中，没有什么是可以保证的）
