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());