一、问题
下面是面试HashMap常见的一些问题
1、HashMap、HashTable、ConcurrentHashMap的区别是什么？
2、哪几个参数比较重要？
3、HashMap触发Resize操作后通过位运算来减少时间开销的大致流程是怎样的？
4、JDK的迭代过程中对HashMap的元素存储进行了结构上的优化，其存储方式与查询时间的复杂度如何？
由此可见，HashMap非常重要，今天着重讲讲HashMap的一些底层知识；
让我们来看看HashMap的源码；
======翻译不易，转载请注明出处======
二、总结
1、Hash Map基于实现了Map接口，提供了所有map的操作方法，并允许null的key和null的value；
2、HashMap类粗略的等同于Hashtable，但是它不是同步的（unsynchronized），并且允许nulls；该类不保证映射的顺序;尤其注意，不能保证保持固定不变的顺序；；
3、HashMap实现类为基本的操作（例如get，set方法）提供持久的表现；假设，hash功能将括号里的元素都分散了；集合中的迭代器要求一定倍数比例的HashMap类的实例(一定数量的括号)的容量能力来增加他的容量大小(key-value映射)；因此，如果看重迭代器的性能的话，初始化时设置较小的容量非常重要（否则，加载器会非常慢）；
4、一个HashMap的实例（instance）有两个参数会影响他的表现；即【初始化容量】和【加载因子】（load factor）；【容量】是hash table中“篮子”的数量；这里可以把HashMap理解为一个超市里的“存储柜”，“存储柜”里有好多个篮子（寄存东西的小格子），篮子越多，装的东西越多，每个篮子都有一个编号，这个编号，就是key；每个篮子里装的东西，就是value；你告诉这个大箱子一个key，他就会根据你提供的key去获取篮子里的值；如果没有这个编号，他就获取不到任何东西；但是，篮子越多，对系统性能要求就越高，程序运行也就越慢，越卡；【初始化容量】是当hash table 创建时就直接决定了；【加载因子】是衡量hash table被允许在装满之前自动扩容的能力；当一个hash table中的许多entries键值对 超过【加载因子】的乘积（product不光有产品的意思，还有乘积的意思），并且当前容量时；hash table会重新哈希（rehashed）（就是说，内部数据构造器会重构）；因此，hash table会获得大约2倍的容量；
5、作为一个普通的规则，默认的加载因子0.75会提供一个在时间和容量花费上很折中（tradeoff）的数值；大于0.75会减少空间上线消耗，但是会增加查找时间花费（大多数HashMap类的操作中的反射，包括get和set方法）；当设置初始容量时，预计map中一定数量的键值对和加载因子必须纳入计算，才能最小化rehash操作的次数。如果【初始化容量】比键值对数目/加载因子的最大值还要大，那么，rehash操作将永远不能发生；
6、如果一个HashMap的实例中存储太多映射，创建一个足够大的容量值得HashMap来让键值对被有效地存储，而不是让他一直自动rehash来扩展表格；就是说，能一次性装进去，就一次性装，不要老是让Hash table 一次次扩容；**PS:就跟公司组织员工出去旅游一样，能一个大车将员工全部装走最好，即使空点座位，当然，座位空太多也浪费；不要一车装不下，还要打电话叫一个小车来，更不要出现再叫一个一样的大车过来这种情况；** 注意，使用许多键值对的key是同一个hashCode,绝对会降低hash table的性能；为了改善这种影响，当key是可比较的，这个类将会使用key值排序，来帮助断裂关系；
7、注意，HashMap的实现是异步的；如果多个线程同时访问一个hashMap，并且至少一个线程改变的这个map的结构，那么，其必须同步；（一个结构上的改变可以是任何操作，例如add或delete一个或多个键值对；仅仅更改与实例已经包含的键关联的值并不是结构修改）；这主要是通过在一个对象上的自然压缩map同步完成的；
8、如果有这样一个对象存在，这个map必须使用synchronizedMap的打包（wrapper）方法；这个打包方法最好在创建的时候，来防止访问异步的map；
Map m = Collections.synchronizedMap(new HashMap(...));
9、迭代器会通过HashMap类中集合的视图方法返回是fail-fast：任何时候，当迭代器创建后，map的结构发生改变，除了通过迭代器自己的remove方法，其他情况，迭代器会抛出ConcurrentModificationException这个异常；因此，当面临并发改变，迭代器会快速失效并清除，而不是武断地冒险，防止以后在不确定的时间发生不确定的行为；
10、注意，迭代器的fail-fast行为不会保证，通俗地讲，不可能在异步并发修改出现，做出任何强制主动操作；fail-fast迭代器会在底层尽可能抛出【ConcurrentModificationException】异常；因此，它是否会错误地写程序取决于这个为了正确性而抛出的异常，迭代器的fail-fast行为必须在发现bugs时只有时使用；
三、附录
java.util.HashMap源代码；
/**
* Hash table based implementation of the Map interface. This
* implementation provides all of the optional map operations, and permits
* null values and the null key. (The HashMap
* class is roughly equivalent to Hashtable, except that it is
* unsynchronized and permits nulls.) This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
*
*
This implementation provides constant-time performance for the basic
* operations (get and put), assuming the hash function
* disperses the elements properly among the buckets. Iteration over
* collection views requires time proportional to the "capacity" of the
* HashMap instance (the number of buckets) plus its size (the number
* of key-value mappings). Thus, it's very important not to set the initial
* capacity too high (or the load factor too low) if iteration performance is
* important.
*
*
An instance of HashMap has two parameters that affect its
* performance: initial capacity and load factor. The
* capacity is the number of buckets in the hash table, and the initial
* capacity is simply the capacity at the time the hash table is created. The
* load factor is a measure of how full the hash table is allowed to
* get before its capacity is automatically increased. When the number of
* entries in the hash table exceeds the product of the load factor and the
* current capacity, the hash table is rehashed (that is, internal data
* structures are rebuilt) so that the hash table has approximately twice the
* number of buckets.
*
*
As a general rule, the default load factor (.75) offers a good
* tradeoff between time and space costs. Higher values decrease the
* space overhead but increase the lookup cost (reflected in most of
* the operations of the HashMap class, including
* get and put). The expected number of entries in
* the map and its load factor should be taken into account when
* setting its initial capacity, so as to minimize the number of
* rehash operations. If the initial capacity is greater than the
* maximum number of entries divided by the load factor, no rehash
* operations will ever occur.
*
*
If many mappings are to be stored in a HashMap
* instance, creating it with a sufficiently large capacity will allow
* the mappings to be stored more efficiently than letting it perform
* automatic rehashing as needed to grow the table. Note that using
* many keys with the same {@code hashCode()} is a sure way to slow
* down performance of any hash table. To ameliorate impact, when keys
* are {@link Comparable}, this class may use comparison order among
* keys to help break ties.
*
*
Note that this implementation is not synchronized.
* If multiple threads access a hash map concurrently, and at least one of
* the threads modifies the map structurally, it must be
* synchronized externally. (A structural modification is any operation
* that adds or deletes one or more mappings; merely changing the value
* associated with a key that an instance already contains is not a
* structural modification.) This is typically accomplished by
* synchronizing on some object that naturally encapsulates the map.
*
* If no such object exists, the map should be "wrapped" using the
* {@link Collections#synchronizedMap Collections.synchronizedMap}
* method. This is best done at creation time, to prevent accidental
* unsynchronized access to the map:
* Map m = Collections.synchronizedMap(new HashMap(...));
*
*
The iterators returned by all of this class's "collection view methods"
* are fail-fast: if the map is structurally modified at any time after
* the iterator is created, in any way except through the iterator's own
* remove method, the iterator will throw a
* {@link ConcurrentModificationException}. Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than risking
* arbitrary, non-deterministic behavior at an undetermined time in the
* future.
*
*
Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification. Fail-fast iterators
* throw ConcurrentModificationException on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: the fail-fast behavior of iterators
* should be used only to detect bugs.
*
*
This class is a member of the
* Java Collections Framework.
*
* @param
the type of keys maintained by this map * @param
the type of mapped values *
* @author Doug Lea
* @author Josh Bloch
* @author Arthur van Hoff
* @author Neal Gafter
* @see Object#hashCode()
* @see Collection
* @see Map
* @see TreeMap
* @see Hashtable
* @since 1.2
*/