jdk1.8 HashMap简介翻译

翻译的不好,还请见谅..


* Hash table based implementation of the <tt>Map</tt> interface. This

* implementation provides all of the optional map operations, and permits

* <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>

* class is roughly equivalent to <tt>Hashtable</tt>, 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.

*

* HashMap基于HashTable,实现了Map接口.该实现提供了所有的map操作,并且允许空的值或键.

* (HashMap大致上等同于HashTable,但HashMap是非同步的,并且允许空键值) HashMap不保证

* 映射的顺序 ,尤其不保证顺序会随着时间的推移而不改变.

*

* <p>This implementation provides constant-time performance for the basic

* operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function

* disperses the elements properly among the buckets. Iteration over

* collection views requires time proportional to the "capacity" of the

* <tt>HashMap</tt> 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.

*

* 如果hash操作将所有元素放在hash桶中合适的地方,那么该实现则可以提供常数时间内执行get或put的操作.

* 如果想迭代整个集合视图,花费的时间与(HashMap实例的容量(桶的数量)加上它的大小(键值对的数量))成正比.

* 因此,如果注重迭代性能,切记不要将初始容量设置得太高(或将加载因子设置得太低).

*

*

* <p>An instance of <tt>HashMap</tt> has two parameters that affect its

* performance: <i>initial capacity</i> and <i>load factor</i>. The

* <i>capacity</i> 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

* <i>load factor</i> 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 <i>rehashed</i> (that is, internal data

* structures are rebuilt) so that the hash table has approximately twice the

* number of buckets.

*

* 一个HashMap实例通常有两个参数影响它的性能:1.初始容量;2.加载因子.

* 容量:Hashtable中桶的数量,初始容量即哈希表创建时的容量.

* 加载因子:当hashTable中的数据数量达到什么程度时,才允许自动扩容.

* 当hashtable中的条目数量超过加载因子和当前容量时,hashTable会rehashed(重构内部数据结构),

* 重哈希后,hashTable中桶的数量大约是之前的两倍.

*

*

* <p>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 <tt>HashMap</tt> class, including

* <tt>get</tt> and <tt>put</tt>). 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.

*

* 一般来说,加载因子的默认值是0.75,这个值权衡了时间和空间的消耗.更高的值会降低

* 空间的开销,但会增加查找成本(该现象会出现在大多数HashMap操作中,包括get和put).

* 当设置map的初始容量时,应将map中预期的条目数量和map的加载因子加入考虑范围,

* 以减少rehashed操作.如果 初始容量>条目总数/加载因子,将不会出现rehashed操作.

*

* <p>If many mappings are to be stored in a <tt>HashMap</tt>

* 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.

*

* 如果一个HashMap实例中要被排序的数据比较多,则创建HashMap实例时,应该分配足够大的空间给它.

* 这要比它自己多次rehashed去扩容来得有效率.注意,如果多个key的hashCode是相同的,那么hashTable的

* 效率绝对是要慢得多的.为了改善这样的影响,当所有的key实现了Comparable接口时,本类会使用比较指令

* 来减弱这样的影响.

*

*

*

* <p><strong>Note that this implementation is not synchronized.</strong>

* If multiple threads access a hash map concurrently, and at least one of

* the threads modifies the map structurally, it <i>must</i> 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.

*

*

* 注意,HashMap不是线程同步的.如果有多个线程并发访问一个hashMap,并且至少有一个

* 线程对它进行了结构修改操作, 对于这样的情况,你需要在外部进行同步(结构修改指的是增加

* 或删除一个或多个映射.如果仅仅是改变了一个实例中已存在的键值关系,这不算结构修改.).

* 这通常需要通过在一些已经封装好的map上使用同步操作来完成.

*

*

*

* 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:<pre>

* Map m = Collections.synchronizedMap(new HashMap(...));</pre>

*

* 如果没有这样的对象存在,那么map应该使用Collections.synchronizedMap()包裹起来.

* 上述操作应在map创建时使用,以防map意外地被多个线程同时访问.

*

* <p>The iterators returned by all of this class's "collection view methods"

* are <i>fail-fast</i>: if the map is structurally modified at any time after

* the iterator is created, in any way except through the iterator's own

* <tt>remove</tt> 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.

*

* 该类的集合视图方法返回的迭代器是fail-fast的:在迭代器被创建后,如果map是结构化的,无论在何时,

* 使用何种方法(除了迭代器本身的remove方法)来修改它,都会抛出ConcurrentModificationException.

* (也就是HashMap的expectedModCount和modCount对不上的时候会抛出上述异常.)

* 因此,面对并发修改操作时,迭代器会迅速且清晰地报错.而不是冒着在不确定的时间做不确定的操作的风险.

*

*

* <p>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 <tt>ConcurrentModificationException</tt> on a best-effort basis.

* Therefore, it would be wrong to write a program that depended on this

* exception for its correctness: <i>the fail-fast behavior of iterators

* should be used only to detect bugs.</i>

*

* 注意,迭代器的fail-fast行为是不能保证的.一般来说,保证非同步的同步操作是不太可能的.

* 在最优基础上,Fail-fast迭代器会抛出ConcurrentModificationException.

* 因此,写一个为了自身正确性而依赖于这个异常的程序是不对的.迭代器的fail-fast行为应该

* 只是用来检测bug而已.

*