标签：map capacity HashMap 映射 JDK1.7 源码 哈希 迭代
【源英文】
JDK1.7 HashMap注释译文
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 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
【译文】
Map接口基于哈希表的实现。此实现提供所有可选的map操作，并允许空值和空键。 （HashMap类与Hashtable大致等效，不同之处在于它是不同步的，并且允许为null。）该类不保证映射的顺序；它不保证映射的顺序。 特别是，它不能保证顺序会随着时间的推移保持恒定。
假设哈希函数将元素正确分散在存储桶中，则此实现为基本操作（获取和放置）提供恒定时间的性能。 集合视图上的迭代所需的时间与HashMap实例的“容量”（存储桶数）及其大小（键-值映射数）成正比。 因此，如果迭代性能很重要，则不要将初始容量设置得过高（或负载因子过低），这一点非常重要。
HashMap的实例具有两个影响其性能的参数：初始容量和负载因子。容量是哈希表中存储桶的数量，初始容量只是创建哈希表时的容量。负载因子是散列表的容量自动增加之前允许其填充的完整程度的度量。当哈希表中的条目数超过负载因子和当前容量的乘积时，哈希表将被重新哈希（即，内部数据结构将被重建），因此哈希表的存储桶数约为两倍。
通常，默认负载因子（.75）在时间和空间成本之间提供了一个很好的折衷方案。较高的值会减少空间开销，但会增加查找成本（在HashMap类的大多数操作中都得到体现，包括get和put）。设置其初始容量时，应考虑映射中的预期条目数及其负载因子，以最大程度地减少重新哈希操作的数量。如果初始容量大于最大条目数除以负载因子，则将不会进行任何哈希操作。
如果将许多映射存储在HashMap实例中，则创建具有足够大容量的映射将比让其根据需要增长表的自动重新哈希处理更有效地存储映射。
请注意，此实现未同步。如果多个线程同时访问一个哈希映射，并且至少有一个线程在结构上修改该映射，则必须在外部进行同步。 （结构修改是添加或删除一个或多个映射的任何操作；仅更改与实例已经包含的键相关联的值不是结构修改。）通常通过在自然封装了地图的某个对象上进行同步来完成此操作。 。
如果不存在这样的对象，则应使用{@link Collections＃synchronizedMap Collections.synchronizedMap}方法“包装”地图。最好在创建时完成此操作，以防止意外的非同步访问地图：Map m = Collections.synchronizedMap（new HashMap（…））;
该类的所有“集合视图方法”返回的迭代器都是快速失败的：如果在创建迭代器后的任何时间对结构进行结构修改，则除了通过迭代器自己的remove方法之外，该迭代器都将抛出{ @link ConcurrentModificationException}。因此，面对并发修改，迭代器会快速干净地失败，而不是冒着在未来不确定的时间冒任意，不确定的行为的风险。
请注意，迭代器的快速失败行为无法得到保证，因为通常来说，在存在不同步的并发修改的情况下，不可能做出任何严格的保证。 快速失败的迭代器会尽最大努力抛出ConcurrentModificationException。 因此，编写依赖于此异常的程序的正确性是错误的：迭代器的快速失败行为应仅用于检测错误。
此类是 Java集合框架的成员。
@param 此映射维护的键的类型
@param 映射值的类型
标签：map,capacity,HashMap,映射,JDK1.7,源码,哈希,迭代 来源： https://blog.csdn.net/qq_42106010/article/details/115448120
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