HashMap源码解析(零) HashMap类的源码注释介绍
先看一下整体的HashMap类的介绍:
* 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.
基于Map接口实现. HashMap实现了所有对于map的操作,而且允许空的Key和空的Value.(HashMap类和HashTable类几乎完全一样,不同之处仅在于HashTable是线程安全的且不允许Null值). HashMap不保证有序,甚至不能保证顺序不变.
* 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.
迭代器访问HashMap的时间和表的容量有关,所以不要一开始设置过大的容量.也不要把装填一字设置的过大或过小
* 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.
HashMap主要由两个参数决定:初始容量和装填因子. 容量是指桶的数量. 装填因子决定了当容量达到多少比例时进行扩容.
* 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.
比例因子默认为0.75 .和容量一起决定何时扩容.
* 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.
HashMap要增加大量元素时,以足够大的容量创建映射将允许映射更有效地存储，而不是让它根据需要执行自动重新散列来增长表. 太多的碰撞会降低效率.
* 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.
*
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.
HahsMap不是线程安全的