HashMap源码注释翻译
/*
* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
*/
package java.util;
import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.Serializable;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;
import java.util.function.Function;
* 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.
*
* 译：HashMap 实现了Map接口（基于哈希表）。HashMap提供了所有可选的map操作，允许
* key和values为null。HashMap和Hashtable类似，不同之处在于HashMap是非同步的，允许
* 空值。HashMap不能保证map的顺序；特别指出，不能保证这个顺序一直保持不变。
*
* 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.
*
* 译：假设哈希函数适当地打乱桶中的元素为前提，HashMap为get/put基本操作提供了恒定
* 的时间性能。（中间这句还不能准确的翻译，大概意思是迭代的性能依赖于map的容量和键值
* 对的数量，进而推导出下面这句）因此，如果迭代性能要求高的话，不能设置容量太大或者
* 装载因子太低变得非常重要的。
*
*
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.
*
* 译：一个HashMap实例有两个参数影响它的性能：初始容量和装载因子。容量是哈希表中桶的
* 数量，初始容量是哈希表创建时的容量。装载因子用来衡量HashMap满的程度，就是决定hash
* 表中用掉多少容量时,进行自动扩充容量的因子.当哈希表中entry的数量超过装载因子和当前
* 容量的乘积时，哈希表内部结构将会重新构建，使得哈希表容量约是桶数量的两倍。
*
*
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.
*
* 译：作为一般规则，默认装载因子（0.75）在时间和空间成本之间提供了很好的
* 折中。更高的值减少了空间开销，但增加了查找成本（反映在HashMap的大部分
* 操作，包括get和put）。当设定初始容量时，应当考虑map中entry的预期数量
* 和装载因子，以便使rehash操作最小。如果初始容量大于最大条目数除以装载因子
* ，rehash操作将不会发生。
*
*
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.
*
* 译：如果你预计有很多元素将存入map,那么主动设置capacity大一点,有助于减少
* rehashing的次数,从而提高性能。如果很多关键字的哈希值相同，会降低哈希表的
* 性能。为了降低这个影响，当键支持java.lang.Comparable时，HashMap可以对
* 关键字做次排序。
*
*
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.
*
* 译：值得注意的是，HashMap是不同步的。如果多个线程同时访问哈希映射，并且至少一个
* 线程在结构上修改map，则必须在外部对其进行同步。结构修改是添加或删除一个或多个映
* 射的任何操作；仅仅更改与键相关联的值不是结构修改。（这句不知道怎么翻译合适）
*
* 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(...));
*
* 译：如果没有这样的对象,那么需要用synchronizedMap来封装.
*
* 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.
*
* 译：所有集合视图方法返回的迭代器都有fail-fast机制:如果map在iterator创建之后
* 结构进行了改变（除了通过迭代器自身remove方法）则会抛出ConcurrentModificationException。
* 因此，在面临并发修改时，迭代器会快速而干净地失败，远比不管情况,不确定操作带来风险要好。
*
*
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.
*
* 译：注意,一般不可能保证在一个线程不安全的操作中这种fail-fast机制.
* 抛出ConcurrentModificationException只能说是尽最大努力.
* 因此在自己的代码里不要依赖抛出这个异常.它只用于发现bug!
*
*
This class is a member of the
*
* Java Collections Framework.
*
* 译：地址:http://docs.oracle.com/javase/8/docs/technotes/guides/collections/index.html
* 这里可以获得官方对容器的详细解释,值得一读.
*
/**
* @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
*/
public class HashMap extends AbstractMap
implements Map, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;
}
翻译了开头注释，感觉很吃力。如果不懂里面的原理，正文中的注释翻译比较难。未完待续
翻译参考文章：https://www.cnblogs.com/killbug/p/7679043.html（有部分翻译参考自这篇文章）
HashMap原理：https://www.cnblogs.com/chengxiao/p/6059914.html（可参考这篇文章，感觉图文并茂，易懂）