HashMap源码解析（jdk1.7）
2022-05-20 本文已影响0人
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HashMap类源码
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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package java.util;
import java.io.*;
/**
* 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.
*
* map接口基于hash表的实现。
* 这个实现提供了所有可选的map操作，允许null键和null值。
* 这个HashMap类大致相当于Hashtable，除了它是线程不安全的和允许null。
* 这个类不保证map的顺序，尤其它不保证随着时间推移，顺序一直保持不变。
*
* <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.
*
* 这个实现为get和put方法提供了常数时间的性能，假设哈希函数将元素正确地分散在桶中。
* 集合视图的迭代需要的时间与 HashMap 实例的“容量”（桶的数量）加上它的大小（键值映射的数量）成正比。
* 因此，如果迭代性能重要的话，不要设置初始容量太大或者加载因子太小。
*
* <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的实例有两个参数影响它的性能：初始化容量和加载因子。
* 容量是hash表中桶的数量，初始化容量只是创建hash表时的容量。
* 加载因子是衡量哈希表在其容量自动增加之前允许达到的程度。
* 当hash表中entry的数量超过了加载因子和当前容量的乘积，hash表就会重新进行散列（也就是内部的数据结构重新构建），桶的大小大约是变成两倍。
*
* <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.
*
* 作为基本规则，默认的加载因子0.75在时间成本和空间成本之间提供了一个好的权衡。
* 更高的值会降低空间成本，但是增加了查找成本（影响了HashMap大部分操作，包括get和put）。
* 当设置初始化容量时，map的被期望的entry的数量和加载因子应该被考虑到，以便最小化重新散列操作的次数。
* 如果初始化容量高于entry的最大数量除以加载因子，则rehash操作永远不会发生。
*
* <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.
*
* 如果许多映射将会被存储在一个HashMap实例中，那么与让它有需要的时候自动rehash，用一个足够大的容量创建实例将会更有效率的存储。
*
* <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.
*
* 注意这个实现是线程不安全的。
* 如果多个线程并发访问同一个map，并且至少有一个线程会修改map的结构，它必须在外部进行同步。
* （结构化的修改是任何一个添加或修改一个或多个映射的操作，仅仅改变一个已经存在的key的value不是一个结构化的修改。）
* 这通常是通过同步一些自然封装map的对象来完成的。
*
* 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，map应该使用Collections.synchronizedMap方法进行包装。
* 这最好在创建时完成，以防止偶然的非同步的访问该map。
*
* <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.
*
* 所有该类的集合视图方法返回的迭代器都是快速失败的。
* 如果在迭代器被创建后在任何时候结构化的修改map，
* 除了通过迭代器自己的 remove 方法之外的任何方式，迭代器都会抛出一个ConcurrentModificationException异常。
* 因此，迭代器快速而干净地失败，而不是在未来不确定的时间冒任意的、非确定性的行为的风险。
*
* <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：迭代器快速失败的行为不能像它说的那样被保证，
* 一般来说，在存在不同步的并发修改的情况下不可能做出任何硬保证。
* 快速失败的迭代器会尽最大努力抛出ConcurrentModificationException。
* 因此，编写一个依赖于这个异常的正确性的程序是错误的：迭代器的快速失败行为应该只用于检测bug。
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @param <K> the type of keys maintained by this map
* @param <V> 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<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable
{
/**
* The default initial capacity - MUST be a power of two.
* 默认初始化容量16，必须是2的幂次方
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* The maximum capacity, used if a higher value is implicitly specified
* by either of the constructors with arguments.
* MUST be a power of two <= 1<<30.
* 最大容量，如果任何一个带参数的构造函数隐式指定了更高的值，则使用该最大值
* 必须是2的幂次方，且必须小于等于2的30次方
*/
static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* The load factor used when none specified in constructor.
* 当没有在构造函数指定时，使用该默认加载因子
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* An empty table instance to share when the table is not inflated.
* 当数组没有初始化时共享的空数组实例
*/
static final Entry<?,?>[] EMPTY_TABLE = {};
/**
* The table, resized as necessary. Length MUST Always be a power of two.
* 数组，根据需要调整大小，数组长度必须是2的幂次方
*/
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
/**
* The number of key-value mappings contained in this map.
* map中实际元素的个数
*/
transient int size;
/**
* The next size value at which to resize (capacity * load factor).
* 下次数组扩容的阈值，等于数组容量*加载因子
* @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
//如果 table == EMPTY_TABLE 那么这是膨胀时创建表的初始容量
int threshold;
/**
* The load factor for the hash table.
* hash表的加载因子
* @serial
*/
final float loadFactor;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
* 这个hashmap已经在结构上被修改的次数，
* 结构上的修改指的是改变hashmap映射数量或修改它内部结构（比如重新计算hash值），
* 这个字段被用来当使用迭代器迭代时快速失败。
*/
transient int modCount;
/**
* The default threshold of map capacity above which alternative hashing is
* used for String keys. Alternative hashing reduces the incidence of
* collisions due to weak hash code calculation for String keys.
* map容量的默认阈值，替代散列用于字符串键。
* 由于String类型键的散列码计算较弱，替代散列降低了冲突的发生率
* <p/>
* This value may be overridden by defining the system property
* {@code jdk.map.althashing.threshold}. A property value of {@code 1}
* forces alternative hashing to be used at all times whereas
* {@code -1} value ensures that alternative hashing is never used.
* 这个值可能被定义在系统属性{@code jdk.map.althashing.threshold}覆盖，
* 值为1会强制替代散列在每时每刻被使用，然而值为-1会确保替代散列从来不会被使用。
*/
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
/**
* holds values which can't be initialized until after VM is booted.
* 保证在虚拟机启动会才进行初始化
*/
private static class Holder {
/**
* Table capacity above which to switch to use alternative hashing.
* 切换到使用替代散列的表容量
*/
static final int ALTERNATIVE_HASHING_THRESHOLD;
static {
String altThreshold = java.security.AccessController.doPrivileged(
new sun.security.action.GetPropertyAction(
"jdk.map.althashing.threshold"));
int threshold;
try {
// 如果设置了系统属性值（jdk.map.althashing.threshold）则使用系统属性值，
// 否则使用默认的ALTERNATIVE_HASHING_THRESHOLD_DEFAULT
threshold = (null != altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;
// disable alternative hashing if -1
// 如果阈值为-1，则设置为Integer.MAX_VALUE
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}
// 阈值必须是正整数
if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentException failed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);
}
ALTERNATIVE_HASHING_THRESHOLD = threshold;
}
}
/**
* A randomizing value associated with this instance that is applied to
* hash code of keys to make hash collisions harder to find. If 0 then
* alternative hashing is disabled.
* 一个和当前实例关联的随机值被应用到key的hasn值计算中，以降低hash冲突。
* 如果设置为0那么替代散列将被禁用
*/
transient int hashSeed = 0;
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
* 指定初始化容量和加载因子的空hashmap的构造函数
*
* @param initialCapacity the initial capacity 初始化容量
* @param loadFactor the load factor 加载因子
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive 如果初始化容量是负数或者加载因子是非正数则抛出异常
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
// 自定义的容量大于最大容量则被设置成最大容量
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
threshold = initialCapacity;
init();
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
* 指定初始化容量，使用默认加载因子0.75的构造函数
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
* hashmap无参构造，指定默认的初始化容量16和默认的加载因子0.75
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
* 用指定map来构造一个新的hashmap。
* 使用默认的加载因子0.75，一个足够容纳指定map大小的初始化容量。
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
inflateTable(threshold);
putAllForCreate(m);
}
/**
* 获取大于等于number的最小的2的幂次方数
* @param number 必须是非负数
* @return
*/
private static int roundUpToPowerOf2(int number) {
// assert number >= 0 : "number must be non-negative";
// 如果number >= 最大容量，返回最大容量；如果 <= 1，返回1；
// 否则当 1 < number < MAXIMUM_CAPACITY, 返回Integer.highestOneBit((number - 1) << 1),
// Integer.highestOneBit(i)返回小于等于i的最大的2的幂次方，
// number=15,Integer.highestOneBit((number - 1) << 1) -> 16
// number=16,Integer.highestOneBit((number - 1) << 1) -> 16
// number=17,Integer.highestOneBit((number - 1) << 1) -> 32
// number-1是为了处理number正好是2的幂次方的情况
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}
/**
* Inflates the table.
* 初始化数组
*/
private void inflateTable(int toSize) {
// Find a power of 2 >= toSize
// 获取大于等于toSize的最小的2的幂次方数
int capacity = roundUpToPowerOf2(toSize);
// 取容量*加载因子和最大容量+1中的较小者作为阈值
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
// 初始化table
table = new Entry[capacity];
initHashSeedAsNeeded(capacity);
}
// internal utilities
/**
* Initialization hook for subclasses. This method is called
* in all constructors and pseudo-constructors (clone, readObject)
* after HashMap has been initialized but before any entries have
* been inserted. (In the absence of this method, readObject would
* require explicit knowledge of subclasses.)
* 为子类预留的初始化钩子函数，
* 在初始化hashmap之后但在任何一个元素插入之前，在所有构造函数和伪构造函数中调用该方法。
* （如果没有该方法，readObject需要明确了解子类）
*/
void init() {
}
/**
* Initialize the hashing mask value. We defer initialization until we
* really need it.
* 初始化散列掩码值。当我们真正需要它时在进行初始化。
*/
final boolean initHashSeedAsNeeded(int capacity) {
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean switching = currentAltHashing ^ useAltHashing;
// 当switching为true时重新设置hashSeed，有以下两种情况：
// 1.currentAltHashing = true（hashSeed != 0）,useAltHashing = false（容量 < Holder.ALTERNATIVE_HASHING_THRESHOLD）,
// 2.currentAltHashing = false（hashSeed == 0）,useAltHashing = true（容量 >= Holder.ALTERNATIVE_HASHING_THRESHOLD）,
if (switching) {
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}
/**
* Retrieve object hash code and applies a supplemental hash function to the
* result hash, which defends against poor quality hash functions. This is
* critical because HashMap uses power-of-two length hash tables, that
* otherwise encounter collisions for hashCodes that do not differ
* in lower bits. Note: Null keys always map to hash 0, thus index 0.
* 检索对象散列码并将补充散列函数应用于结果散列，以防止质量差的散列函数。
* 这很关键，因为 HashMap 使用长度为二的幂的哈希表，否则会遇到低位没有差异的 hashCode 的冲突。
* 注意：空键总是映射到哈希 0，因此索引 0。
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String) k);
}
h ^= k.hashCode();
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
/**
* Returns index for hash code h.
* 返回指定hash值在指定大小数组中的位置。
*/
static int indexFor(int h, int length) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
// 相当于取余 h % length
return h & (length-1);
}
/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map
*/
public int size() {
return size;
}
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
public boolean isEmpty() {
return size == 0;
}
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
* 返回指定key的value
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
* 更正式地说，如果此映射包含从键 {@code k} 到值 {@code v} 的映射，
* 使得 {@code (key==null ? k==null : key.equals(k))}，然后这个方法返回 {@code v};
* 否则返回 {@code null}。 （最多可以有一个这样的映射。）
*
* <p>A return value of {@code null} does not <i>necessarily</i>
* indicate that the map contains no mapping for the key; it's also
* possible that the map explicitly maps the key to {@code null}.
* The {@link #containsKey containsKey} operation may be used to
* distinguish these two cases.
* 返回值为null不能表明map中不包含该key，也有可能是key的value就是null。
* containsKey方法会区分两种情况。
*
* @see #put(Object, Object)
*/
public V get(Object key) {
if (key == null)
return getForNullKey();
Entry<K,V> entry = getEntry(key);
return null == entry ? null : entry.getValue();
}
/**
* Offloaded version of get() to look up null keys. Null keys map
* to index 0. This null case is split out into separate methods
* for the sake of performance in the two most commonly used
* operations (get and put), but incorporated with conditionals in
* others.
* 查找空键的get方法。
* 空键映射到数组的0号位置。
* 为了提高两个最常用操作（get 和 put）的性能，这种 null 情况被拆分为单独的方法，但在其他操作中与条件结合。
*/
private V getForNullKey() {
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
}
/**
* Returns <tt>true</tt> if this map contains a mapping for the
* specified key.
* 如果map包含指定key则返回true
*
* @param key The key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return getEntry(key) != null;
}
/**
* Returns the entry associated with the specified key in the
* HashMap. Returns null if the HashMap contains no mapping
* for the key.
* 返回map中指定key的entry。
* 如果map中不包含该key，则返回null。
*/
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
* 在map将指定的key和value进行关联。
* 如果map已经包含了该key的，则旧的值将会被替换成新的。
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
* 该方法返回与指定key关联的旧的value，如果map中没有该key的话则返回null。
* null返回值也可以表示map中过去与该key关联的值就是null。
*/
public V put(K key, V value) {
// 如果当前数组==EMPTY_TABLE表示数组还没有初始化，那么先初始化数组，再继续插入
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
// 1.7hashmap允许key和value为null
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
// 遍历数组指定位置的链表，找到指定key，并替换value，将旧值返回
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
// 之前没有的话则插入一个新的
addEntry(hash, key, value, i);
return null;
}
/**
* Offloaded version of put for null keys
* 插入key为null的元素，插到了数组中索引为0的位置
*/
private V putForNullKey(V value) {
// 遍历数组中索引为0的位置的链表，找到key为nul的元素，替换value，返回旧值
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
//链表中没找到key为null的元素，则插入新的元素
addEntry(0, null, value, 0);
return null;
}
/**
* This method is used instead of put by constructors and
* pseudoconstructors (clone, readObject). It does not resize the table,
* check for comodification, etc. It calls createEntry rather than
* addEntry.
* 这个方法被用来替代put方法，该方法不会扩容数组，检查修改次数等。
* 它调用createEntry方法而不是addEntry方法。
*/
private void putForCreate(K key, V value) {
int hash = null == key ? 0 : hash(key);
int i = indexFor(hash, table.length);
/**
* Look for preexisting entry for key. This will never happen for
* clone or deserialize. It will only happen for construction if the
* input Map is a sorted map whose ordering is inconsistent w/ equals.
* 寻找指定key已存在的entry，存在则替换value。
* 对于clone or deserialize永远不会发生。
*/
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
e.value = value;
return;
}
}
createEntry(hash, key, value, i);
}
private void putAllForCreate(Map<? extends K, ? extends V> m) {
// 遍历指定map中的所有元素放入当前map中
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}
/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
* 将map中的数据放入一个更大容量的数组中。
* 当map中key的数量达到了阈值，这个方法会被自动调用。
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
* 如果当前数组容量已经达到了最大值，这个方法不会扩容，但是会将阈值设置成Integer.MAX_VALUE。
* 这防止了将来被再次调用。
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
* 数组新容量大小，必须是2的幂次方；必须大于当前容量，除非当前容量已经是最大值，
* 这种情况下新容量是多少就无关紧要了。
*
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
// 如果当前数组容量已经达到了最大值，这个方法不会扩容，但是会将阈值设置成Integer.MAX_VALUE。
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
// 创建新数组
Entry[] newTable = new Entry[newCapacity];
// 将旧数组中的元素全部转移到新数组中
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
/**
* Transfers all entries from current table to newTable.
* 将旧数组中的所有元素转移到新数组中
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
// 遍历旧数组
for (Entry<K,V> e : table) {
// 遍历数组中每一个位置的链表
while(null != e) {
Entry<K,V> next = e.next;
// 是否需要重新计算hash值，默认是不需要的，除非自定义了系统属性（jdk.map.althashing.threshold），并且满足相关条件。
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
// 计算每一个节点在新数组中的位置。
int i = indexFor(e.hash, newCapacity);
// 采用头插法，将每一个元素设置为新的头结点，之前的头结点作为next
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
/**
* Copies all of the mappings from the specified map to this map.
* These mappings will replace any mappings that this map had for
* any of the keys currently in the specified map.
* 把指定map中的元素全部放入当前map中。
*
*
* @param m mappings to be stored in this map
* @throws NullPointerException if the specified map is null
*/
public void putAll(Map<? extends K, ? extends V> m) {
int numKeysToBeAdded = m.size();
if (numKeysToBeAdded == 0)
return;
// 如果当前map的数组还未初始化，先初始化
if (table == EMPTY_TABLE) {
inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
}
/*
* Expand the map if the map if the number of mappings to be added
* is greater than or equal to threshold. This is conservative; the
* obvious condition is (m.size() + size) >= threshold, but this
* condition could result in a map with twice the appropriate capacity,
* if the keys to be added overlap with the keys already in this map.
* By using the conservative calculation, we subject ourself
* to at most one extra resize.
* 如果指定map的元素个数大于当前map的阈值则扩容当前map。
* 这是保守的，明显的条件应该是指定map的大小+当前map的大小的和是否 >= 阈值，
* 但是这个条件可能导致当前map有两倍的合适的容量，因为有可能有些key已经存在当前map中。
* 使用保守的计算，我们控制自己最多一个额外的扩容。
*/
if (numKeysToBeAdded > threshold) {
int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);
if (targetCapacity > MAXIMUM_CAPACITY)
targetCapacity = MAXIMUM_CAPACITY;
int newCapacity = table.length;
while (newCapacity < targetCapacity)
newCapacity <<= 1;
if (newCapacity > table.length)
resize(newCapacity);
}
for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
put(e.getKey(), e.getValue());
}
/**
* Removes the mapping for the specified key from this map if present.
* 如果map中存在指定key的话则删除该key的映射。
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
* 返回指定key的旧值，如果返回null,可能表示没有该key，也可能表示该key的值就是null。
*/
public V remove(Object key) {
Entry<K,V> e = removeEntryForKey(key);
return (e == null ? null : e.value);
}
/**
* Removes and returns the entry associated with the specified key
* in the HashMap. Returns null if the HashMap contains no mapping
* for this key.
* 删除指定key,并返回对应的entry。
* 如果没有指定key，则返回null。
*/
final Entry<K,V> removeEntryForKey(Object key) {
if (size == 0) {
return null;
}
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
// 链表中的前一个节点
Entry<K,V> prev = table[i];
// 链表中遍历的当前节点
Entry<K,V> e = prev;
while (e != null) {
// 当前节点的下一个节点
Entry<K,V> next = e.next;
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) {
modCount++;
size--;
// 找到指定key,如果前一个节点等于当前节点，表示是头节点，则将下一个节点作为新的头结点，否则将前一个节点的next指针指向当前节点的next
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
/**
* Special version of remove for EntrySet using {@code Map.Entry.equals()}
* for matching.
* EntrySet特殊版本的删除，使用Map.Entry.equals()进行匹配
*/
final Entry<K,V> removeMapping(Object o) {
if (size == 0 || !(o instanceof Map.Entry))
return null;
Map.Entry<K,V> entry = (Map.Entry<K,V>) o;
Object key = entry.getKey();
int hash = (key == null) ? 0 : hash(key);
int i = indexFor(hash, table.length);
Entry<K,V> prev = table[i];
Entry<K,V> e = prev;
while (e != null) {
Entry<K,V> next = e.next;
if (e.hash == hash && e.equals(entry)) {
modCount++;
size--;
if (prev == e)
table[i] = next;
else
prev.next = next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}
return e;
}
/**
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
* 清空map,用null填充数组，大小设置为0
*/
public void clear() {
modCount++;
Arrays.fill(table, null);
size = 0;
}
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
* 如果map中至少有一个key的value是指定的value，则返回true
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
*/
public boolean containsValue(Object value) {
if (value == null)
return containsNullValue();
Entry[] tab = table;
// 遍历数组
for (int i = 0; i < tab.length ; i++)
// 遍历数组中每一个位置的链表，只要有一个value与指定的value相等，则返回true，否则返回false
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
return false;
}
/**
* Special-case code for containsValue with null argument
* 是否包含null的value的特殊方法
*/
private boolean containsNullValue() {
Entry[] tab = table;
// 遍历数组
for (int i = 0; i < tab.length ; i++)
// 遍历数组中每一个位置的链表，只要有一个value是null，则返回true;否则返回false
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value == null)
return true;
return false;
}
/**
* Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
* values themselves are not cloned.
* 返回当前map的浅拷贝，key和value与当前map中是同一个引用
*
* @return a shallow copy of this map
*/
public Object clone() {
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedException e) {
// assert false;
}
if (result.table != EMPTY_TABLE) {
result.inflateTable(Math.min(
(int) Math.min(
size * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY),
table.length));
}
result.entrySet = null;
result.modCount = 0;
result.size = 0;
result.init();
result.putAllForCreate(this);
return result;
}
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash;
/**
* Creates new entry.
*/
Entry(int h, K k, V v, Entry<K,V> n) {
value = v;
next = n;
key = k;
hash = h;
}
public final K getKey() {
return key;
}
public final V getValue() {
return value;
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
Object k1 = getKey();
Object k2 = e.getKey();
if (k1 == k2 || (k1 != null && k1.equals(k2))) {
Object v1 = getValue();
Object v2 = e.getValue();
if (v1 == v2 || (v1 != null && v1.equals(v2)))
return true;
}
return false;
}
public final int hashCode() {
return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
}
public final String toString() {
return getKey() + "=" + getValue();
}
/**
* This method is invoked whenever the value in an entry is
* overwritten by an invocation of put(k,v) for a key k that's already
* in the HashMap.
*/
void recordAccess(HashMap<K,V> m) {
}
/**
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap<K,V> m) {
}
}
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
* 用指定的key、value、hash code新增一个新的节点到指定的位置。
* 如果合适的话，扩容数组是这个方法的责任。
*
* Subclass overrides this to alter the behavior of put method.
* 子类重写这个方法可以改变put方法的行为。
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
// jdk7中扩容数组需要两个条件：当前map实际大小 >= 阈值，并且新节点将要插入数组中的位置已经有元素了。
// 先扩容再插入新节点
if ((size >= threshold) && (null != table[bucketIndex])) {
// 数组扩容为之前的两倍，保证始终是2的幂次方
resize(2 * table.length);
// 重新计算hash值
hash = (null != key) ? hash(key) : 0;
// 重新计算在新数组中的位置
bucketIndex = indexFor(hash, table.length);
}
createEntry(hash, key, value, bucketIndex);
}
/**
* Like addEntry except that this version is used when creating entries
* as part of Map construction or "pseudo-construction" (cloning,
* deserialization). This version needn't worry about resizing the table.
* 与 addEntry 类似，只是在创建条目作为 Map 构造或“伪构造”的一部分时使用此版本。
* 这个版本不需要担心扩容数组。
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
* 子类重写这个方法去改变HashMap(Map), clone, and readObject 方法的行为。
*/
void createEntry(int hash, K key, V value, int bucketIndex) {
// 先获取数组中指定位置的元素（头节点）
Entry<K,V> e = table[bucketIndex];
// 采用头插法，创建新节点并将之前的头节点作为next，新节点作为新的头结点放到数组中的指定位置。
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
private abstract class HashIterator<E> implements Iterator<E> {
Entry<K,V> next; // next entry to return
int expectedModCount; // For fast-fail
int index; // current slot
Entry<K,V> current; // current entry
HashIterator() {
expectedModCount = modCount;
if (size > 0) { // advance to first entry
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
}
public final boolean hasNext() {
return next != null;
}
final Entry<K,V> nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry<K,V> e = next;
if (e == null)
throw new NoSuchElementException();
if ((next = e.next) == null) {
Entry[] t = table;
while (index < t.length && (next = t[index++]) == null)
;
}
current = e;
return e;
}
public void remove() {
if (current == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Object k = current.key;
current = null;
HashMap.this.removeEntryForKey(k);
expectedModCount = modCount;
}
}
private final class ValueIterator extends HashIterator<V> {
public V next() {
return nextEntry().value;
}
}
private final class KeyIterator extends HashIterator<K> {
public K next() {
return nextEntry().getKey();
}
}
private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {
public Map.Entry<K,V> next() {
return nextEntry();
}
}
// Subclass overrides these to alter behavior of views' iterator() method
Iterator<K> newKeyIterator() {
return new KeyIterator();
}
Iterator<V> newValueIterator() {
return new ValueIterator();
}
Iterator<Map.Entry<K,V>> newEntryIterator() {
return new EntryIterator();
}
// Views
private transient Set<Map.Entry<K,V>> entrySet = null;
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*/
public Set<K> keySet() {
Set<K> ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}
private final class KeySet extends AbstractSet<K> {
public Iterator<K> iterator() {
return newKeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return HashMap.this.removeEntryForKey(o) != null;
}
public void clear() {
HashMap.this.clear();
}
}
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*/
public Collection<V> values() {
Collection<V> vs = values;
return (vs != null ? vs : (values = new Values()));
}
private final class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return newValueIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
}
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation, or through the
* <tt>setValue</tt> operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations. It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a set view of the mappings contained in this map
*/
public Set<Map.Entry<K,V>> entrySet() {
return entrySet0();
}
private Set<Map.Entry<K,V>> entrySet0() {
Set<Map.Entry<K,V>> es = entrySet;
return es != null ? es : (entrySet = new EntrySet());
}
private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
public Iterator<Map.Entry<K,V>> iterator() {
return newEntryIterator();
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry<K,V> e = (Map.Entry<K,V>) o;
Entry<K,V> candidate = getEntry(e.getKey());
return candidate != null && candidate.equals(e);
}
public boolean remove(Object o) {
return removeMapping(o) != null;
}
public int size() {
return size;
}
public void clear() {
HashMap.this.clear();
}
}
/**
* Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
* serialize it).
*
* @serialData The <i>capacity</i> of the HashMap (the length of the
* bucket array) is emitted (int), followed by the
* <i>size</i> (an int, the number of key-value
* mappings), followed by the key (Object) and value (Object)
* for each key-value mapping. The key-value mappings are
* emitted in no particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
// Write out number of buckets
if (table==EMPTY_TABLE) {
s.writeInt(roundUpToPowerOf2(threshold));
} else {
s.writeInt(table.length);
}
// Write out size (number of Mappings)
s.writeInt(size);
// Write out keys and values (alternating)
if (size > 0) {
for(Map.Entry<K,V> e : entrySet0()) {
s.writeObject(e.getKey());
s.writeObject(e.getValue());
}
}
}
private static final long serialVersionUID = 362498820763181265L;
/**
* Reconstitute the {@code HashMap} instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold (ignored), loadfactor, and any hidden stuff
s.defaultReadObject();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}
// set other fields that need values
table = (Entry<K,V>[]) EMPTY_TABLE;
// Read in number of buckets
s.readInt(); // ignored.
// Read number of mappings
int mappings = s.readInt();
if (mappings < 0)
throw new InvalidObjectException("Illegal mappings count: " +
mappings);
// capacity chosen by number of mappings and desired load (if >= 0.25)
int capacity = (int) Math.min(
mappings * Math.min(1 / loadFactor, 4.0f),
// we have limits...
HashMap.MAXIMUM_CAPACITY);
// allocate the bucket array;
if (mappings > 0) {
inflateTable(capacity);
} else {
threshold = capacity;
}
init(); // Give subclass a chance to do its thing.
// Read the keys and values, and put the mappings in the HashMap
for (int i = 0; i < mappings; i++) {
K key = (K) s.readObject();
V value = (V) s.readObject();
putForCreate(key, value);
}
}
// These methods are used when serializing HashSets
int capacity() { return table.length; }
float loadFactor() { return loadFactor; }
}
Integer.highestOneBit(int i)方法
/**
* Returns an {@code int} value with at most a single one-bit, in the
* position of the highest-order ("leftmost") one-bit in the specified
* {@code int} value. Returns zero if the specified value has no
* one-bits in its two's complement binary representation, that is, if it
* is equal to zero.
*
* @return an {@code int} value with a single one-bit, in the position
* of the highest-order one-bit in the specified value, or zero if
* the specified value is itself equal to zero.
* @since 1.5
*
* 返回小于等于i的最大的2的幂次方，
* Integer.highestOneBit(15) -> 8
* Integer.highestOneBit(16) -> 16
* Integer.highestOneBit(17) -> 16
*/
public static int highestOneBit(int i) {
// HD, Figure 3-1
i |= (i >> 1);
i |= (i >> 2);
i |= (i >> 4);
i |= (i >> 8);
i |= (i >> 16);
return i - (i >>> 1);
}
