Map之HashMap源码分析

public class HashMap<K,V>
extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable

HashMap继承自AbstractMap，实现了Map接口，有map的基本操作；实现了Cloneable接口，使得该类是可克隆的；实现了Serializable接口，说明该类支持序列化。HashMap是线程不安全的。源码版本为JDK1.7.0_75

HashMap的数据结构



紫色的对应Entry<K,V>[] table，绿色的是Entry<K,V>对象队列，解决hash冲突。

在put时，首先调用hashCode方法获得哈希值，在调用indexFor方法获取该key存放在table数组的下标index，新建entry插入到index所指向的链表的队首。在删除时也是通过相同的方法找到该key所在的index，然后遍历链表，将key相同、hash值相等的entry删除。

HashMap还对map存放的key-value数目进行限制（使用threshold），且要求数组大小（capacity）必须是2的幂次方（考虑indexFor方法），当容量无法将entry全部保存或者entry数目超过threshold时，根据loadfactor使用resize对容量进行扩大，其中threshold=capacity*loadfactor。

HashMap中允许存放key为null的entry，对于key为null，其index为0。

inflateTable方法是在当table==EMPTY_TABLE时才用到,用于初始化table。

成员变量

/**
* 默认初始化空间——必须为2的倍数
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;// aka 16

/**
* 最大空间，若在构造方法中没有给定，则使用该值——必须为2的倍数
* 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.
*/
static final int MAXIMUM_CAPACITY = 1 << 30;

/**
* 若在构造时没有指出则使用该值作为加载因子
* The load factor used when none specified in constructor.
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
* 当表格没有膨胀，则空表实例
* An empty table instance to share when the table is not inflated.
*/
static final Entry<?,?>[] EMPTY_TABLE = {};

/**
* 哈希表，有必要时需resize。长度总是2的倍数
* The table, resized as necessary. Length MUST Always be a power of two.
*/
transient Entry<K,V>[]table = (Entry<K,V>[]) EMPTY_TABLE;

/**
* map中包含的key-value映射数目
* The number of key-value mappings contained in this map.
*/
transient int size;

/**
* 调整大小的值？？临界值，当实际大小超过临界值时，会进行resize,threshold=capacity*load factor
* The next size value at which to resize (capacity * load factor).
* @serial
*/
// If table == EMPTY_TABLE then this is the initial capacity at which the
// table will be created when inflated.
int threshold;

/**
* 哈希表的加载因子
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;

/**
* HashMap进行结构上修改的次数。用来是对HashMap的集合上的迭代器fast-fail
* 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).
*/
transient int modCount;

/**
* 当使用string作为键值的哈希时的map容量的默认阈值。。。
* 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.
* <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.
*/
static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
/**
* 随机值，用在keys的hash code上，使得哈希冲突较难发生。若该值为0，则alternative hash不可用（我理解的是二次哈希）
* 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.
*/
transient int hashSeed = 0;

冲突机会越大，则查找效率将降低。loadFactor为加载因子，表示hash表中元素的填满程度，加载因子越大，空间利用率高，但冲突机会就加大了，链表长度会越来越长，查找效率将降低；反之，加载因子越小，填满的元素越少，冲突机会减少，但空间浪费了。

因此，需要在“冲突机会”与“空间利用率”之间寻找平衡——类似“时空”平衡。

参考：

/article/5049984.html

私有静态内部类Holder

/**
* 保存那些直到虚拟机启动时才能初始化的值
* holds values which can't be initialized until after VM is booted.
*/
private static class Holder {

/**
* 当转向使用alternative hashing时，table的容量
* Table capacity above which to switch to use alternative hashing.
*/
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 {
threshold = (null !=altThreshold)
? Integer.parseInt(altThreshold)
: ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;

// disable alternative hashing if -1
if (threshold == -1) {
threshold = Integer.MAX_VALUE;
}

if (threshold < 0) {
throw new IllegalArgumentException("value must be positive integer.");
}
} catch(IllegalArgumentExceptionfailed) {
throw new Error("Illegal value for 'jdk.map.althashing.threshold'",failed);
}

ALTERNATIVE_HASHING_THRESHOLD =threshold;
}
}

构造函数

/**
* 指定了初始容量和加载因子，构造一个空HashMap
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param  initialCapacity the initial capacity
* @param  loadFactor      the load factor
* @throws IllegalArgumentException if the initial capacity is negative
*         or the load factor is nonpositive
*/
public HashMap(intinitialCapacity, floatloadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
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).
*
* @param  initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(intinitialCapacity) {
this(initialCapacity,DEFAULT_LOAD_FACTOR);
}

/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (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>.
*
* @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);
}

inflateTable，该方法在table==EMPTY_TABLE时被调用，用于初始化table，并将capacity提至2的幂次方。

private static int roundUpToPowerOf2(intnumber) {
// assert number >= 0 : "number must be non-negative";
return number >= MAXIMUM_CAPACITY
? MAXIMUM_CAPACITY
: (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;
}
/**
* Inflates the table.
*/
private void inflateTable(inttoSize) {
// Find a power of 2 >= toSize
int capacity = roundUpToPowerOf2(toSize);

threshold = (int) Math.min(capacity *loadFactor, MAXIMUM_CAPACITY + 1);
table = new Entry[capacity];
initHashSeedAsNeeded(capacity);
}

// internal utilities

/**
* 为子类初始化hook（钩子）。该方法在构造方法和伪构造方法（clone，readObject）中初始化HashMap之后entry被插入之前调用的。没有该方法，readObject将需要子类信息。
* 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.)
*/
void init() {
}

/**
* 初始化哈希mask值。直到真的需要用到hashseed时，才初始化它。
* Initialize the hashing mask value. We defer initialization until we
* really need it.
*/
final boolean initHashSeedAsNeeded(intcapacity) {
boolean currentAltHashing = hashSeed != 0;
boolean useAltHashing = sun.misc.VM.isBooted() &&
(capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);
boolean switching = currentAltHashing ^ useAltHashing;
if (switching) {
hashSeed = useAltHashing
? sun.misc.Hashing.randomHashSeed(this)
: 0;
}
return switching;
}

hashing相关

/**
* 获得对象哈希码，并将一个补充的哈希函数应用到哈希结果以防poor哈希函数。
*  注意：键为null总是映射到哈希0，因此索引为0。
* 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.
*/
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
return sun.misc.Hashing.stringHash32((String)k);
}

h ^= k.hashCode();
// 通过若干次移位、异或操作，把hashCode的1的位置变得“松散，均匀”，
//  以免在计算index时不均匀
//  等价于操作 h ^ (h >>> 4) ^ (h >>> 7) ^ (h >>> 12) ^ (h >>> 16) ^ (h >>> 20) ^ (h >>> 24) ^ (h >>> 27);
// 将hashcode用十六进制表示为…nmlkjihgfedcba，则a'=a^b^c…,b'=b^c^d…,c'=c^d^e…
// 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);
returnh ^ (h >>> 7) ^ (h >>> 4);
}

/**
* 返回哈希码h的索引
*   因为length为2的幂次方，因此length-1使用二进制表示所有位都是1，即111111..，h&(length-1)得到的数可以均匀分布到table数组上，例如length=8，则1&7=1,2&7=2,3&7=3,…7&7=7,8&7=0,9&7=1..，可以看出该公式相当于h%length，注意的是针对length要为2的幂次方。
* Returns index for hash code h.
*/
static int indexFor(inth, intlength) {
// assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
return h & (length-1);
}

Get key相关

/**
* 返回给定key在map上对应的value，若map中没有该key，则返回null。
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <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.)
*
* <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.
*
* @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();
}

/**
* 获取key为null所对应的value。前面提到key为null对应的索引为0，因此从table[0]
*  开始遍历。
* 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.
*/
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.
*
* @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.
*/
final Entry<K,V> getEntry(Objectkey) {
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;
}

Put 相关

/**
* 根据key值将key-value对插入到map中，若key已存在，则更新value。返回插入前map
* 中key所对应的value值。
* 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.
*
* @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>.)
*/
public V put(K key, V value) {
if (table ==EMPTY_TABLE) {
inflateTable(threshold);
}
if (key ==null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
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++;
addEntry(hash, key, value, i);
return null;
}

/**
* 设置key为null时的value值。
* Offloaded version of put for null keys
*/
private V putForNullKey(Vvalue) {
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++;
addEntry(0, null, value, 0);
return null;
}

/**
* 不同于put，该方法不需要resize table，构造方法或伪构造方法(clone，readObject)调用该方法。
* 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.
*/
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.
*/
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) {
for (Map.Entry<? extends K, ? extends V>e : m.entrySet())
putForCreate(e.getKey(), e.getValue());
}

Resize

/**
* 将原来map内容重哈希到一个更大容量的新数组上。当map中key数目达到threshold时，自动调用该方法。若当前容量为MAXIMUM_CAPACITY，则不resize map，设置threshold为Integer.MAX_VALUE。
* 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.
*
* 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.
*
* @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).
*/
void resize(intnewCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity ==MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}

Entry[] newTable = new Entry[newCapacity];
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity *loadFactor, MAXIMUM_CAPACITY + 1);
}

/**
* 将当前表中所有entry转换到newTable中。rehash标志转换时是否需要重哈希。
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null !=e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash =null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash,newCapacity);
e.next =newTable[i];
newTable[i] =e;
e = next;
}
}
}

将m中的key-value对全都复制到map中，在put之前，首先对table进行resize，若要添加的键-值对大于threshold，则对table进行resize。

/**
* 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.
*
* @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;

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.
*/
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());
}

删除。。。删除特定key，首先根据key，计算hash，得到索引，获取所在的“桶”，遍历桶内的链表，删除hash相等且key相等的entry。

/**
* Removes the mapping for the specified key from this map if present.
*
* @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>.)
*/
public V remove(Objectkey) {
Entry<K,V> e = removeEntryForKey(key);
return (e ==null ? null :e.value);
}

/**
* 删除并返回key所对应的entry，若map中不存在该key，返回null。
* Removes and returns the entry associated with the specified key
* in the HashMap.  Returns null if the HashMap contains no mapping
* for this key.
*/
final Entry<K,V> removeEntryForKey(Objectkey) {
if (size == 0) {
return null;
}
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;
Object k;
if (e.hash ==hash &&
((k = e.key) == key || (key !=null && key.equals(k)))) {
modCount++;
size--;
if (prev ==e)
table[i] =next;
else
prev.next =next;
e.recordRemoval(this);
return e;
}
prev = e;
e = next;
}

return e;
}

/**
* 根据Map.Entry.equals方法找出与对象o相匹配的entry，将其删除。
* Special version of remove for EntrySet using {@code Map.Entry.equals()}
* for matching.
*/
final Entry<K,V> removeMapping(Objecto) {
if (size == 0 || !(oinstanceof 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;
}

/**
* 删除map中所有映射，使用Arrays.fill方法将table数组的值都设为null
* Removes all of the mappings from this map.
* The map will be empty after this call returns.
*/
public void clear() {
modCount++;
Arrays.fill(table, null);
size = 0;
}

containXXX类方法

/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value.
*
* @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;
for (inti = 0; i < tab.length ; i++)
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
*/
private boolean containsNullValue() {
Entry[] tab = table;
for (inti = 0; i < tab.length ; i++)
for (Entry e = tab[i] ;e != null ;e = e.next)
if (e.value ==null)
return true;
return false;
}

clone方法

/**
* Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map
*/
public Object clone() {
HashMap<K,V> result = null;
try {
result = (HashMap<K,V>)super.clone();
} catch (CloneNotSupportedExceptione) {
// 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;
}

静态内部类Entry

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 (!(oinstanceof 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();
}

/**
* 当entry的值通过调用put方法被重写时该方法将被调用
* 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) {
}

/**
* 当entry从table中删除时该方法将被调用
* This method is invoked whenever the entry is
* removed from the table.
*/
void recordRemoval(HashMap<K,V>m) {
}
}

entry相关操作

/**
* 添加一个新的entry。
* 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.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(inthash, K key, Vvalue, intbucketIndex) {
if ((size >=threshold) && (null !=table[bucketIndex])) {
resize(2 * table.length);
hash = (null !=key) ? hash(key) : 0;
bucketIndex = indexFor(hash,table.length);
}

createEntry(hash, key, value, bucketIndex);
}

/**
* 创建entry，并将其添加到table[bucketIndex]队首中。
* 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.
*
* Subclass overrides this to alter the behavior of HashMap(Map),
* clone, and readObject.
*/
void createEntry(inthash, K key, Vvalue, intbucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] =new Entry<>(hash,key, value,e);
size++;
}

迭代。。。。（没看）

私有内部类KeySet及相关方法

// 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(); } }

私有内部类Value及其相关方法

/** * 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; // values属于AbstractMap的成员变量 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(); } }

私有内部类EntrySet及相关方法

/** * 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 (!(oinstanceof 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(); // HashMap.this值获取HashMap本身，所调用的clear即HashMap的方法 } }

参考：

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/article/1427759.html