HashMap源码阅读（一）
HashMap继承抽象类AbstractMap，AbstractMap抽象类实现了Map接口
一、HashMap中的静态常量
//默认初始容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大长度
static final int MAXIMUM_CAPACITY = 1 << 30;
//负载因子，map中存储的数据在达到负载因子时需要进行扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//哈希桶中存储的链表长度的阈值，当链表长度达到阈值时会转化为红黑树-->树化
static final int TREEIFY_THRESHOLD = 8;
//当哈希桶中存储的链表的长度小于该阈值时，如果发生了树化，则会将树砖换成链表-->反树化
static final int UNTREEIFY_THRESHOLD = 6;
//用于指示哈希表进行重新哈希操作时，何时会将链表转换为红黑树
static final int MIN_TREEIFY_CAPACITY = 64;
二、HashMap中的Node节点
Node节点是用于存储存储哈希表中的键值对的结构通过nent变量，将出现冲突的元素连成一个链表
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;//元素的hash值
final K key;//元素的Key
V value;//元素的值
Node<K,V> next;//该元素所连接的下一个节点
//构造方法
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
//替换Node里面的value
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
return o instanceof Map.Entry<?, ?> e
&& Objects.equals(key, e.getKey())
&& Objects.equals(value, e.getValue());
}
}
三、HashMap中的成员变量
//用于存储所有链表的头节点
transient Node<K,V>[] table;
//保存缓存
transient Set<Map.Entry<K,V>> entrySet;
//map的实际长度
transient int size;
transient int modCount;
//用于调整容量的笑一个容量值
int threshold;
//实际的负载因子
final float loadFactor;
四、HashMap的构造函数
public HashMap(int initialCapacity, float loadFactor) {
//判断初始长度是否合法
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " + initialCapacity);
//判断设置的初始长度是否大于hash表所设置的最大的长
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
//判断负载因子是否合法
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +loadFactor);
this.loadFactor = loadFactor;
//初始长度需要是2^n形式
this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
//构造方法中传入一个map创建对象
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
//将参数中的map添加到当前的map中
putMapEntries(m, false);
}
五、Map中的简单方法
1、将一个map里面的多有值添加到当前map中
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // 初始化
//获取数组的最小长度
float ft = ((float)s / loadFactor) + 1.0F;
//判断获取的最小长度是否大于map所支持的最大长度
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
} else {
//判断传入的map的长度是否大于实际的，并进行扩容
while (s > threshold && table.length < MAXIMUM_CAPACITY)
resize();
}
//遍历参数中的map并将map里面的键值对存储在当前的map中
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
2、查询map的长度和判空
//查询map长度
public int size() {
return size;
}
//判断map是否为空
public boolean isEmpty() {
return size == 0;
}
3、根据键进行查询
3.1根据键查找值
//根据key查询value
public V get(Object key) {
Node<K,V> e;
return (e = getNode(key)) == null ? null : e.value;
}
3.2根据键查找Node
final Node<K,V> getNode(Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n, hash; K k;
//判断table[(n - 1) & (hash = hash(key))]中是否为null,并对数据进行赋值
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & (hash = hash(key))]) != null) {
//判断第一个结点
if (first.hash == hash &&
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
//判断该节点是否树化
if (first instanceof TreeNode)
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
//没有树化，通过遍历查找key
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
3.3判断键是否存在
public boolean containsKey(Object key) {
return getNode(key) != null;
}
4、添加
4.1添加
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/**
* hash:key的hash值
* key
* value
* onlyIfAbsent：为true是，出现一样的key不会覆盖value
* evict：为true时，处于创建模式
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {
//p为当前节点-->currentNode
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判断table是否为空，若为空执行resize方法进行初始化
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//判断(n - 1) & hash下标处是否为空，若为空则添加节点为头节点，直接创建
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
//判断key与头节点的key是否相同
e = p;
else if (p instanceof TreeNode)
//判断链表是否树化,直接向树中添加节点
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
//判断是否是最后一个节点
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//判断是否达到树化的阈值，对链表树化，或对数组进行扩容
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//判断当前节点的key是否与key相同
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//e = p.next,改行代码等价于p = p.next
p = e;
}
}
//e != null => map中存在key：将旧值改为新值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//判断是否达到扩容的阈值
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
4.2将一个map添加到当前map中
public void putAll(Map<? extends K, ? extends V> m) {
putMapEntries(m, true);
}
5、扩容和初始化方法
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//旧的table的长度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//threshole：下一个容量
int oldThr = threshold;
//newCal:新table的长度，newThr：新的threshold
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
//将就的值全部填入新的数组中
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;//垃圾回收
if (e.next == null)//只有一个节点
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)//已树化，将树进行拆分
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
6、链表长度过长，树化或扩容
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
//
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
hd.treeify(tab);
}
}
7、删除
public V remove(Object key) {
Node<K,V> e;
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
//matchValue：当value相同时删除否则不删除
final Node<K,V> removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node<K,V>[] tab; Node<K,V> p; int n, index;
//判断是否为空（数组、key所对应的下标处）
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node<K,V> node = null, e; K k; V v;
//判断头节点key
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
else if ((e = p.next) != null) {
if (p instanceof TreeNode)
node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
else {
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
p = e;
} while ((e = e.next) != null);
}
}
//判断key对应的node是否存在
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
if (node instanceof TreeNode)
((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
else if (node == p)
tab[index] = node.next;
else
p.next = node.next;
++modCount;
--size;
afterNodeRemoval(node);
return node;
}
}
return null;
}
8、清空链表
public void clear() {
Node<K,V>[] tab;
modCount++;
if ((tab = table) != null && size > 0) {
size = 0;
for (int i = 0; i < tab.length; ++i)
tab[i] = null;
}
}
9、判断链表中是否存在某个值
public boolean containsValue(Object value) {
Node<K,V>[] tab; V v;
if ((tab = table) != null && size > 0) {
for (Node<K,V> e : tab) {
for (; e != null; e = e.next) {
if ((v = e.value) == value ||
(value != null && value.equals(v)))
return true;
}
}
}
return false;
}