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KeyV 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valuepublic 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keydo {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-->currentNodeNode<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 1sttreeifyBin(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.nextp = e;}}//e != null => map中存在key：将旧值改为新值if (e != null) { // existing mapping for keyV 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thresholdint 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 thresholdnewCap = oldThr;else {               // zero initial threshold signifies using defaultsnewCap = 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 orderNode<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;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.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keyif (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;elsep.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;
}