public class HashMap extends AbstractMap
implements Map, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;
//默认的初始容量和最大容量以及扩容后的容量都必须是2的幂
//默认大小16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//默认装载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//桶中存放元素个数大于这个数会转化成红黑树存储
static final int TREEIFY_THRESHOLD = 8;
//红黑树上节点个数小于这个值时,树转换成链表
static final int UNTREEIFY_THRESHOLD = 6;
//hash冲突链表转变为红黑树时,需要先判断此时数组容量
//若是因为数组容量太小导致的冲突太多,则不进行链表转换为红黑树操作,转而使用resize()对hashmap进行扩容
static final int MIN_TREEIFY_CAPACITY = 64;
static class Node implements Map.Entry {
//节点的hash值
final int hash;
//key值
final K key;
//value值
V value;
//指向下一个节点
Node next;
Node(int hash, K key, V value, Node 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; }
//一个节点的hash值是key和value分别取hash再做异或运算
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
//使用新值替换旧值并返回旧值
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
//对比两个note是否相等
public final boolean equals(Object o) {
if (o == this)
return true;
//对象是Entry实例
if (o instanceof Map.Entry) {
Map.Entry e = (Map.Entry)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
//为了使高位数据发挥作用
//hashCode 方法产生的 hash 是 int 类型,32 位宽。前16位为高位,后16位为低位,所以要右移16位
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
static Class comparableClassFor(Object x) {
if (x instanceof Comparable) {
Class c; Type[] ts, as; Type t; ParameterizedType p;
if ((c = x.getClass()) == String.class)
return c;
if ((ts = c.getGenericInterfaces()) != null) {
for (int i = 0; i < ts.length; ++i) {
if (((t = ts[i]) instanceof ParameterizedType) &&
((p = (ParameterizedType)t).getRawType() ==
Comparable.class) &&
(as = p.getActualTypeArguments()) != null &&
as.length == 1 && as[0] == c)
return c;
}
}
}
return null;
}
@SuppressWarnings({"rawtypes","unchecked"})
static int compareComparables(Class kc, Object k, Object x) {
return (x == null || x.getClass() != kc ? 0 :
((Comparable)k).compareTo(x));
}
//返回一个比给定整数大且最接近的2的幂次方整数
//不断的把第一个1开始后面的位变成1
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
//存储元素的数组,transient关键字表示该属性不能被序列化
transient Node[] table;
//将数据转换成set的另一种存储形式,这个变量主要用于迭代功能。
transient Set> entrySet;
//元素数量
transient int size;
//统计该map修改的次数
transient int modCount;
//临界值,也就是元素数量达到临界值时,会进行扩容
int threshold;
//加载因子,这个是变量
final float loadFactor;
public HashMap(int initialCapacity, float loadFactor) {
//初始化的容量不能小于0
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
//初始化容量大于最大容量的话就变成最大容量
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
///检查加载因子是否小于0或不合法
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
//初始化阈值
this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
}
public HashMap(Map m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
final void putMapEntries(Map m, boolean evict) {
int s = m.size();
if (s > 0) {
//初始化数组为空
if (table == null) {
//用负载因子计算
float ft = ((float)s / loadFactor) + 1.0F;
// 与最大容量作比较 返回对应的int类型值
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
//扩容
else if (s > threshold)
resize();
//插入处理
for (Map.Entry e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
public V get(Object key) {
Node e;
//也是调用getNode方法来完成
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node getNode(int hash, Object key) {
//first 头结点,e 临时变量,n 长度,k key
Node[] tab;
Node first, e;
int n;
K k;
//(n - 1) & hash是求模运算,fist即是数组下标节点
if ((tab = table) != null && (n = tab.length) > 0 &&
(first = tab[(n - 1) & hash]) != null) {
//对比第一个节点的hash,key如果相等就返回
if (first.hash == hash &&
((k = first.key) == key || (key != null && key.equals(k))))
return first;
//不是头结点
if ((e = first.next) != null) {
//判断是否是红黑树
if (first instanceof TreeNode)
//如果是红黑树就在红黑树中找
return ((TreeNode)first).getTreeNode(hash, key);
do {//是链表,遍历链表,对比hash和key值,相等就返回
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
public boolean containsKey(Object key) {
return getNode(hash(key), key) != null;
}
public V put(K key, V value) {
//四个参数,第一个hash值,第四个参数表示如果该key存在值,如果为null的话,
// 则插入新的value,最后一个参数,在hashMap中没有用,可以不用管,使用默认的即可
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node[] tab; //哈希数组
Node p; //哈希桶的头结点
int n, i;//n为hashmap长度,i为计算出的数组下标
//获取长度并进行扩容,数组一开始没有加载,put后才开始加载
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
//计算出的结点若没有值,就放入数据,并且将头结点给p
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
//头结点已经有结点,发生哈希冲突
else {
Node e;//临时节点
K k;//存放当前节点的key
//hash值,key值都相等,覆盖
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//hash值不等于首节点,判断是否是红黑树节点
//如果是红黑树节点,则在红黑树中进行添加,如果该节点已经存在,则返回该节点
else if (p instanceof TreeNode)
e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
//hash值不等于首节点,且不是红黑树节点,则是链表节点
else {
//遍历该链表
for (int binCount = 0; ; ++binCount) {
//p.next为空,说明到尾部,在尾部添加新的节点
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//判断是否要转换为红黑树结构
if (binCount >= TREEIFY_THRESHOLD - 1)
treeifyBin(tab, hash);
break;
}
//如果有重复的节点,则结束循环
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//有重复的key,则用待插入值进行覆盖,返回旧值
if (e != null) {
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
//修改次数
++modCount;
//实际长度+1,判断是否大于临界值,大于则扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
//扩容方法
final Node[] resize() {
Node[] oldTab = table;//获取没插入前的哈希数组
int oldCap = (oldTab == null) ? 0 : oldTab.length;//old的长度
int oldThr = threshold;//old的阈值
int newCap, newThr = 0;//新的长度和阈值
//oldCap > 0 说明不是首次初始化,因为hashmap是懒加载
if (oldCap > 0) {
//大于最大值
if (oldCap >= MAXIMUM_CAPACITY) {
//阈值变为整数最大值
threshold = Integer.MAX_VALUE;
return oldTab;
}
//扩容两倍,并且扩容后的长度要小于最大值,old长度也要大于16(初始值)
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
//临界值也扩容为old的临界值2倍
newThr = oldThr << 1;
}
//这种情况是oldcap<0,但已经初始化,类似于将元素删除完了的情况,但临界值还在
else if (oldThr > 0)
newCap = oldThr;
//首次初始化
else {
//16
newCap = DEFAULT_INITIAL_CAPACITY;
//临界值=容量*加载因子
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//阈值没有复制
if (newThr == 0) {
//new的临界值
float ft = (float)newCap * loadFactor;
//判断是否new容量是否大于最大值,临界值是否大于最大值
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
//最终改变阈值的位置
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
//初始化新的数组
Node[] newTab = (Node[])new Node[newCap];
//赋予当前的table
table = newTab;
//此处old中的元素,遍历到new中
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
//临时变量
Node e;
//当前哈希桶位置部位空
if ((e = oldTab[j]) != null) {
//变为空,方便回收
oldTab[j] = null;
//下一个节点没有元素
if (e.next == null)
//变量存入newtab
newTab[e.hash & (newCap - 1)] = e;
//若节点为红黑树,则存在冲突,哈希桶中有多个元素
else if (e instanceof TreeNode)
//把此树转移到newtab
((TreeNode)e).split(this, newTab, j, oldCap);
//此处表示为链表结构,同样把链表转移到newCap中,就是把链表遍历后,把值转过去
else {
Node loHead = null, loTail = null;
Node hiHead = null, hiTail = null;
Node 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;
}
}
}
}
}
//返回扩容后的hashMap
return newTab;
}
final void treeifyBin(Node[] tab, int hash) {
int n, index; Node e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
TreeNode hd = null, tl = null;
do {
TreeNode 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);
}
}
public void putAll(Map m) {
putMapEntries(m, true);
}
public V remove(Object key) {
Node e;
//调用removeNode(hash(key), key, null, false, true)进行删除,第三个value为null,
// 表示,把key的节点直接都删除了,不需要用到值,如果设为值,则还需要去进行查找操作
return (e = removeNode(hash(key), key, null, false, true)) == null ?
null : e.value;
}
//第一参数为哈希值,第二个为key,第三个value,第四个为是为true的话,
// 则表示删除它key对应的value,不删除key,第四个如果为false,则表示删除后,不移动节点
final Node removeNode(int hash, Object key, Object value,
boolean matchValue, boolean movable) {
Node[] tab;//哈希数组
Node p;//p 数组下标的节点
int n, index;//n 长度,index 当前数组下标
////哈希数组不为null,且长度大于0,然后获得到要删除key的节点所在是数组下标位置
if ((tab = table) != null && (n = tab.length) > 0 &&
(p = tab[index = (n - 1) & hash]) != null) {
Node node = null, e; // node存储要删除的节点,e 临时变量
K k;//k 当前节点的key
V v;//v 当前节点的value
////如果数组下标的节点正好是要删除的节点,把值赋给临时变量node
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
node = p;
////也就是要删除的节点,在链表或者红黑树上,先判断是否为红黑树的节点
else if ((e = p.next) != null) {
if (p instanceof TreeNode)
//遍历红黑树,找到该节点并返回
node = ((TreeNode)p).getTreeNode(hash, key);
else {//表示为链表节点,一样的遍历找到该节点
do {
if (e.hash == hash &&
((k = e.key) == key ||
(key != null && key.equals(k)))) {
node = e;
break;
}
//如果进入了链表中的遍历,那么此处的p不再是数组下标的节点,
// 而是要删除结点的上一个结点
p = e;
} while ((e = e.next) != null);
}
}
////找到要删除的节点后,判断!matchValue,我们正常的remove删除,!matchValue都为true
if (node != null && (!matchValue || (v = node.value) == value ||
(value != null && value.equals(v)))) {
//如果删除的节点是红黑树结构,则去红黑树中删除
if (node instanceof TreeNode)
((TreeNode)node).removeTreeNode(this, tab, movable);
////如果是链表结构,且删除的节点为数组下标节点,也就是头结点,直接让下一个作为头
else if (node == p)
tab[index] = node.next;
//为链表结构,删除的节点在链表中,把要删除的下一个结点设为上一个结点的下一个节点
else
p.next = node.next;
//修改计数器
++modCount;
//长度减一
--size;
afterNodeRemoval(node);
////返回删除的节点
return node;
}
}
return null;
}