HashMap源码阅读(小白的java进阶)
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构造方法
//构造方法
public HashMap(int initialCapacity, float loadFactor) {
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;
this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
内部类
Node
用Node这个内部类存储键值对
Node<K,V>[] table;
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
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);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
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;
}
}
TreeNode
继承了 LinkedHashMap.Entry<K,V>,但后者又继承了HashMap.Node,所以可以认为TreeNode是Node的子类
//HashMap
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V>
//LinkedHashMap
static class Entry<K,V> extends HashMap.Node<K,V>
重要属性
我们还需要区分三个概念
size : 即table这个数组中已经存放的node个数
capacity: 这个数组能够存放的最大node个数,即table.length
threshold:临界值,到达这个临界值后就需要扩容
//默认初始容量(16)
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//转换成树时的阈值
static final int TREEIFY_THRESHOLD = 8;
//未转换成树时的阈值
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
//用于计算哈希值的函数
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
put方法
最初调用的是public V put(K key, V value)
但这个方法不是最终实现的,调用的是putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict)这个方法
//将指定的key与value存入,如果已经有了key,就覆盖先前的值
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
我们来看putVal
//返回值是值的类型
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
//当为空时,给n赋值
n = (tab = resize()).length;
//判断这个hash值是否存在,不存在则new 一个新的node,此时由于只有一个节点,所以next=null
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {//hash值已经存在了,此时的p就是先前存在的node
Node<K,V> e; K k;
//获取已经存在的该节点的key的hash值,如果hash值相等且key值也相等(要么都为null,要么相同)
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//这里是判断元素是否来自LinkedHashMap,暂且不管
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//到这里以后,说明新插入的数据产生了hash冲突,且不是来自LinkedHashMap
//采用尾插法,把新的节点接到链表的末尾
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
//到达阈值了,转换为红黑树
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//已经存在了这个key值,则直接覆盖旧值,并返回原来的值
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
//这里是空实现
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//到达了阈值进行扩容
if (++size > threshold)
resize();
//这里也是空实现
afterNodeInsertion(evict);
return null;
}
将链表树化
首先判断if (binCount >= TREEIFY_THRESHOLD - 1,看已有的节点是否达到了树化的临界值,到达临界值后调用final void treeifyBin(Node<K,V>[] tab, int hash)进行树化
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;
//这个循环做的事就是把链表转换为树结构
do {
//这里主要是把Node转化为TreeNode,就是返回了一个TreeNode对象
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)
//让目前的树变为红黑树
hd.treeify(tab);
}
}
扩容
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//如果大于了最大容量,就不再扩容
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;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((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;
}
remove方法
关注点
- 构造方法
- put方法
- remove方法
- get()方法
- 几个内部类的作用
- 扩容机制
- 与红黑树的转换
