Java教程
HashMap 1.8 源码
本文主要是介绍HashMap 1.8 源码，对大家解决编程问题具有一定的参考价值，需要的程序猿们随着小编来一起学习吧！
public class HashSet<E> {//1.8版本
private transient HashMap<E,Object> map;
//1-1. 创建一个HashMap对象，并且调用无参构造函数
public HashSet() {
map = new HashMap<>();
}
public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}
public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}
//2-1. 添加第一个元素，调用add方法
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}
//-----------HashMap源码---------------------------
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
transient Node<K,V>[] table;
//1-2. 负载因子赋值为默认值0.75
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
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);
}
//2-2. 进入HashMap的put方法中
public V put(K key, V value) {
//2-3. key = Student{id=1, name='lili'} value=PRESENT,对key进行hash运算
//2-5. 走putVal方法
return putVal(hash(key), key, value, false, true);
}
//2-4. 扰动函数，避免hash碰撞，每个版本的算法不一样。
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//2-5. 此时数组为空，所以进if
if ((tab = table) == null || (n = tab.length) == 0)
//2-6. 走resize方法,resize方法返回一个长度为16的Node[]
//2-14. n = 16
n = (tab = resize()).length;
//2-15. (n - 1) & hash 根据这个表达式算出元素在数组的下标位置
//2-16. (p = tab[i = (n - 1) & hash]) == null 判断数组中该位置是否已经有数据了
if ((p = tab[i = (n - 1) & hash]) == null)
//2-17. 创建一个Node，然后放到数组对应下标的位置上
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))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
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
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//4-1.如果超过扩容边界值12，就扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);//啥也没干
return null;
}
final Node<K,V>[] resize() {
//2-7 oldTab = null;
//4-2. oldTab = new Node[16];
Node<K,V>[] oldTab = table;
//4-3 oldCap = 16
int oldCap = (oldTab == null) ? 0 : oldTab.length;//oldCap = 0
//4-4 oldThr = 12
int oldThr = threshold;// oldThr = 0;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//4-5. (newCap = oldCap << 1 newCap = oldCap*2=32 数组长度扩展为原来的2倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // newThr=24，扩容临界也扩为原来的2倍
}
else if (oldThr > 0)
newCap = oldThr;
else {
//2-8. newCap = 16;
newCap = DEFAULT_INITIAL_CAPACITY;
//2-9. newThr = 12;
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);
}
//2-10. threshold = 12
//4-6. threshold=24
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
//2-11. 创建一个newTab=Node[16]的数组
//4-7 把原来的数组扩展成一个newTab=Node[32]的数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
//2-12.table = newTab=Node[16] 主数组长度默认为16
table = newTab;
//4-8.进入if
if (oldTab != null) {
//4-9.oldCap = 16
/*
整个循环的意思就是对于之前数组中的元素再次根据e.hash & 数组.length公式算出在扩容后的数组的下标位置,
1.如果当前元素没有链表，则按照e.hash & (newCap - 1)，算下标位置
2.如果当前元素下面有链表（链表里每一个元素都重新算），则按照e.hash & oldCap，算下标位置
2-1.如果算出(e.hash & oldCap) == 0，则还放在原来在老数组的对应下标位置；
2-2.如果算出(e.hash & oldCap) ！= 0，则放在j + oldCap的下标位置中；
*/
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;
//1.8 引入的红黑树
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;
}
}
}
}
}
//2-13. 返回一个长度为16的Node[]
return newTab;
}
Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
return new Node<>(hash, key, value, next);
}
final TreeNode<K,V> putTreeVal(HashMap<K,V> map, Node<K,V>[] tab,
int h, K k, V v) {
Class<?> kc = null;
boolean searched = false;
TreeNode<K,V> root = (parent != null) ? root() : this;
for (TreeNode<K,V> p = root;;) {
int dir, ph; K pk;
if ((ph = p.hash) > h)
dir = -1;
else if (ph < h)
dir = 1;
else if ((pk = p.key) == k || (k != null && k.equals(pk)))
return p;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0) {
if (!searched) {
TreeNode<K,V> q, ch;
searched = true;
if (((ch = p.left) != null &&
(q = ch.find(h, k, kc)) != null) ||
((ch = p.right) != null &&
(q = ch.find(h, k, kc)) != null))
return q;
}
dir = tieBreakOrder(k, pk);
}
TreeNode<K,V> xp = p;
if ((p = (dir <= 0) ? p.left : p.right) == null) {
Node<K,V> xpn = xp.next;
TreeNode<K,V> x = map.newTreeNode(h, k, v, xpn);
if (dir <= 0)
xp.left = x;
else
xp.right = x;
xp.next = x;
x.parent = x.prev = xp;
if (xpn != null)
((TreeNode<K,V>)xpn).prev = x;
moveRootToFront(tab, balanceInsertion(root, x));
return null;
}
}
}
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);
}
}
void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }
}
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