HashMap源码(new(),put(),get())

模拟hash桶的put操作：

HashMap的put(key)流程图

demo1

public static void main(String[] args) {
	Map<Integer,Integer> map = new HashMap<>(); // 初始化
	for (int i=0; i<=1000; i++) {
		map.put(i,i); // 添加元素
	}
	System.out.println(map.get(11)); // 获取元素
}

demo2

public static void main(String[] args) {
	HashMap<Integer,Integer> map = new HashMap<>(); // 初始化
	for(int i=0;i<=70;i++){
		map.put(5+16*i,5+16*i); // 模拟hash碰撞
	}
	System.out.println(map);
}

new()：

//负载因子赋为0.75
public HashMap() {
	this.loadFactor = DEFAULT_LOAD_FACTOR; 
}

put()：

//put
public V put(K key, V value) {
	return putVal(hash(key), key, value, false, true);
}

//hash
//若key==null,hash=0
//key.hashCode()和该hash值右移16位进行异或运算，返回为最终的hash值
static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

//putVal
final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
	Node<K,V>[] tab; Node<K,V> p; int n, i;
	
        //初始化长度为16
	if ((tab = table) == null || (n = tab.length) == 0)
		n = (tab = resize()).length;
        //若hash位置为null，则不存在hash冲突
        //(若n=16,则n-1=15=1111,key在索引范围里&(n-1)结果还等于key,key不在索引范围里&(n-1)相当于对n取余)
	if ((p = tab[i = (n - 1) & hash]) == null)
		tab[i] = newNode(hash, key, value, null);
        //hash冲突
	else {
		Node<K,V> e; K k;
                //key相同，替换
		if (p.hash == hash &&
			((k = p.key) == key || (key != null && key.equals(k))))
			e = p;
                //若为树，添加树节点
		else if (p instanceof TreeNode)
			e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
                //若key不相同且不为树，则以链表形式追加到尾部
		else {
			for (int binCount = 0; ; ++binCount) {
				if ((e = p.next) == null) {
					p.next = newNode(hash, key, value, null);
					if (binCount >= TREEIFY_THRESHOLD - 1) 
                                                //尝试调整为红黑树(若node数组长度小于64依然走扩容)
						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;
	//size+1>阈值时，调整node数组长度
	if (++size > threshold)
		resize();
	afterNodeInsertion(evict);
	return null;
}

//调整node数组长度
final Node<K,V>[] resize() {
	Node<K,V>[] oldTab = table;
	int oldCap = (oldTab == null) ? 0 : oldTab.length;
	int oldThr = threshold;
	int newCap, newThr = 0;
	//扩容为2倍，扩容阈值也变为2倍
	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;
	//初始化容量为16,阈值为16*0.75=12,返回长度为16的node数组
	else {               
		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;
	//node数组复制，长度改变为新容量
	if (oldTab != null) {
                //遍历node数组各node
		for (int j = 0; j < oldCap; ++j) {
			Node<K,V> e;
                        //若node为null直接跳过，不为空才处理
			if ((e = oldTab[j]) != null) {
				oldTab[j] = null;
                                //若node没有next,只需要把node上唯一元素按照新hash算法copy到新node数组里
				if (e.next == null)
					newTab[e.hash & (newCap - 1)] = e;
                                //若node为树节点,则按照树节点处理方式copy过去
				else if (e instanceof TreeNode)
					((TreeNode<K,V>)e).split(this, newTab, j, oldCap);                        
                                //否则该节点为链表，则按照链表节点处理方式copy过去
				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;
}

//尝试调整为红黑树(若node数组长度小于64依然走扩容)
final void treeifyBin(Node<K,V>[] tab, int hash) {
	int n, index; Node<K,V> e;
        //若node数组为null或者node数组长度小于64，走扩容
	if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
		resize();
        //只有当node数组长度大于等于64,才转为红黑树
	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);
	}
}

get()：

//get
public V get(Object key) {
	Node<K,V> e;
	return (e = getNode(hash(key), key)) == null ? null : e.value;
}
//先根据key的hash定位，再根据key定位
final Node<K,V> getNode(int hash, Object key) {
	Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
	if ((tab = table) != null && (n = tab.length) > 0 &&
		(first = tab[(n - 1) & hash]) != null) {
		if (first.hash == hash && // always check first node
			((k = first.key) == key || (key != null && key.equals(k))))
			return first;
		if ((e = first.next) != null) {
			if (first instanceof TreeNode)
				return ((TreeNode<K,V>)first).getTreeNode(hash, key);
			do {
				if (e.hash == hash &&
					((k = e.key) == key || (key != null && key.equals(k))))
					return e;
			} while ((e = e.next) != null);
		}
	}
	return null;
}