java集合类史上最细讲解 - HashMap篇

• 1.HashMap的底层机制
• 2.HashMap源码解读

底层存放数据示意图：

k，v是一个Node实现了Map.Entry

jdk8以上底层为数组+链表+红黑树

解读代码：

Map map = new HashMap();
// 添加键值对
map.put("no1","dahe");
map.put("no2","zhangsan");
// Key重复会进行替换
map.put("no1","lisi");

新建HashMap对象，我们步入看看

首先，会进行初始化加载因子，加载因子的值为0.75

public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

---

static final float DEFAULT_LOAD_FACTOR = 0.75f;

开始执行put方法，通过一通计算hash值，传入参数进入putVal方法

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

putVal方法：（不要慌，详细的代码解读可以参见HashSet篇，接下来我们来debug看一下具体的代码执行过程）

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node[] tab; Node p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node 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)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;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

此时的table表为空。我们进行resize操作

if ((tab = table) == null || (n = tab.length) == 0)
    n = (tab = resize()).length;

老样子，按规则进行扩容（规则详见HashSet篇）

final Node[] resize() {
    Node[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int 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 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[] newTab = (Node[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        for (int j = 0; j < oldCap; ++j) {
            Node 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)e).split(this, newTab, j, oldCap);
                else { // preserve order
                    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;
                    }
                }
            }
        }
    }
    return newTab;
}

接下来，通过hash值计算table的某个位置有没有数据存在，如果没有的话，新建一个Node节点，挂在table数组上

if ((p = tab[i = (n - 1) & hash]) == null)
    tab[i] = newNode(hash, key, value, null);

这样一来，第一个数据就添加成功啦！

继续步入，直到第二个元素添加完成，关键来了，开始添加第三个元素

步入步入，到达比较hash值的部分，由于新增的对象的hash值和第一个一致且key值相等，则执行e=p操作

if (p.hash == hash &&
    ((k = p.key) == key || (key != null && key.equals(k))))
    e = p;

继续向下执行，会替换第一个键值对key所对应的值为新增的lisi

if (e != null) { // existing mapping for key
    V oldValue = e.value;
    if (!onlyIfAbsent || oldValue == null)
        e.value = value;
    afterNodeAccess(e);
    return oldValue;
}

分析完毕