【Java】HashMap源碼閱讀（JDK1.8）

哈希表底層採用數組+鏈表（紅黑樹）的數據結構：

static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;    //哈希值
        final K key;    //節點的鍵
        V value;    //節點的值
        Node<K,V> next;    //指向下一個節點的引用
    }

transient Node<K,V>[] table;    //這個就是HashMap數組

由於上面的table數組的每一個Node節點都有一個指向下一個節點的引用，因此這就是底層的數組+鏈表結構

TreeNode結構是繼承自LinkedHashMap.Entry的數據結構，用作紅黑樹的節點，而LinkedHashMap.Entry又是繼承上面的Node結構的，因此由於多臺的原因可以放在一個Node數組當中。

 

哈希函數

根據哈希函數可以將key的哈希值散列到數組下標

static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }

 

插入數據過程

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)    //當數組爲空時要初始化
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)    //判斷哈希散列得到的數組位置是否有該元素，沒有則直接插入
            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))))    //有該節點直接覆蓋
                e = p;
            else if (p instanceof TreeNode)    //如果是紅黑樹節點，則插入紅黑樹中
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {    //否則在鏈表尾部插入
                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)    //判斷是否需要擴容
            resize();
        afterNodeInsertion(evict);
        return null;
    }

 

獲取數據過程

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 &&    //判斷表是否爲空及查詢的鍵值對是否爲空
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // always check first node    //如果第一個節點就是要找的元素，則直接返回
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                if (first instanceof TreeNode)    //如果是紅黑樹節點，調用紅黑樹查詢函數
                    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);    //否則就是鏈表，遍歷鏈表
            }
        }
        return null;    //如果遍歷完沒有則返回null
    }

 

擴容

    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) {
            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    //新的容量爲舊容量的兩倍
        }
        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];    //創建新的數組
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {    //複製舊數組到新的數組當中
                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;
    }