JDK1.8 HashMap

//構造方法
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);
	//加載因子，當加載
        this.loadFactor = loadFactor;
	//當table length大于threshold将resize，threshold = capacity*loadFactor，這裡雖然為2的幾次方
	//在每一次resize的時候将變成threshold = capacity*loadFactor
        this.threshold = tableSizeFor(initialCapacity);
}
//将傳入自定義初始化大小變成2的幾次方
static final int tableSizeFor(int cap) {
        int n = cap - 1;
        n |= n >>> 1;
        n |= n >>> 2;
        n |= n >>> 4;
        n |= n >>> 8;
        n |= n >>> 16;
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}



public HashMap(int initialCapacity){
	this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

public HashMap(){
	this.loadFactor = DEFAULT_LOAD_FACTOR;
}


public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }
    //擷取key的hashcode異或自己高位(前16位)，原因是散列值分布的再松散，要是隻取後幾位的話，
    //碰撞也很會嚴重(因為後面需要&table的長度如果table的長度為16，減1之後二進制為1111，任何數&1111隻會取後4位),
    //這裡将自己的高半區和低關區做異或，混合後的低們摻雜了高位的部分特征，這樣高位的資訊也被變相的保留下來,以此加大低位的随機性
     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;
        if ((tab = table) == null || (n = tab.length) == 0)
        //初始化table
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
        //新插入的值在table[i]的位置沒有發現碰撞，将新插入的值作為首值
            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)
        //如果首值為紅黑樹，将後續的kv插入到紅黑樹中
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
        //将kv插入到連結清單
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
            //如果連結清單長度>=7，将連結清單變成紅黑樹，提高查詢效率
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
            //如果目前key已經存在
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
        //如果目前key已經存在替換value
            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)
        //擴容
            resize();
        afterNodeInsertion(evict);
        return null;
    }



       final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
    //oldCap 初始化為0，後面為2的幾次方(最少4次方)
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
        //擴容
                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"})
    //新的數組
        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)
            //紅黑樹，後續再寫
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
            //這邊比較巧的地方是有些值的hash當table的length變大之後，
            //會産生新的hash值，需要将值重新放入新的table的位置lo低位置，hi為高位置
            //table變成2位，lo為原位置，hi為高位置(不好解釋 --！)
                        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;
    }