HashMap2

HashMap 源码重识

⭐ HashMap 重要的变量

// 默认的初始容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

// 最大的容量，且必须是2的倍数 
static final int MAXIMUM_CAPACITY = 1 << 30;

// 默认的负载因子
static final float DEFAULT_LOAD_FACTOR = 0.75f;

//链表转成红黑树的默认值
static final int TREEIFY_THRESHOLD = 8;

//反向转换回链表的最大值
static final int UNTREEIFY_THRESHOLD = 6;

//可将其分类为树木的最小桌子容量。（否则，如果bin中的节点过多，则将调整表的大小。） 
//至少应为4 * TREEIFY_THRESHOLD,以避免冲突在调整大小和树化阈值之间。
static final int MIN_TREEIFY_CAPACITY = 64;

//该表在首次使用时初始化，并根据需要调整大小。 分配时，长度始终是2的幂。 （在某些操作中，我们还允许长度为零，以允许使用当前不需要的引导机制。）
transient Node<K,V>[] table;

// map 内元素的size
transient int size;

⭐ 构造方法

// 默认负载因子为 0.75 初始化容量是 16   
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

// 指定初始化容量 以及负载因子
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);
}

// 对于给定的目标容量，返回两倍大小的幂。
static final int tableSizeFor(int cap) {
    int n = -1 >>> Integer.numberOfLeadingZeros(cap - 1);
    return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}

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

• 注：

  如果我们传入参数为 new HashMap(10); this.threshold = tableSizeFor(initialCapacity);为16

  当你new HashMap(17); this.threshold = tableSizeFor(initialCapacity);为32

⭐ Put 方法

Map<String,String> map = new HashMap<>(17);

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
	// 初始话数组 			
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    // 当进行第一put时，会通过resize 进行tab的初始化
    // 这时我们的经过 resize() tab.length = 32
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //这里进行 散列，将数据放入 tab 中
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        // 当 key 重复了
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //当节点为tree的时进行
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //循环像链表中添加
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    // 链表的长度 大于 8 转成 tree
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                // key已经存在直接覆盖value
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        // 当key重复，直接替换value 并返回原来的value
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    // 这里将 size 进行 ++ ，并判断与threshold的关系，进行tab的阔容
    // 例如 ： 17 > 32 > 32 * 0.75 > 24 
    // 这里大概的意思就是，我们在构造方法传入 17 ，但是经过计算 threshold =32 ，经过resize 后24
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

⭐ resize()方法

    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        //这是的 threshold 经过构造方法为 32
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
            //超过最大值就等于最大值
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //没超过最大值，就扩充为原来的2倍
            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;
        // 计算新的resize上限
        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;
        //进行数组的大小赋值，
        @SuppressWarnings({"rawtypes","unchecked"})
        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)
                        ((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;
                        } 
                        // 原索引+oldCap放到bucket里
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

⭐ treeifyBin()方法

final void treeifyBin(Node<K,V>[] tab, int hash) {
        int n, index; Node<K,V> e;
        // tab 为 null 或者 length < 64 重新计算长度
        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 {
                // 进行Tree的创建 循环勾结节点
                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)
                //形成具体的树
                hd.treeify(tab);
        }
    }

---

⭐ Get方法

public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

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) {
            //这是特殊的判断，为了减少耗时，每次都乐观的假设一个节点就是目标数据
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
                // 树节点查询
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                // 否则就循环的查询
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }

⭐ getTreeNode

final TreeNode<K,V> getTreeNode(int h, Object k) {
    return ((parent != null) ? root() : this).find(h, k, null);
}

//使用给定的哈希和密钥查找从根p开始的节点。
final TreeNode<K,V> find(int h, Object k, Class<?> kc) {
    TreeNode<K,V> p = this;
    do {
        int ph, dir; K pk;
        TreeNode<K,V> pl = p.left, pr = p.right, q;
        //  重置为 左节点
        if ((ph = p.hash) > h)
            p = pl;
        // 重置为右节点
        else if (ph < h)
            p = pr;
        else if ((pk = p.key) == k || (k != null && k.equals(pk)))
            return p;
        else if (pl == null)
            p = pr;
        else if (pr == null)
            p = pl;
        //说如果实现了 compare 
        else if ((kc != null ||
                  (kc = comparableClassFor(k)) != null) &&
                 (dir = compareComparables(kc, k, pk)) != 0)
            p = (dir < 0) ? pl : pr;
        // 递归
        else if ((q = pr.find(h, k, kc)) != null)
            return q;
        else
            p = pl;
    } while (p != null);
    return null;
}