java源码阅读-hashmap类

/**
     *初始化大小
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * 最大的大小  2的三十次方
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 负载因子  默认0.75
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
  
   /**
    *某一个链表长度超过8之后将其数据结构转化为红黑树
    */
    static final int TREEIFY_THRESHOLD = 8;

/**
     *基础的节点
     */
    static class Node implements Map.Entry {
        final int hash;
        final K key;
        V value;
        Node next;

        Node(int hash, K key, V value, Node next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }

        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry e = (Map.Entry)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

hash算法

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

构造一个空的hashmap

 /**
     * 根据初始化大小和负载因子来构造一个空的hashmap
     *
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    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);

        // Find a power of 2 >= initialCapacity
        int capacity = 1;
        //保证hashmap的大小是2的倍数
        while (capacity < initialCapacity)
            capacity <<= 1;

        this.loadFactor = loadFactor;
        //当hashmap的count到了这个数之后就要resize
        threshold = (int)(capacity * loadFactor);
        //新建桶?
        table = new Entry[capacity];
        init();
    }

public V get(Object key) {
        if (key == null)
            return getForNullKey();
        //获取hash值
        int hash = hash(key.hashCode());
        //根据hash值获取是哪个桶?
        for (Entry e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            //在桶中比较每一项
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
                return e.value;
        }
        return null;
    }
 public V put(K key, V value) {
        if (key == null)
            return putForNullKey(value);
//获取hash值
        int hash = hash(key.hashCode());
//获取应该在桶中的位置
        int i = indexFor(hash, table.length);
        for (Entry e = table[i]; e != null; e = e.next) {
            Object k;
//如果这个key已经存在了  那么替换  并且返回原有的v
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }
        //修改次数+1
        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

比较尴尬了 写到现在发现是1.6版本的 后面开始写1.8的 源码要难看很多,但是是大神写的 有红黑树等等

    void addEntry(int hash, K key, V value, int bucketIndex) {
    Entry e = table[bucketIndex];
//添加到链表最前方
        table[bucketIndex] = new Entry(hash, key, value, e);
//如果size大于了要resize的了  那么resize
        if (size++ >= threshold)
            resize(2 * table.length);
    }

-----------------------------------我是分割线-----------------------------
这里开始写jdk1.8的hashmap源码

  //设置初始化大小,负载因子和threshold的大小
    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);
    }

下面来看threshold的大小是如何设置的
这段代码很巧妙,将cap的从有数开始的每一位置为1 最后再加一 得到得数是2的倍数,<=cap的2倍 》=cap

    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;
    }

有一点比较不解的是尼玛threshold的注释写的是cap*loadFactor啊 fuck
下面我们看get方法 调用了getNode

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

getNode方法如下 套路还是差不多,不过是先判断first节点是不是一个TreeNode,如果是TreeNode那么就要使用红黑树的获取节点的方式了,不然就是一个一个比较

 final Node getNode(int hash, Object key) {
        Node[] tab; Node 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)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;
    }

下面看一下红黑树是怎么获取的

//TODO 过于复杂了  以后再看

下面看一下put方法是怎么回事,调用了putval方法

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

方法一看就比较复杂,待会儿看resize()

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node[] tab; Node p; int n, i;
        //如果桶为空,桶的长度为0 resize   
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //如果key所在的桶中没有元素 那么新建一个NOde就好
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node e; K k;
//如果元素key相同
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
//如果p是红黑树
            else if (p instanceof TreeNode)
//那么往红黑树中添加元素
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else {
//如果不是红黑树
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
//如果这个桶的长度大于6?  那么就把这个桶变成红黑数
                        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;
    }

1.8之后的桶好像不怎么需要resize了
下面要看三个方法 resize,putTreeVal,treeifyBin

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;
            }
//大概是*2这个样子
            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;
//将元素放入新的桶中
        if (oldTab != null) {
//遍历每一个桶
            for (int j = 0; j < oldCap; ++j) {
                Node e;

                if ((e = oldTab[j]) != null) {
//如果桶中元素不为空
                    oldTab[j] = null;
                    if (e.next == null)
//桶中只有e一个元素  将e加入到新的数组中去
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
//如果是红黑树,将树中所有元素加入新的桶
                        ((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;
    }

TODO过于复杂 日后再看

 final TreeNode putTreeVal(HashMap map, Node[] tab,
                                       int h, K k, V v) {
            Class kc = null;
            boolean searched = false;
            TreeNode root = (parent != null) ? root() : this;
            for (TreeNode p = root;;) {
                int dir, ph; K pk;
                if ((ph = p.hash) > h)
                    dir = -1;
                else if (ph < h)
                    dir = 1;
                else if ((pk = p.key) == k || (k != null && k.equals(pk)))
                    return p;
                else if ((kc == null &&
                          (kc = comparableClassFor(k)) == null) ||
                         (dir = compareComparables(kc, k, pk)) == 0) {
                    if (!searched) {
                        TreeNode q, ch;
                        searched = true;
                        if (((ch = p.left) != null &&
                             (q = ch.find(h, k, kc)) != null) ||
                            ((ch = p.right) != null &&
                             (q = ch.find(h, k, kc)) != null))
                            return q;
                    }
                    dir = tieBreakOrder(k, pk);
                }

                TreeNode xp = p;
                if ((p = (dir <= 0) ? p.left : p.right) == null) {
                    Node xpn = xp.next;
                    TreeNode x = map.newTreeNode(h, k, v, xpn);
                    if (dir <= 0)
                        xp.left = x;
                    else
                        xp.right = x;
                    xp.next = x;
                    x.parent = x.prev = xp;
                    if (xpn != null)
                        ((TreeNode)xpn).prev = x;
                    moveRootToFront(tab, balanceInsertion(root, x));
                    return null;
                }
            }
        }

好了 结束了 先不管红黑树怎么实现的了

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