HashMap核心代码解析 (主要是扩容 添加 删除)

首先要记住核心, HashMap的底层是数组+单向链表, 超过THRESHOLD设定的值 对应的数组下标下的链表会转换成红黑树(但是将链表转换成红黑树前会判断，如果当前数组的长度小于 64，那么会选择先进行数组扩容，而不是转换为红黑树）
没有深入了解红黑树 但是不影响看代码 有关tree的就是涉及到红黑树的
删了很多的简单的代码. 总体看下来官方代码就是清晰,认真看完一个其他发现其实也差不多
一定要根据数据结构去看

public class HashMap extends AbstractMap
    implements Map, Cloneable, Serializable {
    private static final long serialVersionUID = 362498820763181265L;
    //初始容量 
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
    // 最大值 超过20亿
    static final int MAXIMUM_CAPACITY = 1 << 30;
   	// 超过容量* LOAD_FACTOR 就会扩容 (16 和 0.75 肯定是经过大量计算得出的)
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    //单个链表长超过就会转换成红黑树
    static final int TREEIFY_THRESHOLD = 8;
    //从树转换回来
    static final int UNTREEIFY_THRESHOLD = 6;
    //hashMap中元素数量大于64时，也会转为红黑树结构。
    static final int MIN_TREEIFY_CAPACITY = 64;

	//就是链表中的每个节点
    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 int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        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;
        }
    }

    static final int hash(Object key) {
        int h;
        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
    }
    //返回最接近的2的幂次方
    static final int tableSizeFor(int cap) {
    }
	//就是存储节点和链表的数组(数据存放的地方)
    transient Node[] table;
    //用于转换进行迭代
    transient Set> entrySet;

    //元素的数量
    transient int size;
    //结构改变的次数
    transient int modCount;
    // 决定何时绝定扩容
    int threshold;
    final float loadFactor;
	//四种构造方法
    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);
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }
    public HashMap(Map m) {
        this.loadFactor = DEFAULT_LOAD_FACTOR;
        putMapEntries(m, false);
    }

    //将Map集合放入
    final void putMapEntries(Map m, boolean evict) {
        int s = m.size();
        if (s > 0) {
            if (table == null) { // pre-size
                float ft = ((float)s / loadFactor) + 1.0F;
                int t = ((ft < (float)MAXIMUM_CAPACITY) ?
                         (int)ft : MAXIMUM_CAPACITY);
                if (t > threshold)
                    threshold = tableSizeFor(t);
            }
            else if (s > threshold)//判断是否扩容
                resize();
            for (Map.Entry e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);
            }
        }
    }
	// 查找对应的key的value
    public V get(Object key) {
        Node e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    //查找key对应的node
    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) { // 盲猜通过tab[(n - 1) & hash]找到是哪个数组下标
            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)//如果是树节点就按照红黑树查找
                    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;
    }
	
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    // 放入节点值或者更新节点值的方法
    //第四个参数表示如果该key存在值，如果为null的话，则插入新的value
    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)// 对应的数组下标没有值才插入 否则p就是对应的下标的头结点
            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)//去红黑树的方法进行更新
                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);//判断是否需要进行转换成红黑树了
                        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;
    }

   //扩容函数
    final Node[] resize() {
        Node[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {// 扩容为int的最大或者扩容两倍
            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 扩容因子也变为两倍
        }
        else if (oldThr > 0) // 如果扩容因子被初始化了就更换
            newCap = oldThr;
        else {               // 代表使用默认的
            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;//根据上面不同的情况判断后进行跟新
        @SuppressWarnings({"rawtypes","unchecked"})
        Node[] newTab = (Node[])new Node[newCap];//扩容
        table = newTab;
        //把就数组遍历到新的数组中 并把旧数组置为null 释放内存
        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 把对应数组下的链表更新过去
                        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;
    }

    // 进行扩容或者 更换转换成树
    final void treeifyBin(Node[] tab, int hash) {
        int n, index; Node e;
        if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)//扩容
            resize();
        else if ((e = tab[index = (n - 1) & hash]) != null) {//转换成红黑树
            TreeNode hd = null, tl = null;
            do {
                //循环链表, 把节点转换成树叶子
                TreeNode 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)
                hd.treeify(tab);
        }
    }


    public V remove(Object key) {
        Node e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }
    // value为值代表可以直接删除,不然需要先进行查找 match表示只有value相等时才能删除, movable表示删除后是否移动节点
    final Node removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node[] tab; Node p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) { //找到要删除key节点对应的数组下标
            Node node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))//先看头结点是否满足
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode) // 检查是否是树叶子
                    node = ((TreeNode)p).getTreeNode(hash, key);
                else {//遍历进行查找
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e; // node是要删除的节点
                            break;
                        }
                        p = e; //p是要删除的节点的上一个节点
                    } while ((e = e.next) != null);
                }
            }
            //正常!matchvalue表示不用判断值
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);//此方法在hashMap中是为了让子类去实现，主要是对删除结点后的链表关系进行处理
                return node;
            }
        }
        return null;
    }

    
    public void clear() {
        Node[] tab;
        modCount++;
        if ((tab = table) != null && size > 0) {
            size = 0;
            for (int i = 0; i < tab.length; ++i)
                tab[i] = null;
        }
    }

    
    public boolean containsValue(Object value) {
        Node[] tab; V v;
        if ((tab = table) != null && size > 0) {
            for (int i = 0; i < tab.length; ++i) {
                for (Node e = tab[i]; e != null; e = e.next) {
                    if ((v = e.value) == value ||
                        (value != null && value.equals(v)))
                        return true;
                }
            }
        }
        return false;
    }

//这三个方法表示的是在访问、插入、删除某个节点之后，进行一些处理，它们在linkedHashMap都有各自的实现(就是子类会根据需要进行设计)
    // Callbacks to allow linkedHashMap post-actions
    void afterNodeAccess(Node p) { }
    void afterNodeInsertion(boolean evict) { }
    void afterNodeRemoval(Node p) { }