HashMap部分源码简析

有参构造

HashMap

/**
     * Constructs an empty <tt>HashMap</tt> with the specified initial
     * capacity and load factor.
     *
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
//容量、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;
    //确保容量是2的整数次幂
    this.threshold = tableSizeFor(initialCapacity);
}

tableSizeFor

/**
     * Returns a power of two size for the given target capacity.
     */
//返回大于输入参数且最近的2的整数次幂的数。比如10，则返回16
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;
}

put方法

put

/**
     * Associates the specified value with the specified key in this map.
     * If the map previously contained a mapping for the key, the old
     * value is replaced.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
     *         (A <tt>null</tt> return can also indicate that the map
     *         previously associated <tt>null</tt> with <tt>key</tt>.)
     */
//调用hash(Object key)函数获取它的hashcode高16位与低16位异或后的值
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

hash

//返回hashcode高16位与低16位异或后的值
//如果key为null，直接返回0，即key为null放置在数组'0'下标处
static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

putVal

/**
     * Implements Map.put and related methods.
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to put
     * @param onlyIfAbsent if true, don't change existing value
     * @param evict if false, the table is in creation mode.
     * @return previous value, or null if none
     */
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //如果table数组未初始化或长度为0，则进行扩容
    if ((tab = table) == null || (n = tab.length) == 0)
        //扩容
        n = (tab = resize()).length;
    //将hash值与数组长度-1做与运算获得所在下标
    //即(hash & (table.length-1)) 等价于 hash % table.length)
    //位运算速度更快
    //如果数组该下标处元素为空，直接放入
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //数组该下标处元素不为空
        //检查与第一个元素是否相等，若相等直接替换
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //否则如果是红黑树，则遍历树
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        //否则如果是链表，则遍历链表
        else {
            //遍历链表
            for (int binCount = 0; ; ++binCount) {
                //如果遍历到链表尾部仍没找到key相等的结点，则在尾部插入，并结束循环
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    //如果长度达到阈值，则转红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //如果在链表中找到key相等的结点，则跳出循环
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        //如果在链表中找到key相等的结点，则返回旧值
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    ++modCount;
    //如果容量达到阈值，则扩容
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

resize方法

resize

/**
     * Initializes or doubles table size.  If null, allocates in
     * accord with initial capacity target held in field threshold.
     * Otherwise, because we are using power-of-two expansion, the
     * elements from each bin must either stay at same index, or move
     * with a power of two offset in the new table.
     *
     * @return the table
     */
final Node<K,V>[] resize() {
    //保存当前的table
    Node<K,V>[] oldTab = table;
    //保存当前table的容量
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    //保存当前的阈值
    int oldThr = threshold;
    //新容量，新阈值
    int newCap, newThr = 0;
    //如果当前容量大于0
    if (oldCap > 0) {
        //如果当前容量大于最大容量（static final int MAXIMUM_CAPACITY = 1 << 30;）
        if (oldCap >= MAXIMUM_CAPACITY) {
            //就将阈值设为最大整型
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //将容量翻倍后小于最大容量 并且 当前容量大于等于默认容量（static final int DEFAULT_INITIAL_CAPACITY = 1 << 4 就是16）
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            //新阈值翻倍
            newThr = oldThr << 1; // double threshold
    }
    //当前容量等于0，阈值大于0
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    //当前容量和当前阈值都为0
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    //新阈值等于0
    if (newThr == 0) {
        //重新设置阈值
        float ft = (float)newCap * loadFactor;
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    //当前阈值改为新阈值
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    //创建一个新容量长度的数组
    Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    //当前数组改为新数组
    table = newTab;
    //旧table不为null
    if (oldTab != null) {
        //遍历整个hashmap数组
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            //如果某一下标处元素不为null
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                //只有一个结点
                if (e.next == null)
                    //直接放入新table中
                    newTab[e.hash & (newCap - 1)] = e;
                //如果为红黑树
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                //如果为链表
                else { // preserve order
                    Node<K,V> loHead = null, loTail = null;
                    Node<K,V> hiHead = null, hiTail = null;
                    Node<K,V> next;
                    //将旧链表拆分成两条新链表
                    do {
                        next = e.next;
                        //拆分链表的条件
                        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);
                    //两条新链表相隔一个oldCap这么远
                    if (loTail != null) {
                        loTail.next = null;
                        newTab[j] = loHead;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    //返回新table
    return newTab;
}