HashMap 原理与源码解析

1、原理

2、源码解析

2.1、构造方法

public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity < 0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    //容量最大为2的30次方，
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    //设置扩容因子
    this.loadFactor = loadFactor;
    //设置大小，大于输入值的最小2的幂方
    this.threshold = tableSizeFor(initialCapacity);
}

2.2、put 方法

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //判断数组结构是否存在，如不存在，建立并获取长度
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //根据key的hash值判断key是否存在于数组中，不存在就放入，存在往下走去链表结构
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //判断hash值是否匹配
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //判断红黑树结构还是链表结构
        else if (p instanceof TreeNode)
            //数据放入红黑树结构
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            for (int binCount = 0; ; ++binCount) {
                //链表遍历
                if ((e = p.next) == null) {
                    //存放数据
                    p.next = newNode(hash, key, value, null);
                    //链表长度大于8，结构转换为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //遇到相同的key，新数据覆盖旧数据
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                p = e;
            }
        }
        /如果是新数据覆盖旧数据，返回旧数据
        if (e != null) { // existing mapping for key
            V oldValue = e.value;
            if (!onlyIfAbsent || oldValue == null)
                e.value = value;
            afterNodeAccess(e);
            return oldValue;
        }
    }
    //修改次数加一
    ++modCount;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

2.3、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;
    //判断数据结构是否存在
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        //是数组中元素，即链表头节点，返回
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        //不是数组中元素，在其后的结构中（链表或红黑树）
        if ((e = first.next) != null) {
            //根据头节点，判断数据结构
            if (first instanceof TreeNode)
                //从红黑树中获取值
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            //遍历列表，获取值
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

2.4、resize 方法（初始化数组结构或者扩容）

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    if (oldCap > 0) {
        //超过最大容量，不能扩容
        if (oldCap >= MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return oldTab;
        }
        //获取新容量，并且设置下次的扩容后容量大小值
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                 oldCap >= DEFAULT_INITIAL_CAPACITY)
            newThr = oldThr << 1; // double threshold
    }
    //获取新容量
    else if (oldThr > 0) // initial capacity was placed in threshold
        newCap = oldThr;
    //原容量为0并且未设置下次扩容后大小
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        //负载因子设置下次扩容后大小
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    //设置下次扩容后大小为0的，重设下次扩容后大小
    if (newThr == 0) {
        float ft = (float)newCap * loadFactor;
        //当前扩容后大小未越界的使用负载因子设置下次扩容后大小，否则最大值无效
        newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    //构造新数组
    @SuppressWarnings({"rawtypes","unchecked"})
        Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        //通过循环将扩容前数组的数据放入扩容后数组
        for (int j = 0; j < oldCap; ++j) {
            Node<K,V> e;
            if ((e = oldTab[j]) != null) {
                oldTab[j] = null;
                //节点转移
                if (e.next == null)
                    newTab[e.hash & (newCap - 1)] = e;
                //红黑树结果转移
                else if (e instanceof TreeNode)
                    ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                //链表结构转移
                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;
                    }
                    if (hiTail != null) {
                        hiTail.next = null;
                        newTab[j + oldCap] = hiHead;
                    }
                }
            }
        }
    }
    return newTab;
}

2.5、remove 方法

public V remove(Object key) {
    Node<K,V> e;
    return (e = removeNode(hash(key), key, null, false, true)) == null ?
        null : e.value;
}
final Node<K,V> removeNode(int hash, Object key, Object value,
                           boolean matchValue, boolean movable) {
    Node<K,V>[] tab; Node<K,V> p; int n, index;
    //判断数据结构是否存在
    if ((tab = table) != null && (n = tab.length) > 0 &&
        //定位hash在数组结构中的位置
        (p = tab[index = (n - 1) & hash]) != null) {
        Node<K,V> node = null, e; K k; V v;
        //是数组中数据，获取
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            node = p;
        //不是数组中数据，在其后数据结构中查找
        else if ((e = p.next) != null) {
            //在红黑树中查找
            if (p instanceof TreeNode)
                node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
            //在链表中查找
            else {
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key ||
                         (key != null && key.equals(k)))) {
                        node = e;
                        break;
                    }
                    p = e;
                } while ((e = e.next) != null);
            }
        }
        //根据hash和key查找到数据，并且value匹配
        if (node != null && (!matchValue || (v = node.value) == value ||
                             (value != null && value.equals(v)))) {
            //从数据结构中移除节点
            if (node instanceof TreeNode)
                ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
            else if (node == p)
                tab[index] = node.next;
            else
                p.next = node.next;
            //修改次数加一
            ++modCount;
            //大小减一
            --size;
            afterNodeRemoval(node);
            return node;
        }
    }
    return null;
}