HashMap源码分析

hash表中存放的元素是Node对象，Node对象是HashMap的静态内部类，Node对象的成员变量有hash，key，value，next节点

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;

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

put（）方法

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

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //初始化hash表长度
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //判断hash表存放的位置是否有元素
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //当前hash表第一个节点有元素，判断key是否相同，进行替换
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //判断是否为红黑树，往红黑树中添加元素
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            //遍历每个节点
            for (int binCount = 0; ; ++binCount) {
                //节点为null
                if ((e = p.next) == null) {
                    //当前位置放入元素
                    p.next = newNode(hash, key, value, null);
                    //判断是否需要升级为红黑树
                    if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                        treeifyBin(tab, hash);
                    break;
                }
                //如果key相同结束循环
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    break;
                //到下一个节点
                p = e;
            }
        }
        //key相同进行值的替换
        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;
}

get（）方法

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

//这返回的是hash表的值--也就是Node<K,V>对象
final Node<K,V> getNode(int hash, Object key) {
    //Node<K,V>[] 就是我们的hash表
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    // 1. 定位键值对所在桶的位置是否存在
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        // always check first node总是检查第一个节点
        //通过hash表中对象的属性，判断hashcode，key是否是我们要的那个
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        //如果第一个节点没返回，就找他的下一个节点
        if ((e = first.next) != null) {
            // 2. 如果 first 是 TreeNode 类型，则调用黑红树查找方法
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {// 2. 对链表进行查找
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}