HashMap 常用函数源码解读

1.常量信息

//默认容量16，必须是2的幂
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//负载因子 即已使用容量超过75%就会触发扩容
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//节点链表长度达到8，会触发由链表变为红黑树的操作
static final int TREEIFY_THRESHOLD = 8;
//红黑树的节点被删除后长度小于6，会触发红黑树变链表的操作
static final int UNTREEIFY_THRESHOLD = 6;
//链表变红黑树要满足容器容量大于64，否则会选择扩容而不是变为红黑树
static final int MIN_TREEIFY_CAPACITY = 64;

2.put 方法

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,  
               boolean evict) {  
    Node[] tab; Node p; int n, i;  
//如果容器为空，则先调用扩容方法，进行一个初始化容器的操作
    if ((tab = table) == null || (n = tab.length) == 0)  
        n = (tab = resize()).length;  
// 如果容器中相同hash的位置为空，则在此位置新建一个节点
// hashMap中获取元素位置的方式为元素hash值与容量为模，即hash除容量的余数
    if ((p = tab[i = (n - 1) & hash]) == null)  
        tab[i] = newNode(hash, key, value, null);  
    else {  
        Node e; K k;  
//如果该位置不为空，且key值引用或内容相同，则替换元素
        if (p.hash == hash &&  
            ((k = p.key) == key || (key != null && key.equals(k))))  
            e = p;  
//如果节点是树节点，则执行树节点插入方法
        else if (p instanceof TreeNode)  
            e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);  
        else {  
//如果节点是链表节点，则先遍历链表
            for (int binCount = 0; ; ++binCount) {  
                if ((e = p.next) == null) {  
                    p.next = newNode(hash, key, value, null);  
//插入节点后容量满足TREEIFY_THRESHOLD ，则将该条链表变为树形结构
                    if (binCount >= TREEIFY_THRESHOLD - 1)  
                        treeifyBin(tab, hash);  
                    break;  
                }  
                if (e.hash == hash &&  
                    ((k = e.key) == key || (key != null && key.equals(k))))  
//如果链表节点的key和输入的key相同，则替换元素
                    break;  
                p = e;  
            }  
        }  
//替换元素
        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;  
}

3.扩容方法

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;  
        }  
// 新的容量是旧的容量的两倍
        else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&  
                 oldCap >= DEFAULT_INITIAL_CAPACITY)  
// 扩容门限也相应的提升为之前的两倍
            newThr = oldThr << 1; // double threshold  
  }  
    else if (oldThr > 0) // initial capacity was placed in threshold  
  newCap = oldThr;  
    else {               // zero initial threshold signifies using defaults  
//首次扩容，容量初始化为16，门限初始化为16*0.75
  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;  
//开始向新的数组复制数据
    @SuppressWarnings({"rawtypes","unchecked"})  
        Node[] newTab = (Node[])new Node[newCap];  
    table = newTab;  
    if (oldTab != null) {  
        for (int j = 0; j < oldCap; ++j) {  
            Node e;  
            if ((e = oldTab[j]) != null) {  
                oldTab[j] = null;  
                if (e.next == null)
//单节点直接在新数组的对应的位置放上数据  
                    newTab[e.hash & (newCap - 1)] = e;  
                else if (e instanceof TreeNode)  
//树形链表使用树形方法拆分链表
                    ((TreeNode)e).split(this, newTab, j, oldCap);  
                else { // preserve order  
//链表结构则将链表拆分为两条，一条在当前位置，一条在当前位置+之前容量的位置
                    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;  
//高位为0，放在原位置
                        newTab[j] = loHead;  
                    }  
                    if (hiTail != null) {  
                        hiTail.next = null;  
//高位为1，放在原位置偏移oldCap的位置
                        newTab[j + oldCap] = hiHead;  
                    }  
                }  
            }  
        }  
    }  
    return newTab;  
}

详细解释一下 hashmap 获取 key 存放位置的方法，和扩容后拆分链表的方法