java基础数据结构-map源码分析

1.HashMap

HashMap实际上是一个节点数组table，只不过数组的索引是通过hash算法计算出来的。每一个节点都包含一个key，一个value，key的hash值，以及下一个节点next。jdk1.7中解决碰撞问题是通过链表实现的，所以会存next节点。图1是map中节点的定义。

图1

• 1.1 put(K key, V value)
  • a) 先通过hash(key)求出key的hash值，源码见图2和图3

图2

图3

• b) 判断当前map是否初始化，已初始化则跳过此步骤，没有初始化则调用resize()方法初始化map，此步骤中需要注意的几个点，全局变量和常量：threshold(变量，表示下次扩容时的map长度),loadFactor (常量，值为0.75，threshold=map.size()*loadFactor)。当map.size() == threshold时就会调用resize()方法进行扩容，扩容一倍。源码见图4，resize()源码较长直接贴出来。

图4

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;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            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;
        @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)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                        ((TreeNode)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        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;
    }

• c) 通过(table.length - 1) & hash计算value的索引，table.length通过位运算计算，始终为2的n次方，索引通过&计算得出，始终在数组范围内。当此索引的节点为null时，直接实例化一个节点存进去，如果此索引的节点值不为null，则会对比此节点key的值，key值相同则覆盖，否则遍历此索引上的节点链表，在链表的尾部存入新的节点。源码见图5。

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)  //计算当前节点的索引，并判断节点是否为null，为null则直接创建新的节点
            tab[i] = newNode(hash, key, value, null);
        else { // 此key的索引位置已有节点
            Node e; K k;
            //判断key是否重复
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else { //key值不重复，存链表
                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);
                        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;
        // 当size > threshold时，重置map的size
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

• 1.2 get(Object key)
  • a) 先根据hash(key)算法求key的hash值
  • b) 然后通过(table.length - 1) & hash求出value所处的位置
  • c) 通过比对该位置的key值是否相等来获取value值，如果不等且节点不为空，jdk1.7中走链表进行匹配查找value。