源码分析八( hashmap工作原理)

首先从一条简单的语句开始，创建了一个hashmap对象：

 Map<String,String> hashmap = new HashMap<String,String>(); 

调用hashmap类无参数构造方法，使用默认转载因子，

public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

默认装载因子只有在构造器不指定的情况下使用

/**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

hashmap还有三个重载的构造方法，分别是：

有装载因子入参的构造方法

public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

有初始化容器容量以及装载因子的构造方法

public HashMap(int initialCapacity, float loadFactor) {
        //如果初始容量小于0，则抛出非法初始化容量异常
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        //如果初始容量大于最多容量，则容量为最多容量
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //如果装载因子不大于0或者装载因子不是数字，则抛出装载因子非法异常
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        //根据初始化容量计算hashmap的阈值容量(hashmap真正的容量，必须是2的次方)
        this.threshold = tableSizeFor(initialCapacity);
     }

计算hashmap的容量

// 根据给定的目标容量，计算一个2的次方
    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;
    }

有参数map的构造方法：

public HashMap(Map<? extends K, ? extends V> m) {
            this.loadFactor = DEFAULT_LOAD_FACTOR;
            putMapEntries(m, false);
     }

将map复制一份到本地，这个复制如果对于数值型则为数值，对于引用型则为地址

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
        int s = m.size();
        //原来map的长度大于0处理
        if (s > 0) {
            if (table == null) { // pre-size
                //根据实际数量计算需要多大的容量
                float ft = ((float) s / loadFactor) + 1.0F;
                int t = ((ft < (float) MAXIMUM_CAPACITY) ? (int) ft : MAXIMUM_CAPACITY);
                //如果预估容量大于现有容量，则需要根据预估容量计算一个2的次方数
                if (t > threshold)
                    threshold = tableSizeFor(t);
            }
            //如果实际容量大于阈值则需要扩容
            else if (s > threshold)
                resize();
            //扩容之后，将m的key和value复制一份到本地
            for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
                K key = e.getKey();
                V value = e.getValue();
                putVal(hash(key), key, value, false, evict);
            }
        }
    }

//第一次调用初始化或者扩容
    final Node<K,V>[] resize() {
        //首次调用时为null,则oldCap为0 | 扩容时，
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        //扩容时,如果目前容量已经超过最多容量，那么默认值为int最大，否则容量为现在2倍
        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
        }
        //初始化时newCap = oldThr = 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);
        }
        //初始化时newThr=newCap * loadFactor
        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"})
        //初始化时，创建一个长度为threshold的Node数组,然后返回
            Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> 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<K,V>)e).split(this, newTab, j, oldCap);
                    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;
    }