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hashmap源码解读

文章目录

• hashmap源码解读
• 一、HashMap1.8源码解读
• 二、解读内容
• • 1.初始值
  • 2.Put解读
  • 2.ReSize解读
  • 2.HashMap构造函数解读
• 总结

一、HashMap1.8源码解读

二、解读内容

1.初始值

代码如下（示例）：

    /**
     * The default initial capacity - MUST be a power of two.
     * 默认的初始化容量，必须是2的幂
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     * 最大容量，
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     * 负载因子，主要是计算能容纳的最大元素
     * 计算公式 threshold = 初始化capacity * loadFactor
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     * 链表转换为红黑树的阀值
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     * 红黑树转换为链表的阀值，扩容时才能发生
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     * 进行树化的最小容量，防止在调整容量和形态时发生冲突
     * 4 * TREEIFY_THRESHOLD
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

2.Put解读

代码如下（示例）：

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        //如果数组为空，扩容数组
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        //i = (n - 1) & hash,数组的位置，就是当前位置的元素为空，则往数组赋值
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            //如果当前key=key，直接覆盖元素
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            //如果元素是个红黑树，则往红黑树赋值
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                //否则，是链表
                for (int binCount = 0; ; ++binCount) {
                    //如果往下元素为空，直接往后插入链表
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        //如果链表长度>=7,8是阈值，直接转换为红黑树
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    //如果链表匹配了key,直接覆盖
                    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;
        //如果大小>初始容量，扩容大小=负载因子*默认容量
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

2.ReSize解读

代码如下（示例）：

final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        //如果老的容量>0,老的容量大于等于最大的容量，则直接赋值最大的容量
        if (oldCap > 0) {
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            //在最大的中间时，直接是原来的两倍
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; // double threshold
        }
        //旧的大于0,直接将旧的赋值给新的
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {
            //新的大小，新的扩容阈值
            // 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<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
        table = newTab;
        //上面是计算新的大小，下面是转移旧的数组
        if (oldTab != null) {
            //先遍历旧的数组
            for (int j = 0; j < oldCap; ++j) {
                Node<K,V> e;
                //找到元素
                if ((e = oldTab[j]) != null) {
                    //先将旧的元素的数组引用置空，便于jvm-回收
                    oldTab[j] = null;
                    //如果当前链表元素的后一元素无值，直接重新计算新数组的位置放置当前元素，也就是当前是只有一个元素
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    //如果当前元素是一个红黑树
                    else if (e instanceof TreeNode)
                        ((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        //链表有值>1
                        Node<K,V> loHead = null, loTail = null;
                        Node<K,V> hiHead = null, hiTail = null;
                        Node<K,V> next;
                        do {
                            next = e.next;
                            //java8很巧妙，不用重新计算新的index值，由于是2的n次方，扩容前是 1111，扩容后index是11111，
                            // 所以值hash*(n-1)的值相比于原数组要么是 01111，11111，取决于hash值，也就是最高位，
                            //最高位是0,则数组坐标不变，最高位是1，则数组位置是=10000+原坐标=原长度+原坐标

                            //此表达式，目的是为了该元素在新旧数组的小标是否相同
                            // (oldCap - 1) * e.hash = (2 * oldCap - 1) * e.hash=oldCap * e.hash = 0;
                            //相同就是低位处理元素
                            if ((e.hash & oldCap) == 0) {
                                // 链表为空时，当前节点设置为头节点
                                if (loTail == null)
                                    loHead = e;
                                else
                                    // 不为空时，将尾节点的下一个设置为当前节点
                                    loTail.next = e;
                                loTail = e;
                            }
                            //否则就是高位
                            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.HashMap构造函数解读

代码如下（示例）：

public HashMap(int initialCapacity, float loadFactor) {
        //初始化的值小于0，直接抛异常
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        //初始化的值大于最大容量，初始化值等于定义的最大容量
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        //加载因子小于等于0，抛异常
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        this.loadFactor = loadFactor;
        //重新计算数组大小，赋值的大小如果不是2的n次方，则转换为2的n次方
        this.threshold = tableSizeFor(initialCapacity);
    }

总结

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