HashMap源码详解

亦凉 2022-05-18 08:36 168阅读 0赞

##### 成员变量 #####

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
    
    static final int MAXIMUM_CAPACITY = 1 << 30;// aka 2^30
    
    static final float DEFAULT_LOAD_FACTOR = 0.75f;
    
    static final int TREEIFY_THRESHOLD = 8;
    
    static final int UNTREEIFY_THRESHOLD = 6;
    
    static final int MIN_TREEIFY_CAPACITY = 64;
    /* filed */
    transient Node<K,V>[] table;//table为Node链表构建的数组
    
    transient Set<Map.Entry<K,V>> entrySet;
    
    transient int size;
    
    transient int modCount;
    
    int threshold;
    
    final float loadFactor;//负载因子，默认值为0.75
    //静态内部类
    static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;
        //HashMap中的核心结构，链表
        //每个节点存储，hash值，key、value以及下一个节点
        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }

##### 构造方法初始化 #####

public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        //自定义负载因子
        this.loadFactor = loadFactor;
        this.threshold = tableSizeFor(initialCapacity);
    }
    
    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }
    
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

##### put方法 #####

public V put(K key, V value) {
        // 对key的hashCode()做hash
        return putVal(hash(key), key, value, false, true);
    }
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; 
        Node<K,V> p; 
        int n, i;
        // table为空则创建，创建过程也是在resize方法内完成的
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        // 计算index，如果未产生冲突
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            // 节点存在
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            // 该链为树
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            // 该链为链表
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);//新节点插到最后
                        if (binCount >= TREEIFY_THRESHOLD - 1) // 如果数量超过8个节点，转换为树存储
                            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;
        // 超过load factor*current capacity，resize
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }
    
    final Node<K,V>[] resize() {
        Node<K,V>[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;//原来的table大小
        int oldThr = threshold;//容量*负载因子
        int newCap, newThr = 0;
        if (oldCap > 0) {
            // 超过最大值不再扩充
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            // 没超过最大值，就扩充为原来的2倍
            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 {               // table为空时，设置默认的容量和负载因子
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);//默认为12
        }
        // 计算新的resize上限
        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];//创建table数组
        table = newTab;
        if (oldTab != null) {
            // 把每个bucket都移动到新的buckets中
            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;
                            // 原索引
                            if ((e.hash & oldCap) == 0) {
                                if (loTail == null)
                                    loHead = e;
                                else
                                    loTail.next = e;
                                loTail = e;
                            }
                            // 原索引+oldCap
                            else {
                                if (hiTail == null)
                                    hiHead = e;
                                else
                                    hiTail.next = e;
                                hiTail = e;
                            }
                        } while ((e = next) != null);
                        // 原索引放到bucket里
                        if (loTail != null) {
                            loTail.next = null;
                            newTab[j] = loHead;
                        }
                        // 原索引+oldCap放到bucket里
                        if (hiTail != null) {
                            hiTail.next = null;
                            newTab[j + oldCap] = hiHead;
                        }
                    }
                }
            }
        }
        return newTab;
    }

##### get方法 #####

public V get(Object key) {
        Node<K,V> e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }
    final Node<K,V> getNode(int hash, Object key) {
        Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            // 直接命中
            if (first.hash == hash && // always check first node
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            // 未命中
            if ((e = first.next) != null) {
                // 在树中找
                if (first instanceof TreeNode)
                    return ((TreeNode<K,V>)first).getTreeNode(hash, key);
                // 在链表中找
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        return e;
                } while ((e = e.next) != null);
            }
        }
        return null;
    }