Java源码阅读之HashMap

Java源码阅读之HashMap

1、知识汇总

HashMap的结构如下，绿色框是一个数组，数组的每一个元素都是一个单链表的头结点。红色框就是一个单链表，单链表用来解决冲突，如果不同的key值映射到了数组中的同一位置，则将其放到链表中。

HashMap中的结点重写了hashCode()方法和equals()方法，只有key和value都相等时，才认为两个结点相等，源码如下：

static class Node implements Map.Entry {
    final int hash; // hash值
    final K key;    // key
    V value;        // value
    Node next; // 单链表中，下一个结点

    // 构造方法
    Node(int hash, K key, V value, Node next) {
        this.hash = hash;
        this.key = key;
        this.value = value;
        this.next = next;
    }

    public final K getKey()        { return key; }
    public final V getValue()      { return value; }
    public final String toString() { return key + "=" + value; }

    // 重写hashCode()
    public final int hashCode() {
        return Objects.hashCode(key) ^ Objects.hashCode(value);
    }

    public final V setValue(V newValue) {
        V oldValue = value;
        value = newValue;
        return oldValue;
    }

    // 重写 equals方法
    public final boolean equals(Object o) {
        if (o == this)
            return true;
        if (o instanceof Map.Entry) {
            Map.Entry e = (Map.Entry)o;
            if (Objects.equals(key, e.getKey()) &&
                Objects.equals(value, e.getValue()))
                return true;
        }
        return false;
    }
}

2、构造方法

HashMap初始容量默认为16，加载因子默认为0.75，当HashMap中的元素数量大于当前容量与加载因子的乘积时，HashMap就需要扩容。而扩容是比较耗时的操作，因此使用HashMap前，最好估计出HashMap的容量。
还有一点需要注意，HashMap的容量始终是2的n次方，且这个2的n次方大于HashMap的实际容量。举个例子，假如我们需要在HashMap中存储10个元素，则构造方法会将HashMap的初始容量设置为16（2^4）。

/**
 * 使用初始容量和加载因子初始化HashMap
 */
public HashMap(int initialCapacity, float loadFactor) {
    if (initialCapacity <0)
        throw new IllegalArgumentException("Illegal initial capacity: " +
                                           initialCapacity);
    // 不能超过最大容量(1 <<30)
    if (initialCapacity > MAXIMUM_CAPACITY)
        initialCapacity = MAXIMUM_CAPACITY;
    if (loadFactor <= 0 || Float.isNaN(loadFactor))
        throw new IllegalArgumentException("Illegal load factor: " +
                                           loadFactor);
    this.loadFactor = loadFactor;
    this.threshold = tableSizeFor(initialCapacity);
}

/**
 * 使用初始容量初始化HashMap（加载因子默认为0.75）
 */
public HashMap(int initialCapacity) {
    this(initialCapacity, DEFAULT_LOAD_FACTOR);
}

/**
 * 无参的构造方法，初始容量默认为16
 */
public HashMap() {
    this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

/**
 * 通过Map构建HashMap
 */
public HashMap(Map m) {
    this.loadFactor = DEFAULT_LOAD_FACTOR;
    putMapEntries(m, false);
}

3、扩容：resize()方法

/**
 * 计算新的容量（大于cap的，最小的2的n次方）
 * 话说这段位运算的代码，还没有搞懂，求大神指导
 */
static final int tableSizeFor(int cap) {
    int n = cap - 1;
    n |= n >>> 1; // 无符号右移
    n |= n >>> 2;
    n |= n >>> 4;
    n |= n >>> 8;
    n |= n >>> 16;
    return (n <0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}


/**
 * 初始化容量或把容量扩大2倍
 */
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;
        }
        // 把容量扩大2倍
        else if ((newCap = oldCap <<1)                  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 float)MAXIMUM_CAPACITY ?
                  (int)ft : Integer.MAX_VALUE);
    }
    threshold = newThr;
    @SuppressWarnings({"rawtypes","unchecked"})
    Node[] newTab = (Node[])new Node[newCap];
    table = newTab;
    if (oldTab != null) {
        // 把原来HashMap中的键值对，重新映射到新HashMap中
        for (int j = 0; 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;
}

4、插入、修改：put()方法

put方法有几点需要注意：
1、如果key为null，放到table[0]指向的单链表中
2、如果key不为null，计算hash值和数组索引，然后插入到该索引指向的单链表中（每次插入都是插入到头结点中）

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)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node e; K k;
        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 {
            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;
    if (++size > threshold)
        resize();
    afterNodeInsertion(evict);
    return null;
}

5、删除：remove()方法

/**
 * 删除节点
 */
final Node removeNode(int hash, Object key, Object value,
                           boolean matchValue, boolean movable) {
    Node[] tab; 
    Node p; 
    int n, index;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (p = tab[index = (n - 1) & hash]) != null) {
        Node node = null, e; K k; V v;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            node = p;
        else if ((e = p.next) != null) {
            if (p instanceof TreeNode)
                node = ((TreeNode)p).getTreeNode(hash, key);
            else {
                do {
                    if (e.hash == hash &&
                        ((k = e.key) == key ||
                         (key != null && key.equals(k)))) {
                        node = e;
                        break;
                    }
                    p = e;
                } while ((e = e.next) != null);
            }
        }
        if (node != null && (!matchValue || (v = node.value) == value ||
                             (value != null && value.equals(v)))) {
            if (node instanceof TreeNode)
                ((TreeNode)node).removeTreeNode(this, tab, movable);
            else if (node == p)
                tab[index] = node.next;
            else
                p.next = node.next;
            ++modCount;
            --size;
            afterNodeRemoval(node);
            return node;
        }
    }
    return null;
}

/**
 * 数组中的每个元素被设置为null，等待GC回收
 */
public void clear() {
    Node[] tab;
    modCount++;
    if ((tab = table) != null && size > 0) {
        size = 0;
        for (int i = 0; i             tab[i] = null;
    }
}

6、查询：get()方法

查询主要用到get()方法，有两点需要注意：
1、key为null的键值对，存放在table[0]指向的单链表中
2、key不为null时，根据hash值找到table中的索引，在索引指向的单链表中查找键值对

public V get(Object key) {
    Node e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

/**
 * 先根据hash值定位到数组中的某个位置
 * 然后遍历该位置指向的单链表进行查找
 */
final Node getNode(int hash, Object key) {
    Node[] tab; // table的副本
    Node first, e; 
    int n; 
    K k; 
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

今天就到这里吧，拜拜~