Java集合类-HashMap分析

HashMap的特点

非线程安全

支持序列化

动态扩容

JDK1.6采用数组 + 链表实现，JDK1.8采用数组 + 链表 + 红黑树实现（当链表长度达到8时，再插入新的结点将链表转换为红黑树）

成员变量

常量

static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 默认初始容量16
static final int MAXIMUM_CAPACITY = 1 << 30; // 最大容量2^30
static final float DEFAULT_LOAD_FACTOR = 0.75f; // 默认加载因子
static final int TREEIFY_THRESHOLD = 8;
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;

属性值

transient Node<K,V>[] table; // 拉链法实现
transient Set<Map.Entry<K,V>> entrySet;
transient int size;
transient int modCount;
int threshold; // 临界值 = 容量 * 加载因子
final float loadFactor; // 加载因子

static class Node<K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;
}

拉链法，数组和链表的结合 

 

构造函数

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);
    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
}

基本方法

增/改 put

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;

    // 用(n - 1) & hash 取代取模，效率更高
    if ((p = tab[i = (n - 1) & hash]) == null) // table[i]不存在
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            // table[i]头结点的key为要存储的key，进行覆盖
            e = p;
        else if (p instanceof TreeNode) 
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else { // 在链表尾部加入结点
            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;
}


final Node<K,V>[] resize() {
    Node<K,V>[] 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<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;
                       // 根据hash & oldCap == 0 将原有的链表分为两段
                        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;
}

 

删 remove

public V remove(Object key) {
    Node<K,V> e;
    return (e = removeNode(hash(key), key, null, false, true)) == null ?
        null : e.value;
}


final Node<K,V> removeNode(int hash, Object key, Object value,
                           boolean matchValue, boolean movable) {
    // 局部变量声明
    Node<K,V>[] tab; Node<K,V> p; int n, index;

    // 入参判断
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (p = tab[index = (n - 1) & hash]) != null) {
        Node<K,V> node = null, e; K k; V v;

        // 找到要删除的结点
        // 如果要删除的结点是链表头结点
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            node = p;
        // 如果不是头结点，遍历找到该结点
        else if ((e = p.next) != null) {
            if (p instanceof TreeNode)
                node = ((TreeNode<K,V>)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);
            }
        }

        // 执行删除结点的操作
        if (node != null && (!matchValue || (v = node.value) == value ||
                             (value != null && value.equals(v)))) {
            // 如果是红黑树
            if (node instanceof TreeNode)
                ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
            // 如果是头结点
            else if (node == p)
                tab[index] = node.next;
            // 如果是链表中的普通结点
            else
                p.next = node.next;
            ++modCount;
            --size;
            afterNodeRemoval(node);
            return node;
        }
    }
    return null;
}

 

1、找到要删除结点在数组中的索引，通过hash、key两个值判断结点位置

2、判断结点是否是头结点或是树节点，如果是头结点将其next置为头结点，如果不是删除该结点。

查 get

 

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) {
    // 局部变量声明
    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) {
        
        // 判断是否是链表的头结点
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        
        if ((e = first.next) != null) {
            // 如果是树节点
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            // 遍历链表
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

 

1、通过hash值找到做属的链表

2、判断是否是链表的头结点、是否是树节点，否则遍历链表找到该结点

取hash值

static final int hash(Object key) {
    int h;
    // 对象的hashcode异或hashcode无符号有移16位
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

由hash值找到数组索引

(n - 1) & hash

n是2的幂，效果等同于 hash % (n -1)，但是效率较高，同样为了达到key值均匀的分布

HashMap扩容原理

初始化

默认初始容量为16， 默认加载因子0.75，临界值=12。

动态扩容

1、将容量扩大为两倍，临界值扩大为两倍

2、将链表根据 hash & oldCap 分为两个链表，两个链表在数组中的位置为原索引位  [j] 和 新位置 [j + oldCap]

相关类和方法

一些问题

参考

http://blog.csdn.net/ns_code/article/details/36034955

http://blog.csdn.net/great_smile/article/details/53119489

逻辑操作符号  与&& 或|| 非！

位操作符号 与& 或| 非~ 异或^