HashSet&HashMap浅析

HashSet&HashMap浅析

HashSet特性

1、不能保证元素是有序的
Hashset内部采用hash值进行存储索引，而hash值不保证有序
2、不保存重复元素
由于HashSet底层是将要插入的元素当作map的key进行存储（底层采用HashMap作为数据存储结构），所以不保存相同的数据。

HashSet的构造方法

内部由HashMap支持，当没有指定参数的时候， loadFactor = 0.75 不初始化threshold

public HashSet() {
    map = new HashMap<>();//由hashmap支持，内部存储是一个hashmap
}

使用Collection对象初始化HashSet时，threshold初始值为：max(c.size/0.75+1, 16)

javapublic HashSet(Collection<? extends E> c) {
    map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
    addAll(c);
}

可以自定义threshold和loadFactor的初始大小

public HashSet(int initialCapacity, float loadFactor) {
    map = new HashMap<>(initialCapacity, loadFactor);
}

单独设置threshold的初始化大小

public HashSet(int initialCapacity) {
    map = new HashMap<>(initialCapacity);
}

Jdk1.8会调用tableSizeFor来对initialCapacity进行处理。此方法计算出接近initialCapacity参数的2^n来作为初始化容量

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);
}

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;
}

HashSet添加元素

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

把添加的元素作为内部维护存储数据的map的key
可以看出有个hash的属性，HashMap的底层就是一个Node数组，通过对key的hash值化，在根据这个i = (n - 1) & hash公式获得插入元素的应该存储位置。因此，由于这样的设计，使得HashMap的元素获取为O(1)复杂度。如果发生Hash碰撞，可以看到在Node类中有一个Node的实例Node<K,V> next;,显而易见，会链表的形势来进行存储。数据结构如下

并且，链表中有这样一个优化机制：当链表长度>=8是，链表会进化成红黑树结构

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

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }
    }

HashMap的put()方法

public V put(K key, V value) {
    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;
    // 如果table为初始化或长度为0，进行table的初始化
    // table是一个HashMap$Node内部类数组
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    // 如果 通过下标“i =（n-1）&hash”不为空的，则创建一个Node对象添加进tab[i]
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        // 如果hash值相同且key与Node的key相同，则不做处理
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        // 如果p 是treeNode类型，才用树形结构去进行存储
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        else {
            // 插入在p的链表（p.next）尾部
            for (int binCount = 0; ; ++binCount) {
                if ((e = p.next) == null) {
                    p.next = newNode(hash, key, value, null);
                    // 如果p的链表长度>=7时，将转换为TreeNode结构存储
                    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;
}

HashMap扩容机制

final Node<K,V>[] resize() {
    Node<K,V>[] oldTab = table;
    int oldCap = (oldTab == null) ? 0 : oldTab.length;
    int oldThr = threshold;
    int newCap, newThr = 0;
    //oldTab!=null,则oldCap>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)
            //当oldCap<16时，是不进行threshold*2的
            //如果能进来证明此map是扩容而不是初始化
            newThr = oldThr << 1; // double threshold
    }
    else if (oldThr > 0) // initial capacity was placed in threshold
        //进入这个if代表map构造时采用的有参构造
        newCap = oldThr;
    else {               // zero initial threshold signifies using defaults
        newCap = DEFAULT_INITIAL_CAPACITY;
        newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
    }
    if (newThr == 0) {//当threshold<16,threshold没有扩容，newThr = 0时: threshold扩容为newCap的loadFactor倍
        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;
    //如果“oldTab != null”说明是扩容，否则直接返回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;
}