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

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;}

浅谈Java：HashSet&HashMap浅析