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 1sttreeifyBin(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 keyV 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>0if (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 defaultsnewCap = 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newTabif (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 orderNode<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;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.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;}