HashMap

无序（每次resize的时候都会变）

非线程安全

key和value都看可以为null

使用数组和链表实现

查找元素的时候速度快

几个重要属性：

loadFactor：用来计算threshold

threshold：决定map是否需要扩容，threshold = capacity * loadFactor

构造函数

// 构造函数中初始化了threadhold和loadFactorpublic 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;    threshold = initialCapacity;    init();}

put

public V put(K key, V value) {    if (table == EMPTY_TABLE) {        inflateTable(threshold);    }    if (key == null)        // 添加key为null的元素，因为key不能重复，只能有一个key为null的元素        return putForNullKey(value);    int hash = hash(key);    int i = indexFor(hash, table.length);    // 先查找链表里面是否存在key相同的entry，如果有就直接替换    for (Entry
    
      e = table[i]; e != null; e = e.next) {        Object k;        if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {            V oldValue = e.value;            e.value = value;            e.recordAccess(this);            return oldValue;        }    }    // 如果没有key相同的entry，新加一个entry    modCount++;    addEntry(hash, key, value, i);    return null;}// 取key的哈希值final int hash(Object k) {    int h = hashSeed;    if (0 != h && k instanceof String) {        return sun.misc.Hashing.stringHash32((String) k);    }    h ^= k.hashCode();    // number of collisions (approximately 8 at default load factor).    // 因为hashCode如果写的不好的话可能会使碰撞出现的次数较多，所以使用移位运算再次hash    // 使用这中方法hash的原因：http://www.iteye.com/topic/709945    h ^= (h >>> 20) ^ (h >>> 12);    return h ^ (h >>> 7) ^ (h >>> 4);}void addEntry(int hash, K key, V value, int bucketIndex) {    if ((size >= threshold) && (null != table[bucketIndex])) {        // 如果size大于阈值threshold则扩容        resize(2 * table.length);        hash = (null != key) ? hash(key) : 0;        bucketIndex = indexFor(hash, table.length);    }    // 将entry添加到链表中    createEntry(hash, key, value, bucketIndex);}void resize(int newCapacity) {    Entry[] oldTable = table;    int oldCapacity = oldTable.length;    if (oldCapacity == MAXIMUM_CAPACITY) {        threshold = Integer.MAX_VALUE;        return;    }    Entry[] newTable = new Entry[newCapacity];    // 每次扩容之后都要重新散列元素，因为table.length 变化了    transfer(newTable, initHashSeedAsNeeded(newCapacity));    table = newTable;    threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);}// 新建一个entry，并放入链表的头部void createEntry(int hash, K key, V value, int bucketIndex) {    Entry
     
       e = table[bucketIndex];    table[bucketIndex] = new Entry<>(hash, key, value, e);    size++;}