理解HashMap

来源：互联网发布：意大利古典家具知乎编辑：程序博客网时间：2024/05/23 12:42

所有的构造函数都会重载到下面这个构造函数

 /**     * Constructs an empty <tt>HashMap</tt> with the specified initial     * capacity and load factor.     *     * @param  initialCapacity the initial capacity     * @param  loadFactor      the load factor     * @throws IllegalArgumentException if the initial capacity is negative     *         or the load factor is nonpositive     */    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);        // Find a power of 2 >= initialCapacity        int capacity = 1;        while (capacity < initialCapacity)            capacity <<= 1;        this.loadFactor = loadFactor;        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);        table = new Entry[capacity];        useAltHashing = sun.misc.VM.isBooted() &&                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);        init();    }

里面的capability一定为指数2的幂(原因是为了方便求Hash值)

HashMap内部实现是一个动态数组，所以当需要的容量超过容器现有的容量需要扩容(每次扩容为原来1倍)，所以threshold是用来衡量是否需要扩容的标准。threshold等于capacity*loadFactor 且不大于max_capacity+1(loadFactor默认为0.75, 空间和时间之间的权衡)。

每次增删查移动时候都会先算hashcode求，再求index(指向table位置的hash），调用静态函数indexFor

/**     * Returns index for hash code h.     */    static int indexFor(int h, int length) {        return h & (length-1);    }

当length(当前容量)为指数2的幂时，h(hashCode)&(length-1) = h%length(貌似求余是所有计算效率最低的) 推导:

当length等于2^n时， h大于length的位数一定可以被h整除剩下一定是余数所以h-1&剩下的就直接可以把余数取出来了

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