Java ConcurrentHashMap初始化

初始化ConcurrentHashMap时可以指定map大小，由于ConcurrentHashMap代码默认大小是2^{n，这里需要把用户填的大小转换成2}n
备注：代码基于jdk 1.8.0_91
ConcurrentHashMap初始化代码

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);

这里重点看一下tablSizeFor()这个方法

/**
     * Returns a power of two size for the given target capacity.
     */
    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;
    }

这个方法主要作用是把用户输入的大小cap，转换成不小于cap的最小2 ^ n ,如cap = 9,返回16, cap = 5返回8
主要思路：如cap = 9,二进制 1001, 把后面3位全部变成1， 1111 + 1 = 10000 = 16.这里移位1， 2， 4， 8， 16主要是前面2， 4， 8， 16多个或运算的结果