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  • 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2n,这里需要把用户填的大小转换成2n
    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

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  • 原文地址:https://www.cnblogs.com/luckygxf/p/8486666.html
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