HashMap源码中一个算法tableSizeFor

Posted 2021-02-02 caster-xzn

阅读JDK1.8版本HashMap源码看到的一段代码，返回大于等于指定入参的最小2的幂。

 1     /**
 2      * The maximum capacity, used if a higher value is implicitly specified
 3      * by either of the constructors with arguments.
 4      * MUST be a power of two <= 1<<30.
 5      */
 6     static final int MAXIMUM_CAPACITY = 1 << 30;
 7 
 8     /**
 9      * Returns a power of two size for the given target capacity.
10      */
11     static final int tableSizeFor(int cap) {
12         int n = cap - 1;
13         n |= n >>> 1;
14         n |= n >>> 2;
15         n |= n >>> 4;
16         n |= n >>> 8;
17         n |= n >>> 16;
18         return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
19     }

int n = cap-1；防止入参本身为2的幂，若不进行减一操作，结果将得到本身的2倍；