閱讀JDK1.8版本HashMap原始碼看到的一段程式碼，返回大於等於指定入參的最小2的冪。

 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；防止入參本身為2的冪，若不進行減一操作，結果將得到本身的2倍；