怎樣知道一個數字是不是2的乘方？

回答了上一篇日誌中的一個問題：怎樣知道一個數字是不是2的乘方？

在網上搜索了一下，自己總結後，整理出三個方法，代碼如下：

package testPass; /** *for positive integer only! *@author 孫如意 [e-mail] */ public class IsPowerOfTwo { public static void main (String args[]){ System.out.println("the method of regular expersion: 128 is a power of 2 ? " + method1(128)); System.out.println("the method of bit operation: 128 is a power of 2 ? " + method2(128)); System.out.println("the method of mod operation: 128 is a power of 2 ? " + method3(128)); } /** * based on regular expression * @param num * @return wheather the num is a power of two */ public static boolean method1(Integer num){ String binNum = Integer.toBinaryString(num); //a number which is a power of 2 in binary has only one 1 and starts with it return binNum.matches("10*"); } /** * based on bit operation * @param num * @return wheather the num is a power of two */ public static boolean method2(Integer num) { if ((num & (num-1)) == 0) { return true; } return false; } /** * based on mod operation * @param num * @return wheather the num is a power of two */ public static boolean method3 (Integer num) { if(num > 0){ while(num > 0){ if (num % 2 == 1) { return false; }else { num /= 2; if(num == 1 ) { return true; } } } } return false; } }