/** 
 * 快速计算二进制数中1的个数(Fast Bit Counting) 
 * 该算法的思想如下： 
 * 每次将该数与该数减一后的数值相与，从而将最右边的一位1消掉 
 * 直到该数为0 
 * 中间循环的次数即为其中1的个数 
 * 例如给定"10100“，减一后为”10011",相与为"10000",这样就消掉最右边的1 
 * Sparse Ones and Dense Ones were first described by Peter Wegner in 
 * “A Technique for Counting Ones in a Binary Computer“, 
 * Communications of the ACM, Volume 3 (1960) Number 5, page 322 
 */ 
package al; 
public class CountOnes { 
 public static void main(String[] args) { 
  int i = 7; 
  CountOnes count = new CountOnes(); 
  System.out.println("There are " + count.getCount(i) + " ones in i"); 
 } 
 /** 
  * @author 
  * @param i 待测数字 
  * @return 二进制表示中1的个数 
  */ 
 public int getCount(int i) {   
  int n; 
  for(n=0; i > 0; n++) { 
   i &= (i - 1); 
  }   
  return n;   
 } 
}