00-Java数组遍历性能对比

三种遍历方式性能对比

1. 循环与数组的length比较

   public class JavaMain {
       static Object[] objs = new Object[10000000];
       static int zero() {
           int sum = 0;
           for(int i = 0; i < objs.length; i++) {
               sum ^= objs[i].hashCode();
           }
           return sum;
       }
   }

2. 将数组length存在方法栈中

   public class JavaMain {
       static Object[] objs = new Object[10000000];
       static int one() {
           int sum = 0;
           int len = objs.length;
           for(int i = 0; i < len; i++) {
               sum ^= objs[i].hashCode();
           }
           return sum;
       }
   }

3. for-each循环

   public class JavaMain {
       static Object[] objs = new Object[10000000];
       static int two() {
           int sum = 0;
           for (Object obj : objs) {
               sum ^= obj.hashCode();
           }
           return sum;
       }
   }

测试速度

public class JavaMain {
    static Object[] objs = new Object[10000000];
    static long run(IntSupplier f) {
        long start = System.currentTimeMillis();
        int result = 0;
        for(int i = 0; i < 100; i++) {
            result = f.getAsInt();
        }
        long end = System.currentTimeMillis();
        System.out.println("result = " + result + ", time = " + (end - start) / 1000.0);
        return  end - start;
    }
    public static void main(String[] args) {
        for (int i = 0; i < objs.length; i++) {
            objs[i] = new Object();
            objs[i].hashCode(); // 首次计算hashcode更慢，先缓存
        }
        for (int i = 0; i < 10; i++) {
            long zeroTime = run(JavaMain::zero);
            long oneTime = run(JavaMain::one);
            long twoTime = run(JavaMain::two);
            System.out.printf("2比1快: %.2f%%\n", ((double)oneTime - twoTime) / oneTime * 100);
            System.out.printf("2比0快: %.2f%%\n", ((double)zeroTime - twoTime) / zeroTime * 100);
            System.out.printf("1比0快: %.2f%%\n", ((double)zeroTime - oneTime) / oneTime * 100);
        }
    }
}

测试结果

result = 360970567, time = 1.393
result = 360970567, time = 1.174
result = 360970567, time = 0.886
2比1快: 24.53%
2比0快: 36.40%
1比0快: 18.65%