【Java基础】时间比较 currentTimeMillis 与 nanoTime

nanoTime() 返回的是 正在运行的 Java 虚拟机的高分辨率时间源的当前值，以纳秒为单位。

currentTimeMillis() 返回的是 当前时间与 UTC 1970 年 1 月 1 日午夜之间的差异，以毫秒为单位。

public class Elapsed {
    public static void main(String[] args) {
        long start, end;
        System.out.println("Timing for loop from 0 to 100,000,000");

        start = System.currentTimeMillis();
        System.out.println("start currentTimeMillis: " + start);
        for (long i=0; i<100000000L; i++);
        end = System.currentTimeMillis();
        System.out.println("end currentTimeMillis: " + end);

        System.out.println("Elapsed time: " + (end - start));


        System.out.println("Timing for loop from 0 to 100,000,000");

        start = System.nanoTime();
        System.out.println("nano nanoTime: " + start);

        for (long i=0; i<100000000L; i++);
        end = System.nanoTime();
        System.out.println("end nanoTime: " + end);

        System.out.println("Elapsed time: " + (end - start));
    }
}

输出结果

Timing for loop from 0 to 100,000,000
start currentTimeMillis: 1653033370155
end currentTimeMillis: 1653033370186
Elapsed time: 31
Timing for loop from 0 to 100,000,000
nano nanoTime: 261630972426900
end nanoTime: 261631001267300
Elapsed time: 28840400   // 这里等于28.84 ms

nanoTime()要比currentTimeMillis() 更加精准，但是currentTimeMillis() 转换成Date