如果您从System.currentTimeMillis()获取时间戳，那么您的时间变量应该是长变量。

Byte Stream Reader Elapsed Time for 23.7 MB is 96 secs

import java.io.*;
import java.io.IOException;
import java.util.Scanner;

class ElaspedTimetoCopyAFileUsingByteStream
{

    private long startTime = 0;
    private long stopTime = 0;
    private boolean running = false;


    public void start() 
    {
        this.startTime = System.currentTimeMillis();
        this.running = true;
    }


    public void stop() 
    {
        this.stopTime = System.currentTimeMillis();
        this.running = false;
    }



    public long getElapsedTime() 
    {
        long elapsed;
        if (running) {
             elapsed = (System.currentTimeMillis() - startTime);
        }
        else {
            elapsed = (stopTime - startTime);
        }
        return elapsed;
    }



    public long getElapsedTimeSecs()                 
    {
        long elapsed;
        if (running) 
        {
            elapsed = ((System.currentTimeMillis() - startTime) / 1000);
        }
        else
        {
            elapsed = ((stopTime - startTime) / 1000);
        }
        return elapsed;
    }





    public static void main(String[] args) throws IOException
    {
        ElaspedTimetoCopyAFileUsingByteStream  s = new ElaspedTimetoCopyAFileUsingByteStream();
        s.start();

        FileInputStream in = null;
        FileOutputStream out = null;

      try {
         in = new FileInputStream("vowels.txt");   // 23.7  MB File
         out = new FileOutputStream("output.txt");

         int c;
         while ((c = in.read()) != -1) {
            out.write(c);
         }
      }finally {
         if (in != null) {
            in.close();
         }
         if (out != null) {
            out.close();
         }
      }



        s.stop();
        System.out.println("elapsed time in seconds: " + s.getElapsedTimeSecs());
    }
}

[Elapsed Time for Byte Stream Reader][1]

**Character Stream Reader Elapsed Time for 23.7 MB is 3 secs**

import java.io.*;
import java.io.IOException;
import java.util.Scanner;

class ElaspedTimetoCopyAFileUsingCharacterStream
{

    private long startTime = 0;
    private long stopTime = 0;
    private boolean running = false;


    public void start() 
    {
        this.startTime = System.currentTimeMillis();
        this.running = true;
    }


    public void stop() 
    {
        this.stopTime = System.currentTimeMillis();
        this.running = false;
    }



    public long getElapsedTime() 
    {
        long elapsed;
        if (running) {
             elapsed = (System.currentTimeMillis() - startTime);
        }
        else {
            elapsed = (stopTime - startTime);
        }
        return elapsed;
    }



    public long getElapsedTimeSecs()                 
    {
        long elapsed;
        if (running) 
        {
            elapsed = ((System.currentTimeMillis() - startTime) / 1000);
        }
        else
        {
            elapsed = ((stopTime - startTime) / 1000);
        }
        return elapsed;
    }





    public static void main(String[] args) throws IOException
    {
        ElaspedTimetoCopyAFileUsingCharacterStream  s = new ElaspedTimetoCopyAFileUsingCharacterStream();
        s.start();

         FileReader in = null;                // CharacterStream Reader
      FileWriter out = null;

      try {
         in = new FileReader("vowels.txt");    // 23.7 MB
         out = new FileWriter("output.txt");

         int c;
         while ((c = in.read()) != -1) {
            out.write(c);
         }
      }finally {
         if (in != null) {
            in.close();
         }
         if (out != null) {
            out.close();
         }
      }

              s.stop();
        System.out.println("elapsed time in seconds: " + s.getElapsedTimeSecs());
    }
}


[Elapsed Time for Character Stream Reader][2]


  [1]: https://i.stack.imgur.com/hYo8y.png
  [2]: https://i.stack.imgur.com/xPjCK.png

如果目的只是将粗略的计时信息打印到程序日志中，那么Java项目的简单解决方案不是编写自己的秒表或计时器类，而是使用Apache Commons Lang中的org.apache.commons.lang.time.StopWatch类。

final StopWatch stopwatch = new StopWatch();
stopwatch.start();
LOGGER.debug("Starting long calculations: {}", stopwatch);
...
LOGGER.debug("Time after key part of calcuation: {}", stopwatch);
...
LOGGER.debug("Finished calculating {}", stopwatch);

我发现这段代码在计时时很有用:

public class Et {
    public Et() {
    reset();
    }
    public void reset() {
    t0=System.nanoTime();
    }
    public long t0() {
        return t0;
    }
    public long dt() {
        return System.nanoTime()-t0();
    }
    public double etms() {
    return etms(dt());
    }
    @Override public String toString() {
        return etms()+" ms.";
    }
    public static double etms(long dt) {
        return dt/1000000.; // 1_000_000. breaks cobertura
    }
    private Long t0;
}

如果您正在编写一个必须处理持续时间的应用程序，那么请查看Joda-Time，它有专门处理持续时间、间隔和周期的类。你的getDuration()方法看起来可以返回一个Joda-Time Interval:

DateTime start = new DateTime(2004, 12, 25, 0, 0, 0, 0);
DateTime end = new DateTime(2005, 1, 1, 0, 0, 0, 0);

public Interval getInterval() {
    Interval interval = new Interval(start, end);
}

不幸的是，目前发布的十个答案中没有一个是完全正确的。

如果您正在测量经过的时间，并且希望它是正确的，那么必须使用System.nanoTime()。您不能使用System.currentTimeMillis()，除非您不介意结果是错误的。

The purpose of nanoTime is to measure elapsed time, and the purpose of currentTimeMillis is to measure wall-clock time. You can't use the one for the other purpose. The reason is that no computer's clock is perfect; it always drifts and occasionally needs to be corrected. This correction might either happen manually, or in the case of most machines, there's a process that runs and continually issues small corrections to the system clock ("wall clock"). These tend to happen often. Another such correction happens whenever there is a leap second.

由于nanoTime的目的是测量经过的时间，因此它不受任何这些小修正的影响。它是你想要使用的。当前使用currentTimeMillis进行的任何计时都将关闭——甚至可能为负值。

您可能会说，“这听起来似乎并没有那么重要”，对此我说，也许不是，但总的来说，正确的代码不就是比错误的代码好吗?此外，nanoTime打字时间更短。

之前发布的关于nanoTime通常只有微秒精度的免责声明是有效的。此外，它可能需要超过一微秒的时间来调用，这取决于环境(另一个也是如此)，所以不要期望对非常非常小的间隔进行正确的计时。