如何计算一段程序的运行时间

Calendar calendar = new GregorianCalendar();
        long start = calendar.getTimeInMillis();
        System.out.println("MILLISECOND Start: " + start);
  
        try{
            Thread.sleep(5000);
        }catch(InterruptedException e){
            System.err.println(e.toString());
        }
       
        for (int i=0;i<10000000;i++){ }
         Calendar calendar1 = new GregorianCalendar();
        long end = calendar1.getTimeInMillis();
        long b = end - start;
        System.out.println("MILLISECOND: " + end);
        System.out.println("运行时间: " + b);