How does Java get the current time and how do I format it?

Cause: When writing code, it is often used to get the current timestamp and date, as summarized below

public void TestDate () {
SimpleDateFormat df = new SimpleDateFormat ("Yyyy-mm-dd");//Set Date format

Date date = new Date ();
String datestring = date.tostring ();
Long times = Date.gettime ();
System.out.println ("date.tostring ():" +date.tostring ());
System.out.println ("current timestamp (milliseconds):" + times);

Calendar C = calendar.getinstance ();
System.out.println ("Current date:" + c.get (calendar.date));
SYSTEM.OUT.PRINTLN ("Current year:" + C.get (calendar.year));
Month starting from 0, 0-11
System.out.println ("Current Month:" + c.get (calendar.month));
12-hour system, 0-11
System.out.println ("12 Hour System: Current hour:" + c.get (calendar.hour));
System.out.println ("24 hour System: Current hour:" + c.get (calendar.hour_of_day));
System.out.println ("Current minute:" + c.get (Calendar.minute));
System.out.println ("Current second:" + c.get (Calendar.second));
System.out.println ("One Day of the year:" + C.get (calendar.day_of_year));
System.out.println ("current timestamp (milliseconds):" + C.gettimeinmillis ());
System.out.println ("The ToString () method of the calendar is generally used as debugging:" +c.tostring ());
SimpleDateFormat df = new SimpleDateFormat ("YyyyMMdd");
String currentdatestr = Df.format (New Date ());
System.out.println ("Current date yyyymmdd:" +currentdatestr);

}

　　

Output Result:

Date.tostring (): Wed 11:39:05 CST 2017
Current timestamp (milliseconds): 1501645145298
Current Date: 2
Current year: 2017
Current Month: 7
12-hour System: Current hour: 11
24-hour System: Current hour: 11
Current minute: 39
Current seconds: 5
Day of the year: 214
Current timestamp (milliseconds): 1501645145302
The ToString () method of the calendar is generally used as debugging: java.util.gregoriancalendar[time=1501645145302,arefieldsset=true,areallfieldsset= True,lenient=true,zone=sun.util.calendar.zoneinfo[id= "Asia/shanghai", offset=28800000,dstsavings=0,usedaylight= False,transitions=19,lastrule=null],firstdayofweek=1,minimaldaysinfirstweek=1,era=1,year=2017,month=7,week_of_ Year=31,week_of_month=1,day_of_month=2,day_of_year=214,day_of_week=4,day_of_week_in_month=1,am_pm=0,hour=11, Hour_of_day=11,minute=39,second=5,millisecond=302,zone_offset=28800000,dst_offset=0]
Current date yyyymmdd:20170802

How does Java get the current time and how do I format it?