long currentTimeMillis = System.currentTimeMillis();

Date currentDate = new Date();
long timestamp = currentDate.getTime();

Instant currentInstant = Instant.now();
long timestamp = currentInstant.toEpochMilli();

LocalDateTime localDateTime = LocalDateTime.now();
ZoneId zoneId = ZoneId.systemDefault();
ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, zoneId);
long currentTimestamp = zonedDateTime.toInstant().toEpochMilli();

Timestamp currentTimestamp = new Timestamp(System.currentTimeMillis());
long timestamp = currentTimestamp.getTime();

import java.util.Calendar;
import java.util.Date;
 
public class TimeTest {
    private static long _TEN_THOUSAND=10000;
    public static void main(String[] args) {
        long times=1000*_TEN_THOUSAND;
        long t1=System.currentTimeMillis();
        testSystem(times);
        long t2=System.currentTimeMillis();
        System.out.println(t2-t1);
 
        testCalander(times);
        long t3=System.currentTimeMillis();
        System.out.println(t3-t2);
 
        testDate(times);
        long t4=System.currentTimeMillis();
        System.out.println(t4-t3);
    }
 
    public static void testSystem(long times){//use 188
        for(int i=0;i<times;i++){
            long currentTime=System.currentTimeMillis();
        }
    }
 
    public static void testCalander(long times){//use 6299
        for(int i=0;i<times;i++){
            long currentTime=Calendar.getInstance().getTimeInMillis();
        }
    }
 
    public static void testDate(long times){
        for(int i=0;i<times;i++){
            long currentTime=new Date().getTime();
        }
    }
 
}