一文搞懂Java中的日期类

public class TestTime {
    public static void main(String[] args) {
        long start=System.currentTimeMillis();
        long sum=0;
        for (int i = 0; i < 1000000; i++) {
            sum+=i;
        }
        System.out.println("sum="+sum);
        long end=System.currentTimeMillis();
        System.out.println("耗时:"+(end-start)+"ms");
    }
}

import java.util.Date;

public class DateTest {
    public static void main(String[] args) {
        Date date = new Date();
        System.out.println("现在的系统时间是："+date);
        long time=date.getTime();
        System.out.println("当前时间距离UTC时间的毫秒数："+time);
    }
}

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class SimpleDateFormatTest {
    public static void main(String[] args) throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日HH小时mm分钟ss秒S毫秒");
        System.out.println("使用指定格式的日期字符串："+sdf.format(new Date()));
        String str="2021年1月15日 06小时44分钟05秒 123毫秒";
        Date date = sdf.parse(str);
        System.out.println("日期对象："+date);
    }
}

public class CalendarTest {
    public static void main(String[] args) {
        Calendar c = Calendar.getInstance(TimeZone.getTimeZone("Asia/Shanghai"), Locale.CHINA);
        System.out.println("年:"+c.get(Calendar.YEAR));
        System.out.println("月："+(c.get(Calendar.MONTH)+1));//1月从0开始
        System.out.println("日:"+c.get(Calendar.DAY_OF_MONTH));
        System.out.println("小时:"+c.get(Calendar.HOUR));
        System.out.println("分钟:"+c.get(Calendar.MINUTE));
        System.out.println("秒:"+c.get(Calendar.SECOND));
    }
}

import java.time.LocalDateTime;

public class LocalDateTimeTest {
    public static void main(String[] args) {
        //1、创建一个日期对象
        LocalDateTime now = LocalDateTime.now();
        //2、获取日历字段
        System.out.println("年:"+now.getYear());
        System.out.println("月:"+now.getMonthValue());
        System.out.println("月："+now.getMonth());
        System.out.println("日："+now.getDayOfMonth());
        System.out.println("小时："+now.getHour());
        System.out.println("分钟："+now.getMinute());
        System.out.println("秒："+now.getSecond());
    }
}

import java.time.Instant;
import java.util.Date;

public class InstantTest {
    public static void main(String[] args) {
        //1、创建一个Instant对象
        Instant now = Instant.now();
        System.out.println("now="+now);
        //2、转换方法
        //[1]Instant->Date
        Date d = Date.from(now);
        System.out.println("d="+d);
        //[2]Date->Instant
        Instant instant = d.toInstant();
        System.out.println("instant="+instant);
    }
}

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class DateTimeFormatterTest {
    public static void main(String[] args) {
        LocalDateTime now = LocalDateTime.now();
        DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd a hh小时mm分钟ss秒");
        //格式化日期(日期->文本)
        String format = dtf.format(now);
        System.out.println(format);
        //解析字符串为日期(文本->日期)
        String s="2018-08-08 下午 05小时06分钟43秒";
        LocalDateTime parse = LocalDateTime.parse(s, dtf);
        System.out.println(parse);

    }
}