三种日期工具

配合下面的案例可以更加深度的了解

Date

需要导包java.util.Date

Date d = new Date();

//两种都是获取到现在时间的时间戳
long t1 = d.getTime();//这里返回的是long类型的时间戳
long t2 = System.currentTimeMillis();//同样返回的是long类型的时间戳

Date d3 =new Date(t2);//将long类型转成Date

---

Calendar

需要导包java.util.Calendar

Calendar c = Calendar.getInstance();

c.set(2021, 5 - 1, 18);//这里返回的是Date

		int year = c.get(1);
        System.out.println(year);

        int month = c.get(Calendar.MONTH) + 1;//0代表一月，以此类推应当加一
        System.out.println(month);

        int day = c.get(Calendar.DAY_OF_MONTH);//当月中的第几天
        System.out.println(day);

        int hh = c.get(Calendar.HOUR);//12小时
        hh = c.get(Calendar.HOUR_OF_DAY);//24小时
        System.out.println(hh);

        int mm = c.get(Calendar.MINUTE);
        System.out.println(mm);
        int ss = c.get(Calendar.SECOND);
        System.out.println(ss);
        int wk = c.get(Calendar.DAY_OF_WEEK);
        //1 星期日 2 星期一  3星期二 4 星期三 5 星期四 6星期五 7星期六
        //System.out.println(wk);

---

SimpleDateFormat

需要导包java.text.SimpleDateFormat

Date d = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//定义格式
  
System.out.println(sdf.format(d));//初始化date
System.out.println(d.getTime()); //返回一个时间戳

Date birth = sdf.parse("2001-7-20 0:0:0");定义一个时间点

三种综合案例

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
public class Date01 {
    public static void main(String[] args) {
        Date d = new Date();

        //时间戳 long 数字
        long t1 = d.getTime();
        long t2 = System.currentTimeMillis();

        //将long转成Date
        Date d3 = new Date(t2);

        Calendar c = Calendar.getInstance();

        c.set(2021, 5 - 1, 18);

        //将Calendar 日历类实例，转成Date
        Date d2 = c.getTime();
        SimpleDateFormat s = new SimpleDateFormat("yyyy年MM月dd日");
        System.out.println(s.format(d));
        System.out.println(s.format(d2));
    }
}

public class Date02 {
    public static void main(String[] args) throws ParseException {
        //使用Date 导入并实例化，java.util.Date
        Date d = new Date();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        System.out.println(sdf.format(d));
        System.out.println(d.getTime()); //返回一个时间戳

        long now = d.getTime() + (1000 * 60 * 60 * 24 * 10);
        d.setTime(now);
        System.out.println(sdf.format(d));

        //计算两个日期之间相关的天数
        Date birth = sdf.parse("2001-7-20 0:0:0");
        //将字符串解析转换为Date
        Date ddd = new Date();

        long days = (ddd.getTime() - birth.getTime())/1000/60/60/24;
        System.out.println(days);
    }
}

public class Date03 {
    public static void main(String[] args) throws ParseException {
        Locale.setDefault(Locale.US);
        Calendar c = Calendar.getInstance();

        c.set(2021, 7 - 1, 27);
        //year
        int year = c.get(1);
        System.out.println(year);

        int month = c.get(Calendar.MONTH) + 1;
        System.out.println(month);

        int day = c.get(Calendar.DAY_OF_MONTH);
        System.out.println(day);

        int hh = c.get(Calendar.HOUR);//12小时
        hh = c.get(Calendar.HOUR_OF_DAY);//24小时
        System.out.println(hh);

        int mm = c.get(Calendar.MINUTE);
        System.out.println(mm);
        int ss = c.get(Calendar.SECOND);
        System.out.println(ss);
        int wk = c.get(Calendar.DAY_OF_WEEK);
        //1 星期日 2 星期一  3星期二 4 星期三 5 星期四 6星期五 7星期六
        //System.out.println(wk);
        if (wk == 1) {
            System.out.println("今日是星期日");
        } else if (wk == 2) {
            System.out.println("今日是星期一");
        } else if (wk == 3) {
            System.out.println("今日是星期二");
        } else if (wk == 4) {
            System.out.println("今日是星期三");
        } else if (wk == 5) {
            System.out.println("今日是星期四");
        } else if (wk == 6) {
            System.out.println("今日是星期五");
        } else {
            System.out.println("今日是星期六");
        }


        Locale.setDefault(Locale.CHINA);
        System.out.printf("%tA%n",c);

        Date d = new Date();
        //0 星期日 1 星期一 6 星期六
        int ww = d.getDay();
        System.out.println(ww);
    }
}