Java 时间的使用

1. 获取当前时间戳

方法一：System.currentTimeMillis();  （13位时间戳，long类型）

方法二：new Date().getTime(); （13位时间戳，long类型）

方法三：Calendar.getInstance().getTimeInMillis();   （13位时间戳，long类型）

2. 获取当前/指定时间（Date类型）

Date date = new Date();  //Tue Oct 13 10:47:06 CST 2020
System.out.println(new Date(1000L)); //Thu Jan 01 08:00:01 CST 1970

3. 将日期时间转换为指定格式

public static void demo01() {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date date = new Date();
        String text = sdf.format(date);

        System.out.println(text); // 2020-10-13 10:49:35
        System.out.println(date); // Tue Oct 13 10:49:35 CST 2020
    }

4. 将指定格式日期字符串转化为Date格式

public static void demo02() throws ParseException {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String text = "2020-10-01 07:10:00";
        Date date = sdf.parse(text);
        
        System.out.println(date); // Thu Oct 01 07:10:00 CST 2020
    }

5. 获取指定日历字段中的某个值

private static void demo01() {
        Calendar c = Calendar.getInstance();
        int year = c.get(Calendar.YEAR);
        System.out.println(year); //2020
    }

6. 将日历字段设置为指定值

private static void demo02() {
        Calendar c = Calendar.getInstance();
        c.set(Calendar.YEAR, 2009);
        int year = c.get(Calendar.YEAR);
        System.out.println(year); // 2009
        c.set(2009, 9, 9);
        int month = c.get(Calendar.MONTH);
        System.out.println(month); //9
    }

7. 将日历指定字段增加或减少

private static void demo03() {
        Calendar c = Calendar.getInstance();
        System.out.println(c.get(Calendar.YEAR)); //2020
        c.add(Calendar.YEAR, -2);
        System.out.println(c.get(Calendar.YEAR)); //2018
    }

8. 把日历对象转化为日期对象

private static void demo04() {
        Calendar c = Calendar.getInstance();
        Date date = c.getTime();
        System.out.println(date); //Tue Oct 13 11:32:33 CST 2020
    }