Java Calendar

参考：

	//实例化
		//getInstance()会调用setTimeInMillis(System.currentTimeMillis());
		//最终执行 time = millis; (time是cal的protected long  time 字段)
		Calendar cal = Calendar.getInstance();
		//或者用这一句，效果一样
		cal.setTime(new Date()); // setTimeInMillis(date.getTime()); time =  millis;
	
	//得到当前年份	
		// 下面这句的意思是：得到 实例cal里的time字段的值换算为Calendar.YEAR后的值。注意没有改变cal里面的YEAR字段值
		int year = cal.get(Calendar.YEAR); 
		System.out.println("现在是" + year + "年");//输出： 现在是2021年
		System.out.println("cal是" + cal.YEAR + "年");//输出： cal是1年
		cal.YEAR = 20;//报错	
		//- The final field Calendar.YEAR cannot be assigned
		//- The static field Calendar.YEAR should be accessed in a 

精确到毫秒

// 创建流水号
		Calendar cal = Calendar.getInstance();
		int year = cal.get(Calendar.YEAR);
		int month = cal.get(Calendar.MONTH) + 1;// 因为月份从0开始，所以加一
		int day = cal.get(Calendar.DATE);
		int hour = cal.get(Calendar.HOUR_OF_DAY);
		int minute = cal.get(Calendar.MINUTE); // 获取当前分钟
		int second = cal.get(Calendar.SECOND); // 获取当前秒数
		int millisecond = cal.get(Calendar.MILLISECOND); // 获取毫秒数

		String transSerialNo = "xfrs" + String.format("%04d", year)
				+ String.format("%02d", month) + String.format("%02d", day)
				+ String.format("%02d", hour) + String.format("%02d", minute)
				+ String.format("%03d", millisecond);
		System.out.println(transSerialNo);

输出xfrs202110191618204

一些字段如下：

public final static int YEAR = 1;
public final static int MonTH = 2;
public final static int WEEK_OF_YEAR = 3;
public final static int DATE = 5;

protected int           fields[];
protected long          time;

官方说明：
Like other locale-sensitive classes, Calendar provides a

• class method, getInstance, for getting a generally useful
• object of this type. Calendar’s getInstance method
• returns a Calendar object whose
• calendar fields have been initialized with the current date and time: