Calendar的使用 对于时间的设置，修改，

Java的Calendar的使用 对于时间的设置，修改，

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

public class Test {

	public static void main(String[] args) {
		SimpleDateFormat simpleDateFormat = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
		String data = "2021-07-05";
		String year = data.substring(0, 4);
		String month = data.substring(5, 7);
		String date = data.substring(8, 10);
		Calendar ca = Calendar.getInstance();
		ca.set(Calendar.YEAR, Integer.parseInt(year));
		ca.set(Calendar.MONTH, Integer.parseInt(month)-1);
		ca.set(Calendar.DAY_OF_MONTH, Integer.parseInt(date));
		ca.set(Calendar.HOUR_OF_DAY,19);
		ca.set(Calendar.MINUTE,00);
		ca.set(Calendar.SECOND,00);
		String format = simpleDateFormat.format(ca.getTime());
		System.out.println(format);
		SimpleDateFormat simpleDateFormat2 = new SimpleDateFormat("YYYY-MM-dd HH:mm:ss");
		Calendar ca2 = Calendar.getInstance();
		ca2.set(Calendar.YEAR, Integer.parseInt(year));
		ca2.set(Calendar.MONTH, Integer.parseInt(month)-1);
		ca2.set(Calendar.DAY_OF_MONTH, Integer.parseInt(date)+1);
		ca2.set(Calendar.HOUR_OF_DAY,07);
		ca2.set(Calendar.MINUTE,00);
		ca2.set(Calendar.SECOND,00);
		String format2 = simpleDateFormat2.format(ca2.getTime());
		System.out.println(format2);
	}
	
}

		String data = "2021-07";
		String year = data.substring(0, 4);
		String month = data.substring(5, 7);
		Calendar cal = Calendar.getInstance();
		//设置年份
		cal.set(Calendar.YEAR,Integer.parseInt(year));
		//设置月份
		cal.set(Calendar.MONTH, Integer.parseInt(month)-1);
		//获取某月最小天数
		int firstDay = cal.getActualMinimum(Calendar.DAY_OF_MONTH);
		//设置日历中月份的最小天数
		cal.set(Calendar.DAY_OF_MONTH, firstDay);
		//格式化日期
		SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
		String firstDayOfMonth = sdf.format(cal.getTime());
		System.out.println(firstDayOfMonth);
		
		Calendar cal1 = Calendar.getInstance();
		//设置年份
		cal1.set(Calendar.YEAR,Integer.parseInt(year));
		//设置月份
		cal1.set(Calendar.MONTH, Integer.parseInt(month)-1);
		//获取某月最大天数
		int lastDay = cal1.getActualMaximum(Calendar.DAY_OF_MONTH);
		//设置日历中月份的最大天数
		cal1.set(Calendar.DAY_OF_MONTH, lastDay);
		//格式化日期
		String lastDayOfMonth = sdf.format(cal1.getTime());
		System.out.println(lastDayOfMonth);