java获取当月的第一天或最后一天

方式共有两种，使用java.util.Calendar日历类实现

第一种：

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
Date date = new Date();
Calendar calendar = new GregorianCalendar();
calendar.setTime(date);//获得本月第一天calendar.add(Calendar.MONTH, 0);
calendar.set(Calendar.DAY_OF_MONTH, 1);
String firstDay = sdf.format(calendar.getTime());
System.out.println("firstDay:"+ firstDay);//获得本月最后一天calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH));
String lastDay = sdf.format(calendar.getTime());

第二种：

Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.set(Calendar.DAY_OF_MONTH, 1);
System.out.println (new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime()));
cal.roll(Calendar.DAY_OF_MONTH, -1);
System.out.println (new SimpleDateFormat("yyyy-MM-dd").format(cal.getTime()));

参考文章：https://www.cnblogs.com/dongming-03/p/5628947.html