Calendar日期操作

Calendar calendar = Calendar.getInstance();

//获取当前年份
int year = calendar.get(Calendar.YEAR);

//获取当前月份
int month = calendar.get(Calendar.MONTH) + 1;

//获取本月天数

int days= calendar.getActualMaximum(Calendar.DATE)

  SimpleDateFormat dateFormat= new SimpleDateFormat("yyyy-MM-dd");

//        获取当前时间
        String today = dateFormat.format(calendar.getTime());

//        获取本月第一天

      calendar.add(Calendar.MONTH, 0);
        calendar.set(Calendar.DAY_OF_MONTH, 1);
        String fristDay = dateFormat.format(calendar.getTime());

private static final SimpleDateFormat SDF_YMD = new SimpleDateFormat("yyyy-MM-dd");

//    获取age个月前的第一天
    private String getFirstDayOfMonthAge(Integer age) {
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.MONTH, age);
        calendar.set(Calendar.DAY_OF_MONTH, 1);
        return SDF_YMD.format(calendar.getTime());
    }
    
//    获取age个月前的最后一天
private String getLastDayOfMonthAge(Integer age) {
  Calendar calendar = Calendar.getInstance();
  calendar.add(Calendar.MONTH, age);
  calendar.set(Calendar.DAY_OF_MONTH,calendar.getActualMaximum(Calendar.DATE));
  return SDF_YMD.format(calendar.getTime());
}