public class LastDayOfMonth
{
    /**
     * 获取某月的最后一天
     *
     */
    public static String getLastDayOfMonth(int year,int month)
    {
        Calendar cal = Calendar.getInstance();
        //设置年份
        cal.set(Calendar.YEAR,year);
        //设置月份
        cal.set(Calendar.MONTH, month-1);
        //获取某月最大天数
        int lastDay = cal.getActualMaximum(Calendar.DAY_OF_MONTH);
        //设置日历中月份的最大天数
        cal.set(Calendar.DAY_OF_MONTH, lastDay);
        //格式化日期
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        String lastDayOfMonth = sdf.format(cal.getTime());
          
        return lastDayOfMonth;
    }
  
    /**
     *    main
     */
    public static void main(String[] args)
    {
        String lastDay = getLastDayOfMonth(2021,7);
        System.out.println("获取当前月的最后一天：" + lastDay);
        //获取当前月的最后一天：2021-07-31
    }
  
}