import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.temporal.TemporalAdjusters;

// The main method must be in a class named "Main".
class Main {
    public static void main(String[] args)throws Exception {
       String date = "20210101";
        LocalDate lastDayOfMonth = LocalDate.parse(date, DateTimeFormatter.ofPattern("yyyyMMdd"))
       .with(TemporalAdjusters.lastDayOfMonth());
       System.out.println(lastDayOfMonth);
    }
}