import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeFormatterBuilder;
import java.time.temporal.ChronoField;

class Main {
    public static void main(String[] args) {
        String pattern = "yyyyMMddHH";
        String date = "2021061104";
        DateTimeFormatter formatter = new DateTimeFormatterBuilder()
            .parseLenient()
            .appendPattern(pattern)
            .parseDefaulting(ChronoField.HOUR_OF_DAY, 0)
            .parseDefaulting(ChronoField.MINUTE_OF_HOUR, 0)
            .parseDefaulting(ChronoField.SECOND_OF_MINUTE, 0)
            .toFormatter();
        LocalDateTime ldt = LocalDateTime.parse(date, formatter);
    }
}