import java.util.stream.*;
import java.util.*;
import java.time.*;

public class Main {
    public static void main(String[] args) {
        List.of("20210101", "20220202", "20230303", "2024-04-04")
                .stream()
                .map((s)->s.replaceAll("(\\d{4})(\\d{2})(\\d{2})","$1-$2-$3")) 
                .map(LocalDate::parse) 
                .forEach(System.out::println); // 打印
    }
}

标准输出：

2021-01-01
2022-02-02
2023-03-03
2024-04-04