Java根据日期截取字符串的多种实现方法

public class DateExtractor {
    public static String extractDateFromFile(String fileName) {
        String regex = ".*_(\\d{4})(\\d{2})(\\d{2})_.*"; // 匹配文件名中的日期部分
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(fileName);
        
        if (matcher.matches()) {
            String year = matcher.group(1);
            String month = matcher.group(2);
            String day = matcher.group(3);
            return year + "-" + month + "-" + day;
        }
        
        return null;
    }
    public static void main(String[] args) {
        String fileName = "file_20220303_report.txt";
        String extractedDate = extractDateFromFile(fileName);
        System.out.println("提取的日期信息为：" + extractedDate);
    }
}

public class DateSubstring {
    public static String extractYearFromDateString(String dateStr) {
        return dateStr.substring(0, 4); // 截取字符串的前四位，即年份信息
    }
    public static void main(String[] args) {
        String dateString = "2022-05-20";
        String extractedYear = extractYearFromDateString(dateString);
        System.out.println("提取的年份信息为：" + extractedYear);
    }
}

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
public class LogDateExtractor {
    
    public static String extractAndFormatDateFromFileName(String fileName) {
        String dateStr = fileName.substring(fileName.indexOf("_") + 1, fileName.indexOf(".txt")); // 提取日志文件名中的日期部分
        LocalDate date = LocalDate.parse(dateStr, DateTimeFormatter.ofPattern("yyyyMMdd")); // 将提取的日期字符串解析为LocalDate对象
        return date.format(DateTimeFormatter.ofPattern("yyyy-MM-dd")); // 格式化日期为指定格式
    }
    public static void main(String[] args) {
        String fileName = "log_20220303.txt";
        String formattedDate = extractAndFormatDateFromFileName(fileName);
        System.out.println("日志文件日期信息为：" + formattedDate);
    }
}