Java LocalDate 日期格式化与转换：理解其内部机制与实践

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

public class DateParsingExample {
    public static void main(String[] args) {
        String dateString = "2022-11-20";
        String pattern = "yyyy-MM-dd";

        // 创建一个DateTimeFormatter实例，指定输入字符串的格式
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);

        // 使用formatter解析字符串，得到LocalDate对象
        LocalDate localDate = LocalDate.parse(dateString, formatter);

        System.out.println("解析后的LocalDate: " + localDate); // 输出: 解析后的LocalDate: 2022-11-20
        System.out.println("LocalDate的toString()结果: " + localDate.toString()); // 依然是ISO 8601
    }
}

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

public class DateFormattingExample {
    public static void main(String[] args) {
        LocalDate localDate = LocalDate.of(2022, 11, 20); // 创建一个LocalDate对象

        // 定义目标输出格式
        String targetPattern = "dd-MM-yyyy";
        DateTimeFormatter targetFormatter = DateTimeFormatter.ofPattern(targetPattern);

        // 使用formatter将LocalDate格式化为字符串
        String formattedDateString = localDate.format(targetFormatter);

        System.out.println("原始LocalDate: " + localDate); // 输出: 原始LocalDate: 2022-11-20
        System.out.println("格式化后的字符串: " + formattedDateString); // 输出: 格式化后的字符串: 20-11-2022
    }
}

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.time.format.DateTimeParseException;

public class DateConverter {

    /**
     * 将一个日期字符串从旧格式转换为新格式。
     *
     * @param oldPattern 旧日期字符串的格式模式，例如 "yyyy-MM-dd"
     * @param newPattern 新日期字符串的目标格式模式，例如 "dd-MM-yyyy"
     * @param inputDateString 需要转换的日期字符串
     * @return 转换后的新格式日期字符串，如果解析或格式化失败则返回null
     */
    public static String convertDateFormat(String oldPattern, String newPattern, String inputDateString) {
        try {
            // 1. 定义旧格式的DateTimeFormatter
            DateTimeFormatter oldFormat = DateTimeFormatter.ofPattern(oldPattern);
            // 2. 将输入日期字符串解析为LocalDate对象
            LocalDate localDate = LocalDate.parse(inputDateString, oldFormat);

            // 3. 定义新格式的DateTimeFormatter
            DateTimeFormatter newFormat = DateTimeFormatter.ofPattern(newPattern);
            // 4. 将LocalDate对象格式化为新格式的字符串
            String outputDateString = localDate.format(newFormat);

            return outputDateString;
        } catch (DateTimeParseException e) {
            System.err.println("日期解析或格式化失败: " + e.getMessage());
            return null; // 或者抛出自定义异常
        }
    }

    public static void main(String[] args) {
        String oldDate = "2022-11-20";
        String oldPattern = "yyyy-MM-dd";
        String newPattern = "dd-MM-yyyy";

        String convertedDate = convertDateFormat(oldPattern, newPattern, oldDate);

        if (convertedDate != null) {
            System.out.println("原始日期字符串: " + oldDate + " (" + oldPattern + ")");
            System.out.println("转换后日期字符串: " + convertedDate + " (" + newPattern + ")"); // 输出: 20-11-2022
        }

        // 尝试一个无效的日期字符串
        String invalidDate = "2022/11/20";
        String invalidConvertedDate = convertDateFormat(oldPattern, newPattern, invalidDate);
        System.out.println("无效日期转换结果: " + invalidConvertedDate); // 输出: 日期解析或格式化失败: ...
    }
}

public static LocalDate localDateToAnotherLocalDate(String oldPattern, String newPattern, String input) {
    DateTimeFormatter oldFormat = DateTimeFormatter.ofPattern(oldPattern);
    DateTimeFormatter newFormat = DateTimeFormatter.ofPattern(newPattern);
    LocalDate localDate = LocalDate.parse(input, oldFormat);
    String output = localDate.format(newFormat); // 此时 output 是 "20-11-2022"
    System.out.println();
    // 错误之处：又将已经格式化为字符串的日期，重新解析回LocalDate
    return getLocalDate(output, newPattern); 
}

public static LocalDate getLocalDate(String date, String datePattern) {
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern(datePattern);
    return LocalDate.parse(date, formatter);
}

@Test
void getLocalDateToAnotherLocalDateTest() {
    LocalDate localDate = DateUtil.localDateToAnotherLocalDate("yyyy-MM-dd", "dd-MM-yyyy", "2022-11-20");
    System.out.println("localDate" + localDate.toString()); // 这里的localDate.toString() 依然是 "2022-11-20"
    assertEquals("20-11-2022", localDate.toString()); // 断言会失败
}