Java中string类型的日期计算差值

在 Java 中，可以使用 SimpleDateFormat 类来将字符串转换成日期对象，然后再使用 Calendar 类来进行日期计算。以下是一个示例代码，计算两个日期之间的天数差值：

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class DateUtil {
    
    public static long getDaysBetween(String startDateStr, String endDateStr) {
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        try {
            Date startDate = sdf.parse(startDateStr);
            Date endDate = sdf.parse(endDateStr);
            
            Calendar startCalendar = Calendar.getInstance();
            startCalendar.setTime(startDate);
            Calendar endCalendar = Calendar.getInstance();
            endCalendar.setTime(endDate);
            
            long daysBetween = (endCalendar.getTimeInMillis() - startCalendar.getTimeInMillis()) / (1000 * 60 * 60 * 24);
            return daysBetween;
        } catch (ParseException e) {
            e.printStackTrace();
            return 0;
        }
    }
    
    public static void main(String[] args) {
        String startDateStr = "2021-01-01";
        String endDateStr = "2021-01-05";
        long daysBetween = getDaysBetween(startDateStr, endDateStr);
        System.out.println("Days between " + startDateStr + " and " + endDateStr + ": " + daysBetween);
    }
}

输出结果为：

Days between 2021-01-01 and 2021-01-05: 4

注意：日期格式字符串必须与实际输入的日期字符串格式相同，否则会抛出 ParseException 异常。