使用 LocalDateTime 计算用户年龄

Question

我正在尝试制作一个程序，提示用户输入生日年份（例如 2001 年）。然后做一些数学的事情（现在时间 - 用户写的年份（2021 - 2001））。但是，我收到此错误：

Operator "-" cannot be applied to string int.

如何解决这个错误？

代码：

package com.myApp.User;

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Scanner;

public class GetUserAge {

    public String localtimeYear;
    public int userBirthdayYear;

    public static void main(String[] args) {

        Scanner scannerObject = new Scanner(System.in);
        GetUserAge getUserAgeObject = new GetUserAge();
        LocalDateTime localDateObject = LocalDateTime.now();
        DateTimeFormatter localDateObjectFormatted = DateTimeFormatter.ofPattern("yyyy");

        getUserAgeObject.localtimeYear = localDateObject.format(localDateObjectFormatted);
        // System.out.println(getUserAgeObject.localtimeYear); => 2021
        getUserAgeObject.userBirthdayYear = scannerObject.nextInt();
       System.out.println(calculateTrueBirthdayYear(getUserAgeObject.userBirthdayYear));
    }

    static int calculateTrueBirthdayYear(int userBirthdayYear) {

        GetUserAge getUserAgeObject = new GetUserAge();

        return (getUserAgeObject.localtimeYear - userBirthdayYear);
    }
}

Answer 1

String 不是数字

你的错误一定来自这一行：

return (getUserAgeObject.localtimeYear - userBirthdayYear);

... 因为成员字段localtimeYear 引用了一个String 对象。您不能在 String 对象上使用数字运算符 -。

Year class

如果只想关注年份，请使用 Year class。

Year currentYear = Year.now() ;
Year birthYear = Year.of( 1970 ) ;

算一下。

int approximateYearsOld = currentYear.getValue() - birthYear.getValue() ;

示例 class.

package work.basil.age;

import java.time.Year;

public record Person( String name , Year birth )
{
    public int calculateApproximateAge ( )
    {
        return ( Year.now().getValue() - this.birth.getValue() );
    }
}

用法。

Person alice = new Person( "Alice" , Year.of( 1972 ) );
int approxAge = alice.calculateApproximateAge();

System.out.println( "alice = " + alice );
System.out.println( "approxAge = " + approxAge );

alice = Person[name=Alice, birth=1972]

approxAge = 49

Answer 2

为什么不利用可用的 API 你可以做类似...

LocalDate ld = LocalDate.of(1972, Month.MARCH, 8);
Period p = Period.between(ld, LocalDate.now());
System.out.println(p.getYears());
System.out.println(p.getMonths());
System.out.println(p.getDays());