java - 为什么我的程序返回 null,null,0 ,0？

Question

我的程序应该将用户提交的信息存储到另一个类中。然后应该执行一个小的计算来确定年龄。最后一部分是在 Driver 中创建辅助类的实例，然后使用 toString 打印出所有信息。我的结果每次都是 null、null、0 ,0。我做错了什么？

public class CustomerInfoProgram {

    public static Scanner scan = new Scanner(System.in);
    public static String firstName;
    public static String lastName;
    public static String address;
    public static String phoneNumber;
    public static int dob;
    public static int currentYear;
    public static int age;
    public static String name;

    public static void main(String[] args) {

        Customer in = new Customer();

        getCustomerName();
        getPhone();
        getAddress();
        getDOB();
        getCurrentYear();
        in.toString();

    }

    public static String getCustomerName() {

        System.out.println("What is your first name?");
        firstName = scan.nextLine();

        System.out.println("What is your last name?");
        lastName = scan.nextLine();

        name = firstName + " " + lastName;

        return name;

    }

    public static String getAddress() {

        System.out.println("What is your address?");
        address = scan.nextLine();

        return address;
    }

    public static String getPhone() {

        System.out.println("What is your phone number?");
        phoneNumber = scan.nextLine();

        return phoneNumber;
    }

    public static int getDOB() {

        System.out.println("What year were you born?");
        dob = scan.nextInt();

        return dob;
    }

    public static int getCurrentYear() {

        System.out.println("What is the current year?");
        currentYear = scan.nextInt();

        return currentYear;
    }
}

中学类:

import java.util.Scanner;

public class Customer {

    public static int age;
    public static String allInfo;
    public static String name = CustomerInfoProgram.name;
    public static String address = CustomerInfoProgram.address;
    public static String phoneNumber = CustomerInfoProgram.phoneNumber;
    public static int dob = CustomerInfoProgram.dob;
    public static int currentYear = CustomerInfoProgram.currentYear;

    private int getAge() {

        age = (currentYear - dob);

        return age;
    }

    public String toString() {

        getAge();
        allInfo = (name + " " + address + " " + phoneNumber + " " + dob + " " + age);
        System.out.println(allInfo);

        return toString();
    }
}

Answer 1

public static String name = CustomerInfoProgram.name;

这只在字段初始化时设置 CustomerInfoProgram.name 的值，即加载 Customer 类时一次 - 它不会在 时更新CustomerInfoProgram.name 已更新。

Customer 类(最迟)在您调用 new Customer() 时加载，这发生在您运行任何方法来设置 的值之前CustomerInfoProgram.name，因此当您尝试使用name时它为空。

如果您想使用CustomerInfoProgram.name的当前值，只需直接引用该字段即可。或者，您可以添加一个 getter，它将返回其当前值:

public static String getName() {
  return CustomerInfoProgram.name;
}

然后调用 getter 来代替 name:

allInfo = (getName() + " " + ...

(其他字段完全相同)。

<小时/>

您还将从 toString() 方法获得一个 StackOverflowError:

public String toString() {

    getAge();
    allInfo = (name + " " + address + " " + phoneNumber + " " + dob + " " + age);
    System.out.println(allInfo);

    return toString();
}

您在 toString() 内部无条件调用 toString()，因此这将继续调用自身，直到耗尽堆栈空间。

相反，只需返回allInfo:

public String toString() {

    getAge();  // This actually does nothing - do you intend to use it in the return value?
    return (name + " " + address + " " + phoneNumber + " " + dob + " " + age);
}

并在主方法中调用System.out.println:

System.out.println(in); // Implicitly calls toString().