使用super.displayInfo调用父类方法时账号等信息显示默认值问题

问题原因 #

你在main方法中创建的Account ac实例和后续的Savings/FixedDeposit实例是完全独立的对象，彼此没有任何关联。子类对象的父类字段（账号、客户名、账户类型、支行名称等）从未被赋值，所以打印时会显示Java的默认值（int类型默认0，String类型默认null）。

解决思路 #

不需要单独创建独立的Account对象，直接让子类对象初始化父类的属性即可。更规范的做法是给父类添加构造方法，子类通过构造方法调用父类构造，一次性完成所有属性的初始化。

修正后的完整代码 #

import java.util.*;

class Account {
    int Acno;
    String CustName, type, Branch;

    // 父类构造方法，初始化基础账户信息
    Account(int n, String nm, String ty, String br) {
        Acno = n;
        CustName = nm;
        type = ty;
        Branch = br;
    }

    void displayInfo() {
        System.out.println("Account No: " + Acno + "\nCustomer Name: " + CustName + "\nAccount Type: " + type
                + "\nBranch: " + Branch);
    }
}

class Savings extends Account {
    int Year;
    double Amount, Rate = 3.5, Interest;

    // 子类构造方法，先调用父类构造初始化基础信息，再初始化子类字段
    Savings(int n, String nm, String ty, String br, int y, double am) {
        super(n, nm, ty, br);
        Year = y;
        Amount = am;
    }

    void calIntAmt() {
        Interest = (Amount * Rate * Year) / 100;
    }

    void displayInfo() {
        super.displayInfo();
        System.out.println("Interest: " + Interest);
    }
}

class FixedDeposit extends Account {
    int Year;
    double Amount, Rate, Interest;

    // 子类构造方法，初始化所有属性
    FixedDeposit(int n, String nm, String ty, String br, int y, double am) {
        super(n, nm, ty, br);
        Year = y;
        Amount = am;
    }

    void calIntAmt() {
        if (Year <= 2) {
            Rate = 6.5;
            Interest = (Amount * Rate * Year) / 100;
        } else if (Year > 2 && Year <= 5) {
            Rate = 7.5;
            Interest = (Amount * Rate * Year) / 100;
        } else if (Year > 5 && Year <= 10) {
            Rate = 8.5;
            Interest = (Amount * Rate * Year) / 100;
        } else if (Year >= 10) {
            Rate = 9.0;
            Interest = (Amount * Rate * Year) / 100;
        } else {
            System.out.println("Invalid Year");
        }
    }

    void displayInfo() {
        super.displayInfo();
        System.out.println("Interest: " + Interest);
    }
}

public class Program {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Account Number: ");
        int n = sc.nextInt();

        System.out.print("Customer Name: ");
        String nm = sc.next();

        System.out.print("Account Type: ");
        String t = sc.next();

        System.out.print("Branch Name: ");
        String br = sc.next();

        System.out.print("Amount: ");
        int a = sc.nextInt();

        System.out.print("Year: ");
        int y = sc.nextInt();

        if (t.equalsIgnoreCase("Saving")) {
            Savings s = new Savings(n, nm, t, br, y, a);
            s.calIntAmt();
            s.displayInfo();
        } else if (t.equalsIgnoreCase("FixedDeposit")) {
            FixedDeposit fd = new FixedDeposit(n, nm, t, br, y, a);
            fd.calIntAmt();
            fd.displayInfo();
        } else {
            System.out.println("Invalid Account Type");
        }
        sc.close();
    }
}

额外修正点 #

1. 修复了FixedDeposit类中displayInfo方法只打印"Interest"无具体值的问题
2. 调整了FixedDeposit利息计算的条件，避免重复判断Year=2的情况
3. 修正了输入提示中的拼写错误Btanch→Branch
4. 使用equalsIgnoreCase判断账户类型，避免大小写敏感导致的匹配失败

内容的提问来源于stack exchange，提问作者Dharmik