Java代码输出异常：setter/getter导致重复输出或null值问题排查

你遇到的问题根源有两个：

• 你的setcustomerDiscount方法里直接包含了System.out.print语句，导致在设置折扣时提前输出了折扣值，和后续main方法里的输出连在一起，出现重复内容；
• 当你去掉print语句后，如果消费金额不满足任何折扣条件，customerDiscount字段会被赋值为空字符串，或者如果调用逻辑有疏漏的话会返回null——不过更合理的做法是把折扣计算逻辑移到getter中，因为折扣是基于消费金额动态计算的派生属性，不需要单独存储。

下面是修正后的完整代码，我会标注关键修改点：

修正后的代码 #

Person类（仅修正拼写细节，核心逻辑不变） #

package driver2;
import java.util.Scanner;
import java.text.DecimalFormat;

class Person{
    private String name;
    private String address;
    private String number;
    private double customerPurchase = 0;

    //////// Constructors /////////
    public Person(String name, String address, String number, int customerPurchase){
        this.name = name;
        this.address = address;
        this.number = number;
        this.customerPurchase = customerPurchase;
    }
    public Person(){}

    //////// Accessors /////////
    public String getName(){ return this.name; }
    public String getAddress(){ return this.address; }
    public String getNumber(){ return this.number; }
    public double getcustomerPurchase(){ return this.customerPurchase; }

    //////// Mutators /////////
    public void setName(String n){ this.name = n; }
    public void setAddress(String a){ this.address = a; }
    public void setNumber(String n){ this.number = n; }
    public void setcustomerPurchase(int a){ this.customerPurchase = a; }
    // 父类的空setter可以保留，也可以直接移除
    public void setcustomerDiscount(int r) {}
}

Customer类（核心修改部分） #

class Customer extends Person{
    private String customerNumber;
    private boolean recieveMail;
    // 移除单独存储的customerDiscount字段，折扣无需持久化，动态计算即可

    //////// Constructors /////////
    public Customer(String name, String address, String number, String customerN, boolean rm, int customerPurchase) {
        super(name, address, number, customerPurchase);
        this.customerNumber = customerN;
        this.recieveMail = rm;
    }
    public Customer(){}

    //////// Accessors /////////
    public String getCustomerNumber(){ return this.customerNumber; }
    public boolean getRecieveMail(){ return this.recieveMail; }

    // 核心修改：把折扣计算逻辑移到getter中，每次调用时动态计算
    public String getcustomerDiscount(){ 
        double purchaseAmount = getcustomerPurchase();
        if (purchaseAmount >= 2000){
            return "10%";
        }else if (purchaseAmount >= 1500){
            return "7%";
        }else if (purchaseAmount >= 1000){
            return "6%";
        }else if (purchaseAmount > 500){
            return "5%";
        }else{
            return "0%"; // 也可以返回空字符串，根据业务需求调整
        }
    }

    //////// Mutators /////////
    public void setCustomerNumber(String c){ this.customerNumber = c; }
    public void setRecieveMail(boolean r){ this.recieveMail = r; }

    // 移除原setter的逻辑，现在不需要手动设置折扣
    @Override
    public void setcustomerDiscount(int r){
        // 留空或直接删除这个方法都可以
    }
}

Driver2主类（简化调用流程） #

public class Driver2{
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter name of customer:");
        String name1 = scanner.nextLine();
        System.out.print("Enter address of customer:");
        String address1 = scanner.nextLine();
        System.out.print("Enter phone number of customer:");
        String number1 = scanner.nextLine();
        System.out.print("Enter customer number:");
        String customerNumber = scanner.nextLine();
        System.out.print("Enter yes/no -- does the customer want to recieve mail?:");
        String answer = scanner.nextLine();
        boolean recieveMail = (answer.equals("yes"));
        System.out.print("Enter amount customer has spent:");
        int customerPurchase = scanner.nextInt();
        scanner.close();

        Customer customer = new Customer(name1, address1, number1, customerNumber, recieveMail, customerPurchase);

        System.out.println("\nCustomer: ");
        System.out.println("Name: "+customer.getName());
        System.out.println("Address: "+customer.getAddress());
        System.out.println("Phone Number: "+customer.getNumber());
        System.out.println("Customer Number: "+customer.getCustomerNumber());
        System.out.println("Recieve Mail?: "+customer.getRecieveMail());
        DecimalFormat ft = new DecimalFormat("####");
        ft = new DecimalFormat("$0.00");
        System.out.println("Amount Purchased: "+ft.format(customer.getcustomerPurchase()));

        // 不再需要调用setcustomerDiscount，直接调用getter获取折扣
        System.out.println("Percent off: " + customer.getcustomerDiscount());
    }
}

关键修改说明 #

1. 移除冗余字段：删除了Customer类中customerDiscount成员变量，因为折扣是基于消费金额动态计算的，不需要持久存储。
2. 逻辑迁移到getter：把原来setter里的折扣判断逻辑移到getcustomerDiscount()方法中，每次调用都会根据当前消费金额计算最新折扣，彻底避免了字段未初始化的问题。
3. 清理输出逻辑：移除了setter中的System.out.print，统一在main方法中输出结果，解决了重复输出的问题。
4. 简化调用流程：main方法中不再需要手动调用setcustomerDiscount，直接调用getter就能获取正确的折扣值。

这样修改后，无论消费金额是多少，都能输出符合预期的Percent off: X%格式，不会出现重复或null的问题。

内容的提问来源于stack exchange，提问作者PROJ3CT2501