Java子类方法修改父类属性后值无法保留的问题求助

问题根源 #

你的DebitCard类的withdraw方法中，只是定义了一个**局部变量currentAmount**来存储扣除后的余额数值，但并没有将这个值同步到父类BaseAccount的私有属性currentAmount中。父类的currentAmount是私有变量，只能通过setCurrentAmount()方法修改，你现在的操作只是在方法内部计算了一个临时值，完全没有改变对象的实际余额状态。

解决方法 #

在withdraw方法计算出扣除后的新余额后，调用父类提供的setCurrentAmount()方法，将新余额赋值给对象的currentAmount属性，这样后续调用balance()方法时就能读取到更新后的余额。

修改后的代码 #

修正后的DebitCard类 #

public class DebitCard extends BaseAccount{

    public DebitCard(double opening, double currentAmount, double amount) {
        super(opening, currentAmount, amount);
    }

    public String withdraw(double amount) {
        double newBalance = getCurrentAmount() - amount;
        // 调用set方法更新父类的currentAmount属性
        setCurrentAmount(newBalance);
        return amount + " have been retired. \nBalance: " + newBalance;
    }
}

完整运行代码 #

public class BaseAccount {

    private double opening;
    private double currentAmount = 0.0;
    private double amount;

    public BaseAccount(double opening, double currentAmount, double amount) {
        this.opening = opening;
        this.currentAmount = currentAmount;
        this.amount = amount;
    }

    public double getOpening() {
        return opening;
    }

    public void setOpening(double opening) {
        this.opening = opening;
    }

    public double getCurrentAmount() {
        return currentAmount;
    }

    public void setCurrentAmount(double currentAmount) {
        this.currentAmount = currentAmount;
    }

    public double getAmount() {
        return amount;
    }

    public void setAmount(double amount) {
        this.amount = amount;
    }

    public String opening(double opening) {
        this.opening = opening;
        this.currentAmount = currentAmount + opening;
        return "This account has been opened with " + this.opening;
    }

    public String deposit(double amount) {
        this.currentAmount += amount;
        return "Depositing " + amount;
    }

    public String balance() {
        return "Balance: " + currentAmount;
    }
}

public class DebitCard extends BaseAccount{

    public DebitCard(double opening, double currentAmount, double amount) {
        super(opening, currentAmount, amount);
    }

    public String withdraw(double amount) {
        double newBalance = getCurrentAmount() - amount;
        setCurrentAmount(newBalance);
        return amount + " have been retired. \nBalance: " + newBalance;
    }
}

public class Inheritance {

    public static void main(String[] args) {
        BaseAccount base1 = new BaseAccount(0,0,0);
        System.out.println(base1.opening(500));
        System.out.println(base1.deposit(22.22));
        System.out.println(base1.balance());

        DebitCard debit1 = new DebitCard(0,0,0);
        System.out.println(debit1.opening(400));
        System.out.println(debit1.deposit(33.33));
        System.out.println(debit1.balance());
        System.out.println(debit1.withdraw(33.33));
        System.out.println(debit1.balance());
    }
}

修改后运行结果 #

This account has been opened with 500.0
Depositing 22.22
Balance: 522.22
This account has been opened with 400.0
Depositing 33.33
Balance: 433.33
33.33 have been retired.
Balance: 400.0
Balance: 400.0

可以看到最后一次balance()输出已经正确显示更新后的余额。

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