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Java 继承中子类函数值保持问题详解

心靈之曲

发布： 2025-10-30 20:04:33

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java 继承中子类函数值保持问题详解

本文旨在解决Java继承中子类方法无法正确更新父类属性值的问题。通过分析示例代码，详细解释了局部变量与类成员变量的区别，并提供了修改方案，确保子类方法能够正确修改和保持父类的状态。

在Java的继承关系中，子类可以继承父类的属性和方法。然而，在子类方法中修改父类属性时，可能会遇到无法正确保持修改后的值的问题。这通常是由于对局部变量和类成员变量的理解不够透彻造成的。下面我们将通过一个具体的例子来详细分析这个问题，并提供解决方案。

问题分析

考虑以下场景：我们有一个BaseAccount（基础账户）类和一个DebitCard（借记卡）类，DebitCard类继承自BaseAccount类。我们希望在DebitCard类的withdraw（取款）方法中修改账户余额，但发现余额并没有被正确更新。

以下是示例代码：

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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 currentAmount = getCurrentAmount() - amount;
        return amount + " have been retired. \nBalance: " + currentAmount;
    }
}

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());
    }
}

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运行上述代码，会发现DebitCard的withdraw方法并没有正确更新账户余额。这是因为在withdraw方法中，我们创建了一个新的局部变量currentAmount，而不是修改父类的currentAmount属性。

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public String withdraw(double amount) {
    double currentAmount = getCurrentAmount() - amount; // 创建了一个局部变量
    return amount + " have been retired. \nBalance: " + currentAmount;
}

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解决方案

要解决这个问题，我们需要使用setCurrentAmount方法来修改父类的currentAmount属性。修改后的withdraw方法如下：

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

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在这个修改后的版本中，我们首先计算新的余额newCurrentAmount，然后使用setCurrentAmount方法将这个新值设置到父类的currentAmount属性中。这样，当我们再次调用balance方法时，就会得到正确的余额。

完整的DebitCard类代码如下：

public class DebitCard extends BaseAccount {

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

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

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