Java继承中子类方法保留技巧

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本文旨在解决Java继承中，子类方法修改父类属性后，数值未正确保持的问题。通过分析示例代码，解释了局部变量与成员变量的区别，并提供了修改方案，确保子类方法能够正确更新父类状态，从而实现期望的程序行为。

在Java的继承体系中，子类可以继承父类的属性和方法。然而，在子类方法中直接修改从父类继承来的属性时，可能会遇到数值未正确保持的问题。这通常是由于对变量作用域理解不当造成的。以下将通过一个账户管理的例子，详细讲解如何解决这个问题。

问题分析

考虑一个BaseAccount（基础账户）类和一个继承自BaseAccount的DebitCard（借记卡）类。BaseAccount类维护账户余额，DebitCard类提供取款功能。问题在于，当在DebitCard类的withdraw（取款）方法中执行取款操作后，账户余额并未正确更新。

示例代码

以下是原始代码：

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 openend 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());
    }
}

问题根源

在DebitCard类的withdraw方法中，以下代码是问题的关键：

double currentAmount = getCurrentAmount() - amount;

这行代码创建了一个局部变量 currentAmount，并将其初始化为取款后的余额。然而，它并没有修改BaseAccount类中声明的currentAmount 成员变量。因此，当调用balance()方法时，返回的仍然是未取款前的余额。

解决方案

要解决这个问题，需要使用setCurrentAmount()方法来更新BaseAccount类的currentAmount成员变量。修改后的withdraw方法如下：

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

这段代码首先计算取款后的余额，然后使用setCurrentAmount()方法将新的余额值赋给BaseAccount类的currentAmount成员变量。这样，balance()方法就能返回正确的余额。

完整代码

以下是修改后的完整代码：

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 openend 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;
        setCurrentAmount(currentAmount);
        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());
    }
}

运行结果

修改后的代码运行结果如下：

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，取款后，账户余额已经正确更新。

注意事项

• 务必区分局部变量和成员变量的作用域。
• 在子类方法中修改父类属性时，应使用父类提供的getter和setter方法，以确保状态同步。
• 考虑封装性，尽量避免直接访问父类的私有属性。

总结

通过本文的讲解，我们了解了在Java继承中，子类方法修改父类属性时可能遇到的问题，以及如何使用setter方法来正确更新父类状态。理解变量作用域和封装性是解决此类问题的关键。希望本文能帮助读者更好地理解和应用Java继承。

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