子债父还消除重复代码的重构技巧-《代码不朽》

科技 06-19 来源：极客架构师-码农老吴

大家好，我是码农老吴，欢迎收看架构师书房。今天，我继续给大家解读《代码不朽》这本书。

上期，我们聊完了如何通过提取静态方法，消除重复代码。本期，我们聊聊消除重复代码的第二个方法，提取父类，也就是通过面对对象里面的继承，来消除重复代码。儿子欠了技术债，父亲来还债，子债父还吗。还是上次的案例。

重构之前

类图

接口及类

CheckingAccount：支票账户

SavingsAccount：储蓄账户

Accounts：账户工具类

Transfer：转账类

Accounts：账户工具类

package eu.sig.training.ch04.v2;

public class Accounts {
    @SuppressWarnings("unused")
    public static CheckingAccount findAcctByNumber(String number) {
        return new CheckingAccount();
    }

    // tag::isValid[]
    public static boolean isValid(String number) {
        int sum = 0;
        for (int i = 0; i < number.length(); i++) {
            sum = sum + (9 - i) * Character.getNumericValue(number.charAt(i));
        }
        return sum % 11 == 0;
    }
    // end::isValid[]
}

CheckingAccount：支票账户

package eu.sig.training.ch04.v2;

import eu.sig.training.ch04.BusinessException;
import eu.sig.training.ch04.Money;

// tag::CheckingAccount[]
public class CheckingAccount {
    private int transferLimit = 100;

    public Transfer makeTransfer(String counterAccount, Money amount)
        throws BusinessException {
        // 1. Check withdrawal limit:
        if (amount.greaterThan(this.transferLimit)) {
            throw new BusinessException("Limit exceeded!");
        }
        if (Accounts.isValid(counterAccount)) { // <1>
            // 2. Look up counter account and make transfer object:
            CheckingAccount acct = Accounts.findAcctByNumber(counterAccount);
            Transfer result = new Transfer(this, acct, amount); // <2>
            return result;
        } else {
            throw new BusinessException("Invalid account number!");
        }
    }

}
// end::CheckingAccount[]

SavingsAccount：储蓄账户

package eu.sig.training.ch04.v2;

import eu.sig.training.ch04.BusinessException;
import eu.sig.training.ch04.Money;

// tag::SavingsAccount[]
public class SavingsAccount {
    CheckingAccount registeredCounterAccount;

    public Transfer makeTransfer(String counterAccount, Money amount) 
        throws BusinessException {
        // 1. Assuming result is 9-digit bank account number,
        // validate with 11-test:
        if (Accounts.isValid(counterAccount)) { // <1>
            // 2. Look up counter account and make transfer object:
            CheckingAccount acct = Accounts.findAcctByNumber(counterAccount);
            Transfer result = new Transfer(this, acct, amount); // <2>
            if (result.getCounterAccount().equals(this.registeredCounterAccount)) 
            {
                return result;
            } else {
                throw new BusinessException("Counter-account not registered!");
            }
        } else {
            throw new BusinessException("Invalid account number!!");
        }
    }

}
// end::SavingsAccount[]

Transfer：转账类

package eu.sig.training.ch04.v2;

import eu.sig.training.ch04.Money;

public class Transfer {
    CheckingAccount counterAccount;

    @SuppressWarnings("unused")
    public Transfer(CheckingAccount acct1, CheckingAccount acct2, Money m) {}

    @SuppressWarnings("unused")
    public Transfer(SavingsAccount acct1, CheckingAccount acct2, Money m) {}

    public CheckingAccount getCounterAccount() {
        return this.counterAccount;
    }

}

头脑风暴

CheckingAccount：支票账户和SavingsAccount：储蓄账户，从这两个类的名字上，就可以看出，他们的相似性，从面向对象的角度看，这两个类，应该都属于账户。就可以建立一个新的类，账户类，然后，将两个子类中，重复的代码，提取到父类中，基于继承实现代码复用。

重构之后

类图

接口及类

Account：账户父类

CheckingAccount：支票账户

SavingsAccount：储蓄账户

Account：账户父类

父类的makeTransfer方法，提供了一种默认的转账操作，子类中，如果该方法，没有变化，则可以直接复用，反之，覆盖父类的方法即可。

package eu.sig.training.ch04.v3;

import eu.sig.training.ch04.BusinessException;
import eu.sig.training.ch04.Money;

// tag::Account[]
public class Account {
    public Transfer makeTransfer(String counterAccount, Money amount)
        throws BusinessException {
        // 1. Assuming result is 9-digit bank account number, validate 11-test:
        int sum = 0; // <1>
        for (int i = 0; i < counterAccount.length(); i++) {
            sum = sum + (9 - i) * Character.
                getNumericValue(counterAccount.charAt(i));
        }
        if (sum % 11 == 0) {
            // 2. Look up counter account and make transfer object:
            CheckingAccount acct = Accounts.findAcctByNumber(counterAccount);
            Transfer result = new Transfer(this, acct, amount); // <2>
            return result;
        } else {
            throw new BusinessException("Invalid account number!");
        }
    }
}
// end::Account[]

CheckingAccount：支票账户

子类makeTransfer方法，既有自己的业务逻辑，又通过super关键字调用了父类提供的makeTransfer（）；

package eu.sig.training.ch04.v3;

import eu.sig.training.ch04.BusinessException;
import eu.sig.training.ch04.Money;

// tag::CheckingAccount[]
public class CheckingAccount extends Account {
    private int transferLimit = 100;

    @Override
    public Transfer makeTransfer(String counterAccount, Money amount)
        throws BusinessException {
        if (amount.greaterThan(this.transferLimit)) {
            throw new BusinessException("Limit exceeded!");
        }
        return super.makeTransfer(counterAccount, amount);
    }
}
// end::CheckingAccount[]

SavingsAccount：储蓄账户

同上

package eu.sig.training.ch04.v3;

import eu.sig.training.ch04.BusinessException;
import eu.sig.training.ch04.Money;

// tag::SavingsAccount[]
public class SavingsAccount extends Account {
    CheckingAccount registeredCounterAccount;

    @Override
    public Transfer makeTransfer(String counterAccount, Money amount)
        throws BusinessException {
        Transfer result = super.makeTransfer(counterAccount, amount);
        if (result.getCounterAccount().equals(this.registeredCounterAccount)) {
            return result;
        } else {
            throw new BusinessException("Counter-account not registered!");
        }
    }
}
// end::SavingsAccount[]