重构：Replace Conditional with Polymorphism

You have a conditional that chooses different behavior depending on the type of an object.
Move each leg of the conditional to an overriding method in a subclass. Make the original method
abstract.

当有条件句，它根据对象类型来选择不同的行为，这时就可以将条件句的每一个分支搬移到子类的覆盖方法中来实现。

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假设这是原始代码：

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After using Replace Conditional with Polymorphism:
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// clientclass Employee...    int payAmount() {        return _type.payAmount(this);    }// interfaceclass EmployeeType...    abstract int payAmount(Employee emp);// some concrete classesclass Engineer...    int payAmount(Employee emp) {        return emp.getMonthlySalary();    }class Salesman...    int payAmount(Employee emp) {        return emp.getMonthlySalary() + emp.getCommission();    }class Manager...    int payAmount(Employee emp) {        return emp.getMonthlySalary() + emp.getBonus();    }
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