重构31-Replace conditional with Polymorphism（多态代替条件）

多态（Polymorphism）是面向对象编程的基本概念之一。在这里，是指在进行类型检查和执行某些类型操作时，最好将算法封装在类中，并且使用多态来对代码中的调用进行抽象。

public class OrderProcessor {
    public Double ProcessOrder(Customer customer, List<Product> products) {    
        // do some processing of order 
        Double orderTotal = sum(products);
        Type customerType = customer.GetType();
        if (customerType == typeof(Employee)) {
            orderTotal -= orderTotal * 0.15d;
        } else if (customerType == typeof(NonEmployee)) {
            orderTotal -= orderTotal * 0.05d;
        }
        return orderTotal;
    }
}

如你所见，我们没有利用已有的继承层次进行计算，而是使用了违反SRP原则的执行方式。要进行重构，我们只需将百分率的计算置于实际的customer类型之中。我知道这只是一项补救措施，但我还是会这么做，就像在代码中那样

public abstract class Customer {
}

public class Employee extends Customer {
}

public class NonEmployee extends Customer {
}

public abstract class Customer {
    public abstract Double DiscountPercentage(){
        return null;
    }
}
public class Employee extends Customer {
    @Override
    public Double DiscountPercentage(){
        return 0.15d;
    }
}
public class NonEmployee extends Customer {
    @Override
    public Double DiscountPercentage(){
        return 0.05d;
    }
}
public class OrderProcessor {
    public Double ProcessOrder(Customer customer, List<Product> products) {    // do some processing of order
        Double orderTotal = sum(products);
        orderTotal -= orderTotal * customer.DiscountPercentage();
        return orderTotal;
    }
}

来自为知笔记(Wiz)