replaceconditionalwithpolymorphism

The last day of refactoring comes from Fowlers refactoring catalog and can be found here. This shows one of the foundations of Object Oriented Programming which is Polymorphism. The concept here is that in instances where you are doing checks by type, and performing some type of operation, it’s a good idea to encapsulate that algorithm within the class and then use polymorphism to abstract the call to the code.

public abstract class Customer {
}
public class Employee extends Customer{
}
public class NonEmployee extends Customer {
}

public class Product {
	public double getPrice(){
		return 1.0;
	}
}

public class OrderProcessor {
	double orderTotal = 0;

	public double processOrder(Customer customer, List<Product> products) {
		orderTotal = 0;
		products.forEach(product -> orderTotal += product.getPrice());
		if (customer instanceof Employee) {
			orderTotal -= orderTotal * 0.15;
		} else if (customer instanceof NonEmployee) {
			orderTotal -= orderTotal * 0.05;
		}
		return orderTotal;
	}
}

As you can see here, we’re not leaning on our inheritance hierarchy to put the calculation, or even the data needed to perform the calculation lest we have a SRP violation. So to refactor this we simply take the percentage rate and place that on the actual customer type that each class will then implement. I know this is really remedial but I wanted to cover this as well as I have seen it in code.

public abstract class Customer {
	public abstract double getDiscountPercent();
}
public class Employee extends Customer {

	@Override
	public double getDiscountPercent() {
		return 15;
	}
}
public class NonEmployee extends Customer {

	@Override
	public double getDiscountPercent() {
		return 5;
	}
}
public class Product {
	public double getPrice(){
		return 1.0;
	}
}
public class OrderProcessor {
	double orderTotal = 0;

	public double processOrder(Customer customer, List<Product> products) {
		orderTotal = 0;
		products.forEach(product -> orderTotal += product.getPrice());
		orderTotal -= customer.getDiscountPercent();
		return orderTotal;
	}
}

Original C# code from the book

public abstract class Customer {
}

public class Employee : Customer {
}

public class NonEmployee : Customer {
}

public class OrderProcessor {
	public decimal ProcessOrder(Customer customer, IEnumerable<Product> products) {
		// do some processing of order
		decimal orderTotal = products.Sum(p => p.Price);
		Type customerType = customer.GetType();
		if (customerType == typeof(Employee)) {
			orderTotal -= orderTotal * 0.15m;
		}
		else if (customerType == typeof(NonEmployee)) {
			orderTotal -= orderTotal * 0.05m;
		}
		return orderTotal;
	}
}

//Solution:
public abstract class Customer {
	public abstract decimal DiscountPercentage { get; }
}

public class Employee : Customer {
	public override decimal DiscountPercentage {
		get { return 0.15m; } 
	}
}
public class NonEmployee : Customer {
	public override decimal DiscountPercentage {
		get { return 0.05m; }
	}
}
public class OrderProcessor {
	public decimal ProcessOrder(Customer customer, IEnumerable<Product> products) {
		// do some processing of order
		decimal orderTotal = products.Sum(p => p.Price);
		orderTotal -= orderTotal * customer.DiscountPercentage;
		return orderTotal;
	}
}