Replace conditional with polymorphism

Question

I am trying to understand this clean code practice with an example. Consider a class Product having switch case for discount. I am trying to replace switch statement with polymorphism.

Before code:

class Product {     String priceCode;     int discount;      Product(String priceCode) {         setDiscount(priceCode);     }      public int getDiscount() {         return discount;     }      public void setDiscount(String priceCode) {         switch (priceCode) {             case "CODE1":                 discount = // some logic;             case "CODE2":                 discount = // some other logic;             case "CODE3":                 discount = // some other logic;         }     } } 

In below code as you can see I removed switch statement but I still have if conditions to create an object of discountStrategy. My question is I still have if conditions which I am trying to remove with Polymorphism.

After code:

class Product {     String priceCode;     DiscountStrategy discountStrategy;      Product(String priceCode) {         setDiscount(priceCode);     }      public int getDiscount() {         return discountStrategy.getDiscount();     }      public void setDiscount(String priceCode) {         if (priceCode.equals("CODE1")) {             discountStrategy = new DiscountStrategy1();         } else if (priceCode.equals("CODE2")) {             discountStrategy = new DiscountStrategy2();         }         // ...     } }  interface DiscountStrategy {     public int getDiscount(); }  class DiscountStrategy1 implements DiscountStrategy {     public int getDiscount() {         // calculate & return discount;     } }  class DiscountStrategy2 implements DiscountStrategy {     public int getDiscount() {         // calculate & return discount;     } }  class DiscountStrategy3 implements DiscountStrategy {     public int getDiscount() {         // calculate & return discount;     } } 

Can you please help me understand this concept with better implementation of this example?

Answer 1

I think that Product class must not be aware about the discount creation process, it should only use a discount. So, my suggestion is to create a discount factory with a Map that will hold different discount implementations:

class DiscountFactory {     private static final Map<String, DiscountStrategy> strategies = new HashMap<>();     private static final DiscountStrategy DEFAULT_STRATEGY = () -> 0;      static {         strategies.put("code1", () -> 10);         strategies.put("code2", () -> 20);     }      public DiscountStrategy getDiscountStrategy(String priceCode) {         if (!strategies.containsKey(priceCode)) {             return DEFAULT_STRATEGY;         }         return strategies.get(priceCode);     } } 

After that, the Product class can be simplified:

class Product {     private DiscountStrategy discountStrategy;      Product(DiscountStrategy discountStrategy) {         this.discountStrategy = discountStrategy;     }      public int getDiscount() {         return discountStrategy.getDiscount();     } } 

Functional interface will allow you to create different implementations using lambda expressions:

interface DiscountStrategy {     int getDiscount(); } 

And finally, example of the use of a product together with discount:

DiscountFactory factory = new DiscountFactory(); Product product = new Product(factory.getDiscountStrategy("code1"));