Revert "Refactor PriceCalculator with strategy pattern (#138)"

This reverts commit 04f48d7.

src/main/java/com/github/hcsp/polymorphism/Discount95Strategy.java

@@ -1,8 +1,3 @@
 package com.github.hcsp.polymorphism;
 
-public class Discount95Strategy extends DiscountStrategy {
-    @Override
-    public int discount(int price, User user) {
-        return (int)(price * 0.95);
-    }
-}
+public class Discount95Strategy {}

src/main/java/com/github/hcsp/polymorphism/NoDiscountStrategy.java

@@ -1,8 +1,3 @@
 package com.github.hcsp.polymorphism;
 
-public class NoDiscountStrategy extends DiscountStrategy {
-    @Override
-    public int discount(int price, User user) {
-        return price;
-    }
-}
+public class NoDiscountStrategy {}

src/main/java/com/github/hcsp/polymorphism/OnlyVipDiscountStrategy.java

@@ -1,12 +1,3 @@
 package com.github.hcsp.polymorphism;
 
-public class OnlyVipDiscountStrategy extends DiscountStrategy {
-    @Override
-    public int discount(int price, User user) {
-        if (user.isVip()) {
-            return (int)(price * 0.95);
-        } else {
-            return price;
-        }
-    }
-}
+public class OnlyVipDiscountStrategy {}

src/main/java/com/github/hcsp/polymorphism/PriceCalculator.java

@@ -7,23 +7,22 @@ public class PriceCalculator {
     // OnlyVipDiscountStrategy 只有VIP打95折，其他人保持原价
     // 重构后的方法签名：
     // public static int calculatePrice(DiscountStrategy strategy, int price, User user)
-    public static int calculatePrice(DiscountStrategy discountStrategy, int price, User user) {
-        return discountStrategy.discount(price, user);
-//        switch (discountStrategy) {
-//            case "NoDiscount":
-//                return price;
-//            case "Discount95":
-//                return (int) (price * 0.95);
-//            case "OnlyVip":
-//                {
-//                    if (user.isVip()) {
-//                        return (int) (price * 0.95);
-//                    } else {
-//                        return price;
-//                    }
-//                }
-//            default:
-//                throw new IllegalStateException("Should not be here!");
-//        }
+    public static int calculatePrice(String discountStrategy, int price, User user) {
+        switch (discountStrategy) {
+            case "NoDiscount":
+                return price;
+            case "Discount95":
+                return (int) (price * 0.95);
+            case "OnlyVip":
+                {
+                    if (user.isVip()) {
+                        return (int) (price * 0.95);
+                    } else {
+                        return price;
+                    }
+                }
+            default:
+                throw new IllegalStateException("Should not be here!");
+        }
     }
 }