feat: add SubscriptionTiers to ensure encapsulation

Author: adrianliz

exercises/tiered_pricing/solutions/adrianliz/java/src/main/java/tv/codely/checkout/InvalidSubscriptionTiers.java

@@ -0,0 +1,8 @@
+package tv.codely.checkout;
+
+public final class InvalidSubscriptionTiers extends RuntimeException {
+
+    public InvalidSubscriptionTiers(String message) {
+        super(message);
+    }
+}

exercises/tiered_pricing/solutions/adrianliz/java/src/main/java/tv/codely/checkout/SubscriptionTiers.java

@@ -0,0 +1,27 @@
+package tv.codely.checkout;
+
+import java.util.List;
+
+public final class SubscriptionTiers {
+
+    private final List<SubscriptionTier> tiers;
+
+    public SubscriptionTiers(List<SubscriptionTier> tiers) {
+        validate(tiers);
+        this.tiers = tiers;
+    }
+
+    private static void validate(final List<SubscriptionTier> tiers) {
+        if (tiers == null || tiers.isEmpty()) {
+            throw new InvalidSubscriptionTiers("There must be at least one subscription tier");
+        }
+
+        if (tiers.stream().noneMatch(SubscriptionTier::isLast)) {
+            throw new IllegalArgumentException("There must be a last subscription tier");
+        }
+    }
+
+    public List<SubscriptionTier> tiers() {
+        return tiers;
+    }
+}

exercises/tiered_pricing/solutions/adrianliz/java/src/main/java/tv/codely/checkout/TieredPricing.java

@@ -6,8 +6,12 @@ public class TieredPricing {
 
     private final List<SubscriptionTier> subscriptionTiers;
 
+    public TieredPricing(final SubscriptionTiers subscriptionTiers) {
+        this.subscriptionTiers = subscriptionTiers.tiers();
+    }
+
     public TieredPricing(final List<SubscriptionTier> subscriptionTiers) {
-        this.subscriptionTiers = subscriptionTiers;
+        this(new SubscriptionTiers(subscriptionTiers));
     }
 
     public double getTotalPrice(int subscriptions) {

exercises/tiered_pricing/solutions/adrianliz/java/src/test/java/tv/codely/checkout/TieredPricingShould.java

@@ -2,6 +2,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.List;
@@ -48,6 +49,23 @@ void return_total_price_based_on_number_of_subscriptions() {
         assertEquals(expectedPrice, totalPrice);
     }
 
+    @Test
+    void should_throw_invalid_tiers_if_there_is_no_last_tier() {
+        final var subscriptionTiers = SubscriptionTierMother.randoms();
+        final var lastSubscriptionTier =
+            subscriptionTiers.stream()
+                .filter(SubscriptionTier::isLast)
+                .findFirst()
+                .orElse(null);
+
+        assertNotNull(lastSubscriptionTier);
+    }
+
+    @Test
+    void should_throw_invalid_tiers_if_there_is_no_tiers() {
+        assertThrows(InvalidSubscriptionTiers.class, () -> new TieredPricing(List.of()));
+    }
+
     @Test
     void should_have_a_subscription_tier_range_with_no_upper_limit() {
         final var subscriptionTiers = SubscriptionTierMother.randoms();