Add visitor design pattern with test scenario (#55)

Author: mperor

DesignPatterns/README.md

@@ -15,6 +15,7 @@ with practical examples and best practices for using design patterns to create r
 - [State](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state) 📜
 - [Strategy](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/strategy) 🎯
 - [Template Method](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/template/method) 📋
+- [Visitor](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor) 🧳
 
 ### Creational Design Patterns 🏭
 

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/CorporateCustomer.java

@@ -0,0 +1,4 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+record CorporateCustomer() implements Customer {
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/Customer.java

@@ -0,0 +1,8 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+public sealed interface Customer permits CorporateCustomer, RegularCustomer, PremiumCustomer {
+
+    default <T> T accept(CustomerVisitor<T> visitor) {
+        return visitor.visit(this);
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/CustomerVisitor.java

@@ -0,0 +1,6 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+public interface CustomerVisitor<T> {
+
+    T visit(Customer customer);
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/DiscountLevel.java

@@ -0,0 +1,13 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+enum DiscountLevel {
+    BRONZE(10),
+    SILVER(20),
+    GOLD(30);
+
+    private final int percent;
+
+    DiscountLevel(int percent) {
+        this.percent = percent;
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/DiscountProvider.java

@@ -0,0 +1,17 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+public class DiscountProvider implements CustomerVisitor<DiscountLevel> {
+
+    public DiscountLevel get(Customer customer) {
+        return customer.accept(this);
+    }
+
+    @Override
+    public DiscountLevel visit(Customer customer) {
+        return switch (customer) {
+            case RegularCustomer _ -> DiscountLevel.BRONZE;
+            case CorporateCustomer _ -> DiscountLevel.SILVER;
+            case PremiumCustomer _ -> DiscountLevel.GOLD;
+        };
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/InvitationLetterGenerator.java

@@ -0,0 +1,18 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+public class InvitationLetterGenerator implements CustomerVisitor<Letter> {
+
+    public Letter generate(Customer customer) {
+        return customer.accept(this);
+    }
+
+    @Override
+    public Letter visit(Customer customer) {
+        var message = switch (customer) {
+            case RegularCustomer _ -> "👨 Welcome normal customer!";
+            case CorporateCustomer _ -> "‍💼 Welcome business customer!";
+            case PremiumCustomer _ -> "🧑‍💼 Welcome vip customer!";
+        };
+        return new Letter(message);
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/Letter.java

@@ -0,0 +1,4 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+record Letter(String content) {
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/PremiumCustomer.java

@@ -0,0 +1,4 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+record PremiumCustomer() implements Customer {
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/visitor/RegularCustomer.java

@@ -0,0 +1,4 @@
+package pl.mperor.lab.java.design.pattern.behavioral.visitor;
+
+record RegularCustomer() implements Customer {
+}