Add observer design pattern with test case

DesignPatterns/README.md

@@ -2,15 +2,17 @@
 
 ## Description 📄
 
-This project explores various design patterns and their implementations in modern Java. It aims to provide developers with practical examples and best practices for using design patterns to create robust and maintainable code.
+This project explores various design patterns and their implementations in modern Java. It aims to provide developers
+with practical examples and best practices for using design patterns to create robust and maintainable code.
 
 ## Design Pattern Catalog 📚
 
 ### Behavioral Design Patterns 💪
 
 - [Command](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/command) 📝
-- [Strategy](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/strategy) 🎯
 - [Execute Around Method (EAM)](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/eam) ⭕
+- [Strategy](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/strategy) 🎯
+- [Observer](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer) 👀
 
 ### Creational Design Patterns 🏭
 

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer/Follower.java

@@ -0,0 +1,9 @@
+package pl.mperor.lab.java.design.pattern.behavioral.observer;
+
+public record Follower(String name) implements MessageListener {
+
+    @Override
+    public void accept(String message) {
+        System.out.printf("%s received message: %s%n", name, message);
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer/MessageListener.java

@@ -0,0 +1,7 @@
+package pl.mperor.lab.java.design.pattern.behavioral.observer;
+
+@FunctionalInterface
+public interface MessageListener {
+
+    void accept(String message);
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer/MessagePublisher.java

@@ -0,0 +1,10 @@
+package pl.mperor.lab.java.design.pattern.behavioral.observer;
+
+public interface MessagePublisher {
+
+    void follow(MessageListener participant);
+
+    void unfollow(MessageListener participant);
+
+    void publish(String message);
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer/SocialMediaAccount.java

@@ -0,0 +1,24 @@
+package pl.mperor.lab.java.design.pattern.behavioral.observer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class SocialMediaAccount implements MessagePublisher {
+
+    private List<MessageListener> followers = new ArrayList<>();
+
+    @Override
+    public void follow(MessageListener follower) {
+        followers.add(follower);
+    }
+
+    @Override
+    public void unfollow(MessageListener follower) {
+        followers.remove(follower);
+    }
+
+    @Override
+    public void publish(String message) {
+        followers.forEach(follower -> follower.accept(message));
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/observer/MessageObserverTest.java

@@ -0,0 +1,26 @@
+package pl.mperor.lab.java.design.pattern.behavioral.observer;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import pl.mperor.lab.common.TestUtils;
+
+public class MessageObserverTest {
+
+    @Test
+    public void messagePublisherShouldAllowToFollowHisMessages() {
+        var out = TestUtils.setTempSystemOut();
+
+        var socialMediaAccount = new SocialMediaAccount();
+        socialMediaAccount.follow(new Follower("👨 Adam"));
+        socialMediaAccount.follow(new Follower("👩 Eva"));
+        socialMediaAccount.follow(message -> System.out.printf("🤖 I'm scanning your message: %s%n", message));
+        socialMediaAccount.publish("Hello 🙋 everybody!");
+
+        var printedLines = out.lines();
+        Assertions.assertEquals("👨 Adam received message: Hello 🙋 everybody!", printedLines.getFirst());
+        Assertions.assertEquals("👩 Eva received message: Hello 🙋 everybody!", printedLines.getSecond());
+        Assertions.assertEquals("🤖 I'm scanning your message: Hello 🙋 everybody!", printedLines.getThird());
+
+        TestUtils.resetSystemOut();
+    }
+}