Add mediator design pattern with test case

DesignPatterns/README.md

@@ -12,6 +12,7 @@ with practical examples and best practices for using design patterns to create r
 - [Chain of Responsibility](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/chain/of/responsibility) 🔗
 - [Command](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/command) 📝
 - [Execute Around Method (EAM)](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/eam) ⭕
+- [Mediator](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/mediator) 🗣️
 - [Memento](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/memento) 💾
 - [Observer](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/observer) 👀
 - [State](src/main/java/pl/mperor/lab/java/design/pattern/behavioral/state) 📜

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/mediator/ChatRoom.java

@@ -0,0 +1,20 @@
+package pl.mperor.lab.java.design.pattern.behavioral.mediator;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Predicate;
+
+class ChatRoom {
+
+    private final List<ChatUser> users = new ArrayList<>();
+
+    void addUser(ChatUser user) {
+        users.add(user);
+    }
+
+    void sendMessage(String message, ChatUser sender) {
+        users.stream()
+                .filter(Predicate.not(sender::equals))
+                .forEach(user -> user.receive(message));
+    }
+}

DesignPatterns/src/main/java/pl/mperor/lab/java/design/pattern/behavioral/mediator/ChatUser.java

@@ -0,0 +1,13 @@
+package pl.mperor.lab.java.design.pattern.behavioral.mediator;
+
+record ChatUser(ChatRoom mediator, String name) {
+
+    void send(String message) {
+        System.out.println(name + " sends: " + message);
+        mediator.sendMessage(message, this);
+    }
+
+    void receive(String message) {
+        System.out.println(name + " received: " + message);
+    }
+}

DesignPatterns/src/test/java/pl/mperor/lab/java/design/pattern/behavioral/mediator/ChatMediatorTest.java

@@ -0,0 +1,31 @@
+package pl.mperor.lab.java.design.pattern.behavioral.mediator;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import pl.mperor.lab.common.TestUtils;
+
+public class ChatMediatorTest {
+
+    @Test
+    public void testChatRoomMediating() {
+        var out = TestUtils.setTempSystemOut();
+
+        var chat = new ChatRoom();
+        var alice = new ChatUser(chat, "👩 Alice");
+        var bob = new ChatUser(chat, "👨 Bob");
+
+        chat.addUser(alice);
+        chat.addUser(bob);
+
+        alice.send("Hello everyone!");
+        bob.send("Hi Alice!");
+
+        var outLines = out.lines();
+        Assertions.assertEquals("👩 Alice sends: Hello everyone!", outLines.getFirst());
+        Assertions.assertEquals("👨 Bob received: Hello everyone!", outLines.getSecond());
+        Assertions.assertEquals("👨 Bob sends: Hi Alice!", outLines.getThird());
+        Assertions.assertEquals("👩 Alice received: Hi Alice!", outLines.getForth());
+
+        TestUtils.resetSystemOut();
+    }
+}