Implement the ClientCallback

src/main/java/org/example/client/ClientCallbackImpl.java

+package org.example.client;
+
+import org.example.shared.IClientCallback;
+import java.rmi.RemoteException;
+import java.rmi.server.UnicastRemoteObject;
+
+public class ClientCallbackImpl extends UnicastRemoteObject implements IClientCallback {
+
+    public ClientCallbackImpl() throws RemoteException {
+        super();
+    }
+
+    @Override
+    public void onLeaderboardUpdate(String message) throws RemoteException {
+        System.out.println("\n================================");
+        System.out.println("- Notification from Server:");
+        System.out.println(message);
+        System.out.println("================================");
+    }
+}
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