It works !

ChatApp/app/src/main/java/client/UsernameSingleton.java

@@ -0,0 +1,24 @@
+package client;
+
+public class UsernameSingleton {
+    private static UsernameSingleton instance;
+    private String username;
+
+    private UsernameSingleton() {
+    }
+
+    public static UsernameSingleton getInstance() {
+        if (instance == null) {
+            instance = new UsernameSingleton();
+        }
+        return instance;
+    }
+
+    public String getUsername() {
+        return username;
+    }
+
+    public void setUsername(String username) {
+        this.username = username;
+    }
+}