.

app/src/main/java/com/example/mysmsapp/MainActivity.java

@@ -20,6 +20,8 @@ import androidx.core.view.ViewCompat;
 import androidx.core.view.WindowInsetsCompat;
 
 import java.io.Serializable;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 
 public class MainActivity extends AppCompatActivity {
     private static final String TAG = "MainActivity";