diff --git a/app/src/main/java/org/solovyev/android/calculator/BaseActivity.java b/app/src/main/java/org/solovyev/android/calculator/BaseActivity.java
index e576adc8..f027a9df 100644
--- a/app/src/main/java/org/solovyev/android/calculator/BaseActivity.java
+++ b/app/src/main/java/org/solovyev/android/calculator/BaseActivity.java
@@ -12,10 +12,13 @@ import android.support.design.widget.FloatingActionButton;
 import android.support.v7.app.ActionBar;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.Toolbar;
-import android.view.*;
+import android.view.KeyEvent;
+import android.view.MenuItem;
+import android.view.View;
+import android.view.ViewGroup;
+import android.view.Window;
 import android.widget.TextView;
-import butterknife.Bind;
-import butterknife.ButterKnife;
+
 import org.solovyev.android.Check;
 import org.solovyev.android.calculator.ga.Ga;
 import org.solovyev.android.calculator.history.History;
@@ -28,13 +31,16 @@ import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 import javax.inject.Inject;
 
+import butterknife.Bind;
+import butterknife.ButterKnife;
+
 import static android.content.pm.ActivityInfo.SCREEN_ORIENTATION_PORTRAIT;
 import static android.content.pm.ActivityInfo.SCREEN_ORIENTATION_UNSPECIFIED;
 import static android.view.WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON;
 import static org.solovyev.android.calculator.App.cast;
 import static org.solovyev.android.calculator.Preferences.Gui.keepScreenOn;
 
-public class BaseActivity extends AppCompatActivity implements SharedPreferences.OnSharedPreferenceChangeListener {
+public abstract class BaseActivity extends AppCompatActivity implements SharedPreferences.OnSharedPreferenceChangeListener {
 
     @Nonnull
     protected final Tabs tabs;