diff --git a/app/src/main/java/org/itxtech/daedalus/activity/AppFilterActivity.java b/app/src/main/java/org/itxtech/daedalus/activity/AppFilterActivity.java
index 65cc4a2..15d8bc3 100644
--- a/app/src/main/java/org/itxtech/daedalus/activity/AppFilterActivity.java
+++ b/app/src/main/java/org/itxtech/daedalus/activity/AppFilterActivity.java
@@ -40,7 +40,6 @@ import java.util.Objects;
  * (at your option) any later version.
  */
 public class AppFilterActivity extends AppCompatActivity {
-
     private RecyclerViewAdapter adapter;
 
     @Override
@@ -78,7 +77,7 @@ public class AppFilterActivity extends AppCompatActivity {
         adapter.notifyDataSetChanged();
     }
 
-    private class AppObject {
+    private static class AppObject {
         private String appName;
         private String appPackageName;
         private Drawable appIcon;
@@ -111,13 +110,11 @@ public class AppFilterActivity extends AppCompatActivity {
 
         void updateList(ArrayList<AppObject> appObjects) {
             appList = appObjects;
-
             for (int i = 0; i < appObjects.size(); i++) {
                 if (Daedalus.configurations.getAppObjects().contains(appObjects.get(i).appPackageName)) {
                     checkStatus.put(i, true);
                 }
             }
-
             runOnUiThread(this::notifyDataSetChanged);
         }
 
@@ -158,7 +155,7 @@ public class AppFilterActivity extends AppCompatActivity {
         }
     }
 
-    private class RecyclerViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
+    private static class RecyclerViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
         private ImageView appIcon;
         private TextView appName;
         private CheckBox appCheck;
@@ -172,7 +169,6 @@ public class AppFilterActivity extends AppCompatActivity {
             itemView.setOnClickListener(this);
         }
 
-
         @Override
         public void onClick(View v) {
             if (appCheck.isChecked()) {
diff --git a/app/src/main/java/org/itxtech/daedalus/fragment/DnsTestFragment.java b/app/src/main/java/org/itxtech/daedalus/fragment/DnsTestFragment.java
index c08cf3c..797ef84 100644
--- a/app/src/main/java/org/itxtech/daedalus/fragment/DnsTestFragment.java
+++ b/app/src/main/java/org/itxtech/daedalus/fragment/DnsTestFragment.java
@@ -151,7 +151,6 @@ public class DnsTestFragment extends ToolbarFragment {
                 }
             }
 
-
             private StringBuilder testServer(DnsQuery dnsQuery, Record.TYPE type, AbstractDnsServer server, String domain, StringBuilder testText) {
                 Logger.debug("Testing DNS server " + server.getRealName());
                 testText.append(getString(R.string.test_domain)).append(" ").append(domain).append("\n")
diff --git a/app/src/main/java/org/itxtech/daedalus/provider/ProviderPicker.java b/app/src/main/java/org/itxtech/daedalus/provider/ProviderPicker.java
index cd882bd..b1d56eb 100644
--- a/app/src/main/java/org/itxtech/daedalus/provider/ProviderPicker.java
+++ b/app/src/main/java/org/itxtech/daedalus/provider/ProviderPicker.java
@@ -15,7 +15,7 @@ import org.itxtech.daedalus.service.DaedalusVpnService;
  * the Free Software Foundation, either version 3 of the License, or
  * (at your option) any later version.
  */
-public class ProviderPicker {
+public abstract class ProviderPicker {
     public static final int DNS_QUERY_METHOD_UDP = 0;
     public static final int DNS_QUERY_METHOD_TCP = 1;
     public static final int DNS_QUERY_METHOD_TLS = 2;
diff --git a/app/src/main/java/org/itxtech/daedalus/service/DaedalusVpnService.java b/app/src/main/java/org/itxtech/daedalus/service/DaedalusVpnService.java
index eb410e4..03ad1d8 100644
--- a/app/src/main/java/org/itxtech/daedalus/service/DaedalusVpnService.java
+++ b/app/src/main/java/org/itxtech/daedalus/service/DaedalusVpnService.java
@@ -91,7 +91,7 @@ public class DaedalusVpnService extends VpnService implements Runnable {
         }
     }
 
-    public static void updateUpstreamServers(Context context) {
+    private static void updateUpstreamServers(Context context) {
         String[] servers = DnsServersDetector.getServers(context);
         if (servers != null) {
             if (servers.length >= 2 && (aliasPrimary == null || !aliasPrimary.getHostAddress().equals(servers[0])) &&
@@ -197,6 +197,7 @@ public class DaedalusVpnService extends VpnService implements Runnable {
         stopThread();
         if (receiver != null) {
             unregisterReceiver(receiver);
+            receiver = null;
         }
     }