diff --git a/src/org/kde/kdeconnect/Backends/BaseLink.java b/src/org/kde/kdeconnect/Backends/BaseLink.java
index 622ed411..6bfe9288 100644
--- a/src/org/kde/kdeconnect/Backends/BaseLink.java
+++ b/src/org/kde/kdeconnect/Backends/BaseLink.java
@@ -8,6 +8,7 @@ package org.kde.kdeconnect.Backends;
 
 import android.content.Context;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.WorkerThread;
 
 import org.kde.kdeconnect.Device;
@@ -20,18 +21,17 @@ import java.util.ArrayList;
 
 public abstract class BaseLink {
 
-    protected final Context context;
-
     public interface PacketReceiver {
-        void onPacketReceived(NetworkPacket np);
+        void onPacketReceived(@NonNull NetworkPacket np);
     }
 
+    protected final Context context;
     private final BaseLinkProvider linkProvider;
     private final String deviceId;
     private final ArrayList<PacketReceiver> receivers = new ArrayList<>();
     protected PrivateKey privateKey;
 
-    protected BaseLink(Context context, String deviceId, BaseLinkProvider linkProvider) {
+    protected BaseLink(@NonNull Context context, @NonNull String deviceId, @NonNull BaseLinkProvider linkProvider) {
         this.context = context;        
         this.linkProvider = linkProvider;
         this.deviceId = deviceId;
@@ -39,13 +39,13 @@ public abstract class BaseLink {
 
     /* To be implemented by each link for pairing handlers */
     public abstract String getName();
-    public abstract BasePairingHandler getPairingHandler(Device device, BasePairingHandler.PairingHandlerCallback callback);
+    public abstract BasePairingHandler getPairingHandler(@NonNull Device device, @NonNull BasePairingHandler.PairingHandlerCallback callback);
 
     public String getDeviceId() {
         return deviceId;
     }
 
-    public void setPrivateKey(PrivateKey key) {
+    public void setPrivateKey(@NonNull PrivateKey key) {
         privateKey = key;
     }
 
@@ -53,15 +53,15 @@ public abstract class BaseLink {
         return linkProvider;
     }
 
-    public void addPacketReceiver(PacketReceiver pr) {
+    public void addPacketReceiver(@NonNull PacketReceiver pr) {
         receivers.add(pr);
     }
-    public void removePacketReceiver(PacketReceiver pr) {
+    public void removePacketReceiver(@NonNull PacketReceiver pr) {
         receivers.remove(pr);
     }
 
     //Should be called from a background thread listening for packets
-    protected void packetReceived(NetworkPacket np) {
+    protected void packetReceived(@NonNull NetworkPacket np) {
         for(PacketReceiver pr : receivers) {
             pr.onPacketReceived(np);
         }
@@ -73,5 +73,5 @@ public abstract class BaseLink {
 
     //TO OVERRIDE, should be sync. If sendPayloadFromSameThread is false, it should only block to send the packet but start a separate thread to send the payload.
     @WorkerThread
-    public abstract boolean sendPacket(NetworkPacket np, Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) throws IOException;
+    public abstract boolean sendPacket(@NonNull NetworkPacket np, @NonNull Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) throws IOException;
 }
diff --git a/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java b/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
index 2ae8f5f2..fa72872d 100644
--- a/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
+++ b/src/org/kde/kdeconnect/Backends/BluetoothBackend/BluetoothLink.java
@@ -10,6 +10,7 @@ import android.bluetooth.BluetoothDevice;
 import android.content.Context;
 import android.util.Log;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.WorkerThread;
 
 import org.json.JSONException;
@@ -137,7 +138,7 @@ public class BluetoothLink extends BaseLink {
 
     @WorkerThread
     @Override
-    public boolean sendPacket(NetworkPacket np, Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) throws IOException {
+    public boolean sendPacket(@NonNull NetworkPacket np, @NonNull Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) throws IOException {
         // sendPayloadFromSameThread is ignored, we always send from the same thread!
 
         /*if (!isConnected()) {
diff --git a/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java b/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
index e3a67f91..38ce82d8 100644
--- a/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
+++ b/src/org/kde/kdeconnect/Backends/LanBackend/LanLink.java
@@ -9,6 +9,7 @@ package org.kde.kdeconnect.Backends.LanBackend;
 import android.content.Context;
 import android.util.Log;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.WorkerThread;
 
 import org.json.JSONObject;
@@ -131,7 +132,7 @@ public class LanLink extends BaseLink {
     //Blocking, do not call from main thread
     @WorkerThread
     @Override
-    public boolean sendPacket(NetworkPacket np, final Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
+    public boolean sendPacket(@NonNull NetworkPacket np, @NonNull final Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
         if (socket == null) {
             Log.e("KDE/sendPacket", "Not yet connected");
             callback.onFailure(new NotYetConnectedException());
diff --git a/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java b/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
index 691e127c..65c15446 100644
--- a/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
+++ b/src/org/kde/kdeconnect/Backends/LoopbackBackend/LoopbackLink.java
@@ -8,6 +8,7 @@ package org.kde.kdeconnect.Backends.LoopbackBackend;
 
 import android.content.Context;
 
+import androidx.annotation.NonNull;
 import androidx.annotation.WorkerThread;
 
 import org.kde.kdeconnect.Backends.BaseLink;
@@ -34,7 +35,7 @@ public class LoopbackLink extends BaseLink {
 
     @WorkerThread
     @Override
-    public boolean sendPacket(NetworkPacket in, Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
+    public boolean sendPacket(@NonNull NetworkPacket in, @NonNull Device.SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
         packetReceived(in);
         if (in.hasPayload()) {
             callback.onPayloadProgressChanged(0);
diff --git a/src/org/kde/kdeconnect/Device.java b/src/org/kde/kdeconnect/Device.java
index 834481e1..88b8349d 100644
--- a/src/org/kde/kdeconnect/Device.java
+++ b/src/org/kde/kdeconnect/Device.java
@@ -93,7 +93,7 @@ public class Device implements BaseLink.PacketReceiver {
     }
 
     public interface PluginsChangedListener {
-        void onPluginsChanged(Device device);
+        void onPluginsChanged(@NonNull Device device);
     }
 
     public enum PairStatus {
@@ -626,22 +626,22 @@ public class Device implements BaseLink.PacketReceiver {
     };
 
     @AnyThread
-    public void sendPacket(NetworkPacket np) {
+    public void sendPacket(@NonNull NetworkPacket np) {
         sendPacket(np, -1, defaultCallback);
     }
 
     @AnyThread
-    public void sendPacket(NetworkPacket np, int replaceID) {
+    public void sendPacket(@NonNull NetworkPacket np, int replaceID) {
         sendPacket(np, replaceID, defaultCallback);
     }
 
     @WorkerThread
-    public boolean sendPacketBlocking(NetworkPacket np) {
+    public boolean sendPacketBlocking(@NonNull NetworkPacket np) {
         return sendPacketBlocking(np, defaultCallback);
     }
 
     @AnyThread
-    public void sendPacket(final NetworkPacket np, final SendPacketStatusCallback callback) {
+    public void sendPacket(@NonNull final NetworkPacket np, @NonNull final SendPacketStatusCallback callback) {
         sendPacket(np, -1, callback);
     }
 
@@ -652,7 +652,7 @@ public class Device implements BaseLink.PacketReceiver {
      * @param callback A callback for success/failure
      */
     @AnyThread
-    public void sendPacket(final NetworkPacket np, int replaceID, final SendPacketStatusCallback callback) {
+    public void sendPacket(@NonNull final NetworkPacket np, int replaceID, @NonNull final SendPacketStatusCallback callback) {
         if (packetQueue == null) {
             callback.onFailure(new Exception("Device disconnected!"));
         } else {
@@ -675,7 +675,7 @@ public class Device implements BaseLink.PacketReceiver {
     }
 
     @WorkerThread
-    public boolean sendPacketBlocking(final NetworkPacket np, final SendPacketStatusCallback callback) {
+    public boolean sendPacketBlocking(@NonNull final NetworkPacket np, @NonNull final SendPacketStatusCallback callback) {
         return sendPacketBlocking(np, callback, false);
     }
 
@@ -691,7 +691,7 @@ public class Device implements BaseLink.PacketReceiver {
      * @see BaseLink#sendPacket(NetworkPacket, SendPacketStatusCallback)
      */
     @WorkerThread
-    public boolean sendPacketBlocking(final NetworkPacket np, final SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
+    public boolean sendPacketBlocking(@NonNull final NetworkPacket np, @NonNull final SendPacketStatusCallback callback, boolean sendPayloadFromSameThread) {
 
         /*
         if (!m_outgoingCapabilities.contains(np.getType()) && !NetworkPacket.protocolPacketTypes.contains(np.getType())) {