diff --git a/app/build.gradle b/app/build.gradle
index 4d3059f..45dc106 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -25,4 +25,5 @@ android {
 }
 
 dependencies {
+    implementation "androidx.annotation:annotation:1.1.0"
 }
diff --git a/app/src/main/java/ch/fixme/status/Main.java b/app/src/main/java/ch/fixme/status/Main.java
index 36b3045..4581ac0 100644
--- a/app/src/main/java/ch/fixme/status/Main.java
+++ b/app/src/main/java/ch/fixme/status/Main.java
@@ -59,6 +59,8 @@
 import java.util.Set;
 import java.util.regex.Pattern;
 
+import androidx.annotation.UiThread;
+
 public class Main extends Activity {
 
     // API: https://spaceapi.io/
@@ -92,6 +94,7 @@ public class Main extends Activity {
     private GetImage getImageTask;
 
     @Override
+    @UiThread
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main);
@@ -110,12 +113,14 @@ public void onCreate(Bundle savedInstanceState) {
     }
 
     @Override
+    @UiThread
     protected void onNewIntent(Intent intent) {
         Log.d(TAG, "onNewIntent()=" + intent);
         showHsInfo(intent);
     }
 
     @Override
+    @UiThread
     protected void onDestroy() {
         if (getApiTask != null) {
             getApiTask.cancel(true);
@@ -205,6 +210,7 @@ protected Dialog onCreateDialog(int id) {
     }
 
     @Override
+    @UiThread
     public void startActivity(Intent intent) {
         // http://stackoverflow.com/questions/13691241/autolink-not-working-on-htc-htclinkifydispatcher
         try {
diff --git a/app/src/main/java/ch/fixme/status/Net.java b/app/src/main/java/ch/fixme/status/Net.java
index 9b1e490..628bac3 100644
--- a/app/src/main/java/ch/fixme/status/Net.java
+++ b/app/src/main/java/ch/fixme/status/Net.java
@@ -18,6 +18,9 @@
 import java.net.HttpURLConnection;
 import java.net.URL;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.WorkerThread;
+
 // From CommonsWare and Android Blog
 // https://github.com/commonsguy/cw-android/tree/master/Internet
 // http://android-developers.blogspot.ch/2010/07/multithreading-for-performance.html
@@ -31,11 +34,13 @@ public class Net {
     private HttpURLConnection mUrlConnection;
     private InputStream mInputStream;
 
-    public Net(String urlStr) throws Throwable {
+    @WorkerThread
+    public Net(@NonNull String urlStr) throws Throwable {
         this(urlStr, true);
     }
 
-    public Net(String urlStr, boolean useCache) throws Throwable {
+    @WorkerThread
+    public Net(@NonNull String urlStr, boolean useCache) throws Throwable {
         // Connect to URL
         URL url;
         int responseCode;
@@ -70,6 +75,8 @@ public Net(String urlStr, boolean useCache) throws Throwable {
         mInputStream = mUrlConnection.getInputStream();
     }
 
+    @WorkerThread
+    @NonNull
     public String getString() throws Throwable {
         try {
             BufferedReader r = new BufferedReader(new InputStreamReader(mInputStream));
@@ -87,6 +94,8 @@ public String getString() throws Throwable {
         }
     }
 
+    @WorkerThread
+    @NonNull
     public Bitmap getBitmap() throws Throwable {
         try {
             return BitmapFactory.decodeStream(new FlushedInputStream(mInputStream));