Add @Nonnull annotations

src/main/java/org/schabi/newpipe/extractor/ListExtractor.java

@@ -3,6 +3,7 @@ package org.schabi.newpipe.extractor;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.stream.StreamInfoItemCollector;
 
+import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.util.List;
 
@@ -39,7 +40,9 @@ public abstract class ListExtractor extends Extractor {
         return nextStreamsUrl == null || nextStreamsUrl.isEmpty();
     }
 
+    @Nonnull
     public abstract StreamInfoItemCollector getStreams() throws IOException, ExtractionException;
+
     public abstract NextItemsResult getNextStreams() throws IOException, ExtractionException;
 
     public boolean hasMoreStreams() {

src/main/java/org/schabi/newpipe/extractor/services/soundcloud/SoundcloudChannelExtractor.java

@@ -86,6 +86,7 @@ public class SoundcloudChannelExtractor extends ChannelExtractor {
         return user.getString("description", "");
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws IOException, ExtractionException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/services/soundcloud/SoundcloudChartsExtractor.java

@@ -50,6 +50,7 @@ public class SoundcloudChartsExtractor extends KioskExtractor {
         return new NextItemsResult(collector, nextStreamsUrl);
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws IOException, ExtractionException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/services/soundcloud/SoundcloudPlaylistExtractor.java

@@ -88,6 +88,7 @@ public class SoundcloudPlaylistExtractor extends PlaylistExtractor {
         return playlist.getNumber("track_count", 0).longValue();
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws IOException, ExtractionException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/services/soundcloud/SoundcloudStreamExtractor.java

@@ -71,7 +71,7 @@ public class SoundcloudStreamExtractor extends StreamExtractor {
 
     @Override
     public int getAgeLimit() {
-        return 0;
+        return NO_AGE_LIMIT;
     }
 
     @Override

src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeChannelExtractor.java

@@ -163,6 +163,7 @@ public class YoutubeChannelExtractor extends ChannelExtractor {
         }
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws IOException, ExtractionException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubePlaylistExtractor.java

@@ -143,6 +143,7 @@ public class YoutubePlaylistExtractor extends PlaylistExtractor {
         }
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws IOException, ExtractionException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeStreamExtractor.java

@@ -167,7 +167,7 @@ public class YoutubeStreamExtractor extends StreamExtractor {
     @Override
     public int getAgeLimit() throws ParsingException {
         if (!isAgeRestricted) {
-            return 0;
+            return NO_AGE_LIMIT;
         }
         try {
             return Integer.valueOf(doc.select("meta[property=\"og:restrictions:age\"]")

src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeTrendingExtractor.java

@@ -79,6 +79,7 @@ public class YoutubeTrendingExtractor extends KioskExtractor {
         }
     }
 
+    @Nonnull
     @Override
     public StreamInfoItemCollector getStreams() throws ParsingException {
         StreamInfoItemCollector collector = new StreamInfoItemCollector(getServiceId());

src/main/java/org/schabi/newpipe/extractor/stream/StreamExtractor.java

@@ -39,6 +39,8 @@ import java.util.List;
  */
 public abstract class StreamExtractor extends Extractor {
 
+    public static final int NO_AGE_LIMIT = 0;
+
     public StreamExtractor(StreamingService service, String url) throws IOException, ExtractionException {
         super(service, url);
     }