Make some methods return the specific InfoItem type

- Some methods were returning a broader range of InfoItem types than it should be. For example, a ChannelInfo should return a List containing only StreamInfoItem, instead of the more general InfoItem.
- Renamed and changed return type of ListExtractor.getInfoItems to getInitialPage returning a InfoItemsPage, to be consistent with getPage(url)

Author: mauriciocolli

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

@@ -4,21 +4,45 @@
 
 import javax.annotation.Nonnull;
 import java.io.IOException;
+import java.util.Collections;
 import java.util.List;
 
 /**
  * Base class to extractors that have a list (e.g. playlists, users).
  */
-public abstract class ListExtractor extends Extractor {
+public abstract class ListExtractor<R extends InfoItem> extends Extractor {
 
     public ListExtractor(StreamingService service, String url) {
         super(service, url);
     }
 
+    /**
+     * A {@link InfoItemsPage InfoItemsPage} corresponding to the initial page where the items are from the initial request and
+     * the nextPageUrl relative to it.
+     *
+     * @return a {@link InfoItemsPage} corresponding to the initial page
+     */
     @Nonnull
-    public abstract InfoItemsCollector<? extends InfoItem, ?> getInfoItems() throws IOException, ExtractionException;
+    public abstract InfoItemsPage<R> getInitialPage() throws IOException, ExtractionException;
+
+    /**
+     * Returns an url that can be used to get the next page relative to the initial one.<br/>
+     * <p>Usually, these links will only work in the implementation itself.</p>
+     *
+     * @return an url pointing to the next page relative to the initial page
+     * @see #getPage(String)
+     */
     public abstract String getNextPageUrl() throws IOException, ExtractionException;
-    public abstract InfoItemPage<? extends InfoItem> getPage(final String nextPageUrl) throws IOException, ExtractionException;
+
+    /**
+     * Get a list of items corresponding to the specific requested page.
+     *
+     * @param nextPageUrl any next page url got from the exclusive implementation of the list extractor
+     * @return a {@link InfoItemsPage} corresponding to the requested page
+     * @see #getNextPageUrl()
+     * @see InfoItemsPage#getNextPageUrl()
+     */
+    public abstract InfoItemsPage<R> getPage(final String nextPageUrl) throws IOException, ExtractionException;
 
     public boolean hasNextPage() throws IOException, ExtractionException {
         final String nextPageUrl = getNextPageUrl();
@@ -29,14 +53,34 @@ public boolean hasNextPage() throws IOException, ExtractionException {
     // Inner
     //////////////////////////////////////////////////////////////////////////*/
 
-    public static class InfoItemPage<T extends InfoItem> {
+    /**
+     * A class that is used to wrap a list of gathered items and eventual errors, it
+     * also contains a field that points to the next available page ({@link #nextPageUrl}).
+     */
+    public static class InfoItemsPage<T extends InfoItem> {
+        private static final InfoItemsPage<InfoItem> EMPTY =
+                new InfoItemsPage<>(Collections.<InfoItem>emptyList(), "", Collections.<Throwable>emptyList());
+
+        /**
+         * A convenient method that returns a representation of an empty page.
+         *
+         * @return a type-safe page with the list of items and errors empty and the nextPageUrl set to an empty string.
+         */
+        public static <T extends InfoItem> InfoItemsPage<T> emptyPage() {
+            //noinspection unchecked
+            return (InfoItemsPage<T>) EMPTY;
+        }
+
+
         /**
-         * The current list of items to this result
+         * The current list of items of this page
          */
         private final List<T> itemsList;
 
         /**
-         * Next url to fetch more items
+         * Url pointing to the next page relative to this one
+         *
+         * @see ListExtractor#getPage(String)
          */
         private final String nextPageUrl;
 
@@ -45,11 +89,11 @@ public static class InfoItemPage<T extends InfoItem> {
          */
         private final List<Throwable> errors;
 
-        public InfoItemPage(InfoItemsCollector<T, ?> collector, String nextPageUrl) {
+        public InfoItemsPage(InfoItemsCollector<T, ?> collector, String nextPageUrl) {
             this(collector.getItems(), nextPageUrl, collector.getErrors());
         }
 
-        public InfoItemPage(List<T> itemsList, String nextPageUrl, List<Throwable> errors) {
+        public InfoItemsPage(List<T> itemsList, String nextPageUrl, List<Throwable> errors) {
             this.itemsList = itemsList;
             this.nextPageUrl = nextPageUrl;
             this.errors = errors;
@@ -59,7 +103,7 @@ public boolean hasNextPage() {
             return nextPageUrl != null && !nextPageUrl.isEmpty();
         }
 
-        public List<T> getItemsList() {
+        public List<T> getItems() {
             return itemsList;
         }
 

src/main/java/org/schabi/newpipe/extractor/channel/ChannelExtractor.java

@@ -1,16 +1,12 @@
 package org.schabi.newpipe.extractor.channel;
 
-import edu.umd.cs.findbugs.annotations.NonNull;
 import org.schabi.newpipe.extractor.ListExtractor;
 import org.schabi.newpipe.extractor.StreamingService;
 import org.schabi.newpipe.extractor.UrlIdHandler;
-import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
-import org.schabi.newpipe.extractor.stream.StreamInfoItemsCollector;
 
 import javax.annotation.Nonnull;
-import java.io.IOException;
 
 /*
  * Created by Christian Schabesberger on 25.07.16.
@@ -32,7 +28,7 @@
  * along with NewPipe.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-public abstract class ChannelExtractor extends ListExtractor {
+public abstract class ChannelExtractor extends ListExtractor<StreamInfoItem> {
 
     public ChannelExtractor(StreamingService service, String url) {
         super(service, url);
@@ -44,12 +40,6 @@ protected UrlIdHandler getUrlIdHandler() {
         return getService().getChannelUrlIdHandler();
     }
 
-    @NonNull
-    @Override
-    public abstract StreamInfoItemsCollector getInfoItems() throws IOException, ExtractionException;
-    @Override
-    public abstract InfoItemPage<StreamInfoItem> getPage(String nextPageUrl) throws IOException, ExtractionException;
-
     public abstract String getAvatarUrl() throws ParsingException;
     public abstract String getBannerUrl() throws ParsingException;
     public abstract String getFeedUrl() throws ParsingException;

src/main/java/org/schabi/newpipe/extractor/channel/ChannelInfo.java

@@ -1,6 +1,6 @@
 package org.schabi.newpipe.extractor.channel;
 
-import org.schabi.newpipe.extractor.ListExtractor.InfoItemPage;
+import org.schabi.newpipe.extractor.ListExtractor.InfoItemsPage;
 import org.schabi.newpipe.extractor.ListInfo;
 import org.schabi.newpipe.extractor.NewPipe;
 import org.schabi.newpipe.extractor.StreamingService;
@@ -30,18 +30,12 @@
  * along with NewPipe.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-public class ChannelInfo extends ListInfo {
+public class ChannelInfo extends ListInfo<StreamInfoItem> {
 
     public ChannelInfo(int serviceId, String url, String id, String name) {
         super(serviceId, id, url, name);
     }
 
-
-    public static InfoItemPage<StreamInfoItem> getMoreItems(StreamingService service, String url, String pageUrl)
-            throws IOException, ExtractionException {
-        return service.getChannelExtractor(url).getPage(pageUrl);
-    }
-
     public static ChannelInfo getInfo(String url) throws IOException, ExtractionException {
         return getInfo(NewPipe.getServiceByUrl(url), url);
     }
@@ -52,6 +46,10 @@ public static ChannelInfo getInfo(StreamingService service, String url) throws I
         return getInfo(extractor);
     }
 
+    public static InfoItemsPage<StreamInfoItem> getMoreItems(StreamingService service, String url, String pageUrl) throws IOException, ExtractionException {
+        return service.getChannelExtractor(url).getPage(pageUrl);
+    }
+
     public static ChannelInfo getInfo(ChannelExtractor extractor) throws IOException, ExtractionException {
 
         // important data
@@ -79,7 +77,9 @@ public static ChannelInfo getInfo(ChannelExtractor extractor) throws IOException
             info.addError(e);
         }
 
-        info.setRelatedItems(ExtractorHelper.getInfoItemsOrLogError(info, extractor));
+        final InfoItemsPage<StreamInfoItem> itemsPage = ExtractorHelper.getItemsPageOrLogError(info, extractor);
+        info.setRelatedItems(itemsPage.getItems());
+        info.setNextPageUrl(itemsPage.getNextPageUrl());
 
         try {
             info.setSubscriberCount(extractor.getSubscriberCount());
@@ -92,7 +92,6 @@ public static ChannelInfo getInfo(ChannelExtractor extractor) throws IOException
             info.addError(e);
         }
 
-        info.setNextPageUrl(extractor.getNextPageUrl());
         return info;
     }
 

src/main/java/org/schabi/newpipe/extractor/kiosk/KioskExtractor.java

@@ -20,18 +20,15 @@
  * along with NewPipe.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-import edu.umd.cs.findbugs.annotations.NonNull;
 import org.schabi.newpipe.extractor.ListExtractor;
 import org.schabi.newpipe.extractor.StreamingService;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
-import org.schabi.newpipe.extractor.stream.StreamInfoItemsCollector;
 
 import javax.annotation.Nonnull;
-import java.io.IOException;
 
-public abstract class KioskExtractor extends ListExtractor {
+public abstract class KioskExtractor extends ListExtractor<StreamInfoItem> {
     private String contentCountry = null;
     private final String id;
 
@@ -43,12 +40,6 @@ public KioskExtractor(StreamingService streamingService,
         this.id = kioskId;
     }
 
-    @NonNull
-    @Override
-    public abstract StreamInfoItemsCollector getInfoItems() throws IOException, ExtractionException;
-    @Override
-    public abstract InfoItemPage<StreamInfoItem> getPage(String nextPageUrl) throws IOException, ExtractionException;
-
     /**
      * For certain Websites the content of a kiosk will be different depending
      * on the country you want to poen the website in. Therefore you should

src/main/java/org/schabi/newpipe/extractor/kiosk/KioskInfo.java

@@ -30,16 +30,16 @@
 
 import java.io.IOException;
 
-public class KioskInfo extends ListInfo {
+public class KioskInfo extends ListInfo<StreamInfoItem> {
 
     private KioskInfo(int serviceId, String id, String url, String name) {
         super(serviceId, id, url, name);
     }
 
-    public static ListExtractor.InfoItemPage<StreamInfoItem> getMoreItems(StreamingService service,
-                                                                          String url,
-                                                                          String pageUrl,
-                                                                          String contentCountry) throws IOException, ExtractionException {
+    public static ListExtractor.InfoItemsPage<StreamInfoItem> getMoreItems(StreamingService service,
+                                                                           String url,
+                                                                           String pageUrl,
+                                                                           String contentCountry) throws IOException, ExtractionException {
         KioskList kl = service.getKioskList();
         KioskExtractor extractor = kl.getExtractorByUrl(url, pageUrl);
         extractor.setContentCountry(contentCountry);
@@ -75,7 +75,9 @@ public static KioskInfo getInfo(KioskExtractor extractor) throws ExtractionExcep
 
         KioskInfo info = new KioskInfo(serviceId, id, name, url);
 
-        info.setRelatedItems(ExtractorHelper.getInfoItemsOrLogError(info, extractor));
+        final ListExtractor.InfoItemsPage<StreamInfoItem> itemsPage = ExtractorHelper.getItemsPageOrLogError(info, extractor);
+        info.setRelatedItems(itemsPage.getItems());
+        info.setNextPageUrl(itemsPage.getNextPageUrl());
 
         return info;
     }

src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistExtractor.java

@@ -1,18 +1,14 @@
 package org.schabi.newpipe.extractor.playlist;
 
-import edu.umd.cs.findbugs.annotations.NonNull;
 import org.schabi.newpipe.extractor.ListExtractor;
 import org.schabi.newpipe.extractor.StreamingService;
 import org.schabi.newpipe.extractor.UrlIdHandler;
-import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.exceptions.ParsingException;
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
-import org.schabi.newpipe.extractor.stream.StreamInfoItemsCollector;
 
 import javax.annotation.Nonnull;
-import java.io.IOException;
 
-public abstract class PlaylistExtractor extends ListExtractor {
+public abstract class PlaylistExtractor extends ListExtractor<StreamInfoItem> {
 
     public PlaylistExtractor(StreamingService service, String url) {
         super(service, url);
@@ -24,12 +20,6 @@ protected UrlIdHandler getUrlIdHandler() {
         return getService().getPlaylistUrlIdHandler();
     }
 
-    @NonNull
-    @Override
-    public abstract StreamInfoItemsCollector getInfoItems() throws IOException, ExtractionException;
-    @Override
-    public abstract InfoItemPage<StreamInfoItem> getPage(String nextPageUrl) throws IOException, ExtractionException;
-
     public abstract String getThumbnailUrl() throws ParsingException;
     public abstract String getBannerUrl() throws ParsingException;
 

src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistInfo.java

@@ -1,26 +1,21 @@
 package org.schabi.newpipe.extractor.playlist;
 
-import org.schabi.newpipe.extractor.ListExtractor.InfoItemPage;
+import org.schabi.newpipe.extractor.ListExtractor.InfoItemsPage;
 import org.schabi.newpipe.extractor.ListInfo;
 import org.schabi.newpipe.extractor.NewPipe;
 import org.schabi.newpipe.extractor.StreamingService;
 import org.schabi.newpipe.extractor.exceptions.ExtractionException;
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
+import org.schabi.newpipe.extractor.utils.ExtractorHelper;
 
 import java.io.IOException;
 
-import static org.schabi.newpipe.extractor.utils.ExtractorHelper.getInfoItemsOrLogError;
-
-public class PlaylistInfo extends ListInfo {
+public class PlaylistInfo extends ListInfo<StreamInfoItem> {
 
     public PlaylistInfo(int serviceId, String id, String url, String name) {
         super(serviceId, id, url, name);
     }
 
-    public static InfoItemPage<StreamInfoItem> getMoreItems(StreamingService service, String url, String pageUrl) throws IOException, ExtractionException {
-        return service.getPlaylistExtractor(url).getPage(pageUrl);
-    }
-
     public static PlaylistInfo getInfo(String url) throws IOException, ExtractionException {
         return getInfo(NewPipe.getServiceByUrl(url), url);
     }
@@ -31,6 +26,10 @@ public static PlaylistInfo getInfo(StreamingService service, String url) throws
         return getInfo(extractor);
     }
 
+    public static InfoItemsPage<StreamInfoItem> getMoreItems(StreamingService service, String url, String pageUrl) throws IOException, ExtractionException {
+        return service.getPlaylistExtractor(url).getPage(pageUrl);
+    }
+
     /**
      * Get PlaylistInfo from PlaylistExtractor
      *
@@ -75,8 +74,10 @@ public static PlaylistInfo getInfo(PlaylistExtractor extractor) throws IOExcepti
             info.addError(e);
         }
 
-        info.setRelatedItems(getInfoItemsOrLogError(info, extractor));
-        info.setNextPageUrl(extractor.getNextPageUrl());
+        final InfoItemsPage<StreamInfoItem> itemsPage = ExtractorHelper.getItemsPageOrLogError(info, extractor);
+        info.setRelatedItems(itemsPage.getItems());
+        info.setNextPageUrl(itemsPage.getNextPageUrl());
+
         return info;
     }
 

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

@@ -92,11 +92,11 @@ public String getDescription() {
 
     @Nonnull
     @Override
-    public StreamInfoItemsCollector getInfoItems() throws ExtractionException {
+    public InfoItemsPage<StreamInfoItem> getInitialPage() throws ExtractionException {
         if(streamInfoItemsCollector == null) {
             computeNextPageAndGetStreams();
         }
-        return streamInfoItemsCollector;
+        return new InfoItemsPage<>(streamInfoItemsCollector, getNextPageUrl());
     }
 
     @Override
@@ -123,14 +123,14 @@ private void computeNextPageAndGetStreams() throws ExtractionException {
     }
 
     @Override
-    public InfoItemPage<StreamInfoItem> getPage(final String pageUrl) throws IOException, ExtractionException {
+    public InfoItemsPage<StreamInfoItem> getPage(final String pageUrl) throws IOException, ExtractionException {
         if (pageUrl == null || pageUrl.isEmpty()) {
             throw new ExtractionException(new IllegalArgumentException("Page url is empty or null"));
         }
 
         StreamInfoItemsCollector collector = new StreamInfoItemsCollector(getServiceId());
         String nextPageUrl = SoundcloudParsingHelper.getStreamsFromApiMinItems(15, collector, pageUrl);
 
-        return new InfoItemPage<>(collector, nextPageUrl);
+        return new InfoItemsPage<>(collector, nextPageUrl);
     }
 }