Introduce constants for some Strings that indicate no data in Tab class

Author: TobiGr

app/src/main/java/org/schabi/newpipe/settings/tabs/Tab.java

@@ -36,6 +36,10 @@
 public abstract class Tab {
     private static final String JSON_TAB_ID_KEY = "tab_id";
 
+    private static final String NO_NAME = "<no-name>";
+    private static final String NO_ID = "<no-id>";
+    private static final String NO_URL = "<no-url>";
+
     Tab() {
     }
 
@@ -185,7 +189,9 @@ public int getTabId() {
 
         @Override
         public String getTabName(final Context context) {
-            return "NewPipe"; //context.getString(R.string.blank_page_summary);
+            // TODO: find a better name for the blank tab (maybe "blank_tab") or replace it with
+            //       context.getString(R.string.app_name);
+            return "NewPipe"; // context.getString(R.string.blank_page_summary);
         }
 
         @DrawableRes
@@ -309,7 +315,7 @@ public static class KioskTab extends Tab {
         private String kioskId;
 
         private KioskTab() {
-            this(-1, "<no-id>");
+            this(-1, NO_ID);
         }
 
         public KioskTab(final int kioskServiceId, final String kioskId) {
@@ -357,7 +363,7 @@ protected void writeDataToJson(final JsonSink writerSink) {
         @Override
         protected void readDataFromJson(final JsonObject jsonObject) {
             kioskServiceId = jsonObject.getInt(JSON_KIOSK_SERVICE_ID_KEY, -1);
-            kioskId = jsonObject.getString(JSON_KIOSK_ID_KEY, "<no-id>");
+            kioskId = jsonObject.getString(JSON_KIOSK_ID_KEY, NO_ID);
         }
 
         @Override
@@ -395,7 +401,7 @@ public static class ChannelTab extends Tab {
         private String channelName;
 
         private ChannelTab() {
-            this(-1, "<no-url>", "<no-name>");
+            this(-1, NO_URL, NO_NAME);
         }
 
         public ChannelTab(final int channelServiceId, final String channelUrl,
@@ -440,8 +446,8 @@ protected void writeDataToJson(final JsonSink writerSink) {
         @Override
         protected void readDataFromJson(final JsonObject jsonObject) {
             channelServiceId = jsonObject.getInt(JSON_CHANNEL_SERVICE_ID_KEY, -1);
-            channelUrl = jsonObject.getString(JSON_CHANNEL_URL_KEY, "<no-url>");
-            channelName = jsonObject.getString(JSON_CHANNEL_NAME_KEY, "<no-name>");
+            channelUrl = jsonObject.getString(JSON_CHANNEL_URL_KEY, NO_URL);
+            channelName = jsonObject.getString(JSON_CHANNEL_NAME_KEY, NO_NAME);
         }
 
         @Override
@@ -527,15 +533,15 @@ public static class PlaylistTab extends Tab {
         private LocalItemType playlistType;
 
         private PlaylistTab() {
-            this(-1, "<no-name>");
+            this(-1, NO_NAME);
         }
 
         public PlaylistTab(final long playlistId, final String playlistName) {
             this.playlistName = playlistName;
             this.playlistId = playlistId;
             this.playlistType = LocalItemType.PLAYLIST_LOCAL_ITEM;
             this.playlistServiceId = -1;
-            this.playlistUrl = "<no-url>";
+            this.playlistUrl = NO_URL;
         }
 
         public PlaylistTab(final int playlistServiceId, final String playlistUrl,
@@ -589,8 +595,8 @@ protected void writeDataToJson(final JsonSink writerSink) {
         @Override
         protected void readDataFromJson(final JsonObject jsonObject) {
             playlistServiceId = jsonObject.getInt(JSON_PLAYLIST_SERVICE_ID_KEY, -1);
-            playlistUrl = jsonObject.getString(JSON_PLAYLIST_URL_KEY, "<no-url>");
-            playlistName = jsonObject.getString(JSON_PLAYLIST_NAME_KEY, "<no-name>");
+            playlistUrl = jsonObject.getString(JSON_PLAYLIST_URL_KEY, NO_URL);
+            playlistName = jsonObject.getString(JSON_PLAYLIST_NAME_KEY, NO_NAME);
             playlistId = jsonObject.getInt(JSON_PLAYLIST_ID_KEY, -1);
             playlistType = LocalItemType.valueOf(
                     jsonObject.getString(JSON_PLAYLIST_TYPE_KEY,