added new test case for prev() & changed prev() to work with regional and saved stations

src/main/java/device/radioPlayer/RadioPlayer.java

@@ -127,12 +127,23 @@ public class RadioPlayer implements Device {
 
     @Override
     public void prev() {
-        int currentIndex = savedPlaylist.indexOf(playedStation);
-        int nextIndex = savedPlaylist.size() - 1;
-        if (currentIndex != 0) {
-            nextIndex = (currentIndex - 1);
+        if (regionPlaylist.contains(playedStation)) {
+            int currentIndex = regionPlaylist.indexOf(playedStation);
+            int nextIndex = regionPlaylist.size() - 1;
+            if (currentIndex != 0) {
+                nextIndex = (currentIndex - 1);
+            }
+            playedStation = regionPlaylist.get(nextIndex);
+        } else {
+            {
+                int currentIndex = savedPlaylist.indexOf(playedStation);
+                int nextIndex = savedPlaylist.size() - 1;
+                if (currentIndex != 0) {
+                    nextIndex = (currentIndex - 1);
+                }
+                playedStation = savedPlaylist.get(nextIndex);
+            }
         }
-        playedStation = savedPlaylist.get(nextIndex);
     }
 
     @Override

src/test/java/device/radioPlayer/RadioPlayerTest.java

@@ -76,17 +76,21 @@ class RadioPlayerTest {
         RadioPlayer rp2 = new RadioPlayer();
         RadioPlayer rp3 = new RadioPlayer();
         RadioPlayer rp4 = new RadioPlayer();
+        RadioPlayer rp5 = new RadioPlayer();
         rp1.next();
         rp2.next();
         rp2.prev();
         rp3.prev();
         rp4.changeRegion("BY");
         rp4.next();
+        rp5.changeRegion("BY");
+        rp5.prev();
         return Stream.of(
                 Arguments.of("Next Station Test for saved Stations", rp1, "Teddy"),
                 Arguments.of("Prev Station Test for saved Stations", rp2, "YouFM"),
                 Arguments.of("Prev works on first Station for saved Stations", rp3, "MegaHits"),
-                Arguments.of("changed Region and going to next station", rp4, "Bayern 1")
+                Arguments.of("changed Region and going to next station", rp4, "Bayern 1"),
+                Arguments.of("changed Region and going to prev station", rp5, "Hit Radio N1")
         );
     }