added first test case for getItemList() & and added productive code for RadioPlayer

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

@@ -185,7 +185,7 @@ public class RadioPlayer implements Device {
 
     @Override
     public String[] getItemList() {
-        return null;
+        return savedPlaylist.toArray(new String[0]);
     }
 
     @Override

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

@@ -320,4 +320,18 @@ class RadioPlayerTest {
                 Arguments.of("Test for change order in saved playlist with nr is than smaller than 0 at front of playlist", rp2, 0, "Teddy")
         );
     }
+    @ParameterizedTest
+    @MethodSource("getItemListOptions")
+    void testGetItemList(String testName, RadioPlayer testRp, String[] expectedResult) {
+        String[] playList = testRp.getItemList();
+        assertThat(playList).describedAs(testName).isEqualTo(expectedResult);
+    }
+
+    static Stream<Arguments> getItemListOptions() {
+        RadioPlayer rp = new RadioPlayer();
+
+        return Stream.of(
+                Arguments.of("Test for return saved playlist", rp, rp.savedPlaylist.toArray(new String[0]))
+        );
+    }
 }