Method mute of CDPlayer tested on checkingTheReturnValue -> ShouldReturnAString.

src/main/java/device/cdPlayer/CDPlayer.java

@@ -120,7 +120,7 @@ public class CDPlayer implements Device {
 
     @Override
     public String unmute() {
-        return null;
+        return ("Volume unmute.");
     }
 
     @Override

src/test/java/device/cdPlayer/CDPlayerTest.java

@@ -565,7 +565,17 @@ class CDPlayerTest {
         );
     }
 
-
+    @ParameterizedTest
+    @MethodSource("unmuteTestData")
+    void unmuteTest(String testName,String cases, CDPlayer _cdPlayer,String expectedResult) {
+        assertThat(_cdPlayer.unmute()).describedAs(testName).isEqualTo(expectedResult);
+    }
+    static Stream<Arguments> unmuteTestData () {
+        CDPlayer cdPlayer1 = new CDPlayer();
+        return Stream.of(
+                Arguments.of("[unmute() by checkingTheReturnValue ] => ShouldReturnAString", "Case1", cdPlayer1, "Volume unmute.")
+        );
+    }
 
 
     }