refactor the data service for the consistent behavior in case of failure

src/main/java/click/poweronoff/satellite/api/FeaturesController.java

@@ -23,7 +23,7 @@ public class FeaturesController {
     @RequestMapping(value = "/features", method = RequestMethod.GET, produces = "application/json")
     @ResponseBody
     public List<Feature> features() {
-        return dataService.getAllFeatures();
+        return dataService.getAllFeatures().orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Unable to find any feature"));
     }
 
     @RequestMapping(value = "/features/{id}", method = RequestMethod.GET, produces = "application/json")

src/main/java/click/poweronoff/satellite/service/DataService.java

@@ -10,17 +10,17 @@ public interface DataService {
     /**
      * returned all possible Features as a list
      *
-     * @return list of Features
+     * @return Optional of list of Feature in case of features can
+     * be found or empty Optional
      */
-    List<Feature> getAllFeatures();
+    Optional<List<Feature>> getAllFeatures();
 
     /**
      * returned specific Feature by given id
      *
      * @param featureId feature id
      * @return Optional of Feature in case of the feature can be
-     * found
-     * or an empty Optional
+     * found or an empty Optional
      */
     Optional<Feature> getFeature(final String featureId);
 
@@ -29,7 +29,8 @@ public interface DataService {
      * returned a picture as a byte[] representation for given feature id
      *
      * @param featureId feature id
-     * @return a picture as a byte[]
+     * @return Optional of byte[] representing a picture or
+     * empty optional if no picture can be found for given feature id
      */
     Optional<byte[]> getPicture(final String featureId);
 }

src/main/java/click/poweronoff/satellite/service/DataServiceImpl.java

@@ -3,15 +3,14 @@ package click.poweronoff.satellite.service;
 import click.poweronoff.satellite.domain.Feature;
 import org.springframework.stereotype.Component;
 
-import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 
 @Component
 public class DataServiceImpl implements DataService {
 
-    public List<Feature> getAllFeatures() {
-        return Collections.emptyList();
+    public Optional<List<Feature>> getAllFeatures() {
+        return Optional.empty();
     }
 
     @Override

src/test/java/click/poweronoff/satellite/api/FeaturesControllerTest.java

@@ -33,7 +33,7 @@ public class FeaturesControllerTest {
 
     @Test
     public void getFeaturesShouldReturnDefaultMessage() throws Exception {
-        when(dataService.getAllFeatures()).thenReturn(List.of(createTestFeature()));
+        when(dataService.getAllFeatures()).thenReturn(Optional.of(List.of(createTestFeature())));
         this.mockMvc.perform(get("/features")).andDo(print())
                 .andExpect(status().isOk())
                 .andExpect(content().contentType(MediaType.APPLICATION_JSON))
@@ -46,6 +46,13 @@ public class FeaturesControllerTest {
                 );
     }
 
+    @Test
+    public void getFeaturesShouldReturnNotFound() throws Exception {
+        when(dataService.getAllFeatures()).thenReturn(Optional.empty());
+        this.mockMvc.perform(get("/features"))
+                .andExpect(status().isNotFound());
+    }
+
     @Test
     public void getFeatureByIdShouldReturnAMessage() throws Exception {
         when(dataService.getFeature(anyString())).thenReturn(Optional.of(createTestFeature()));