Make RtChecks more safe

src/main/java/com/jcabi/github/Checks.java

@@ -47,6 +47,6 @@ public interface Checks {
      * @return Checks.
      * @throws IOException If there is any I/O problem.
      */
-    Collection<Check> all() throws IOException;
+    Collection<? extends Check> all() throws IOException;
 
 }

src/main/java/com/jcabi/github/RtChecks.java

@@ -31,8 +31,12 @@
 
 import com.jcabi.http.Request;
 import com.jcabi.http.response.JsonResponse;
+import com.jcabi.http.response.RestResponse;
 import java.io.IOException;
+import java.net.HttpURLConnection;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.Optional;
 import java.util.stream.Collectors;
 import javax.json.JsonObject;
 import javax.json.JsonValue;
@@ -86,23 +90,29 @@ class RtChecks implements Checks {
      * @throws IOException If there is any I/O problem.
      */
     @Override
-    public Collection<Check> all() throws IOException {
+    public Collection<? extends Check> all() throws IOException {
         final Coordinates coords = this.pull.repo().coordinates();
-        return this.request.uri()
+        final RestResponse rest = this.request.uri()
             .path("/repos")
             .path(coords.user())
             .path(coords.repo())
             .path("/commits")
-            .path(this.pull.head().ref())
+            .path(this.pull.head().sha())
             .path("/check-runs")
             .back()
-            .method(Request.GET).fetch().as(JsonResponse.class)
+            .method(Request.GET).fetch()
+            .as(RestResponse.class)
+            .assertStatus(HttpURLConnection.HTTP_OK);
+        final JsonObject object = rest.as(JsonResponse.class)
             .json()
-            .readObject()
-            .getJsonArray("check_runs")
-            .stream()
-            .map(RtChecks::check)
-            .collect(Collectors.toList());
+            .readObject();
+        return Optional.ofNullable(object.getJsonArray("check_runs"))
+            .map(
+                obj -> obj.stream()
+                    .map(RtChecks::check)
+                    .collect(Collectors.toList())
+            )
+            .orElseGet(Collections::emptyList);
     }
 
     /**

src/test/java/com/jcabi/github/RtChecksTest.java

@@ -39,6 +39,7 @@
 import javax.json.Json;
 import org.hamcrest.MatcherAssert;
 import org.hamcrest.Matchers;
+import org.junit.Assert;
 import org.junit.Rule;
 import org.junit.Test;
 import org.mockito.Mockito;
@@ -70,7 +71,7 @@ public void getsAllChecks() throws IOException {
             .next(
                 new MkAnswer.Simple(
                     HttpURLConnection.HTTP_OK,
-                    RtChecksTest.json()
+                    RtChecksTest.jsonWithCheckRuns()
                 )
             )
             .start(this.resource.port())) {
@@ -85,12 +86,64 @@ public void getsAllChecks() throws IOException {
         }
     }
 
+    /**
+     * Checks whether RtChecks can return empty checks if they are absent.
+     *
+     * @throws IOException If some I/O problem happens.
+     */
+    @Test
+    public void returnsEmptyChecksIfTheyAreAbsent() throws IOException {
+        try (final MkContainer container = new MkGrizzlyContainer()
+            .next(
+                new MkAnswer.Simple(
+                    HttpURLConnection.HTTP_OK,
+                    RtChecksTest.empty()
+                )
+            )
+            .start(this.resource.port())) {
+            MatcherAssert.assertThat(
+                ((Checks) new RtChecks(
+                    new JdkRequest(container.home()),
+                    this.repo().pulls().get(0)
+                )).all(),
+                Matchers.iterableWithSize(0)
+            );
+        }
+    }
+
+    /**
+     * Checks whether RtChecks can throw an exception
+     * if response code is not 200.
+     *
+     * @throws IOException If some I/O problem happens.
+     */
+    @Test
+    public void assertsOkResponse() throws IOException {
+        try (final MkContainer container = new MkGrizzlyContainer()
+            .next(
+                new MkAnswer.Simple(
+                    HttpURLConnection.HTTP_NOT_FOUND,
+                    RtChecksTest.jsonWithCheckRuns()
+                )
+            ).start(this.resource.port())
+        ) {
+            final Checks checks = new RtChecks(
+                new JdkRequest(container.home()),
+                this.repo().pulls().get(0)
+            );
+            Assert.assertThrows(
+                AssertionError.class,
+                checks::all
+            );
+        }
+    }
+
     /**
      * Creates json response body.
      *
      * @return Json response body.
      */
-    private static String json() {
+    private static String jsonWithCheckRuns() {
         return Json.createObjectBuilder()
             .add("total_count", Json.createValue(1))
             .add(
@@ -108,6 +161,16 @@ private static String json() {
             .toString();
     }
 
+    /**
+     * Creates json response body without check runs.
+     * @return Json response body.
+     */
+    private static String empty() {
+        return Json.createObjectBuilder()
+            .build()
+            .toString();
+    }
+
     /**
      * Create and return repo for testing.
      * @return Repo
@@ -126,7 +189,7 @@ private Repo repo() throws IOException {
         Mockito.doReturn(pull).when(pulls).get(0);
         Mockito.doReturn(repo).when(pull).repo();
         Mockito.doReturn(ref).when(pull).head();
-        Mockito.doReturn("abcdef1").when(ref).ref();
+        Mockito.doReturn("abcdef1").when(ref).sha();
         return repo;
     }
 }

src/test/java/com/jcabi/github/RtPullTest.java

@@ -266,7 +266,7 @@ public void canFetchChecks() throws IOException {
                     ))
                 .start(this.resource.port())
         ) {
-            final Collection<Check> all = new RtPull(
+            final Collection<? extends Check> all = new RtPull(
                 new ApacheRequest(container.home()),
                 this.repo(),
                 new Random().nextInt()