diff --git a/app/src/main/java/fr/free/nrw/commons/auth/AuthenticatedActivity.java b/app/src/main/java/fr/free/nrw/commons/auth/AuthenticatedActivity.java
index 0db56e66b..1fcec5389 100644
--- a/app/src/main/java/fr/free/nrw/commons/auth/AuthenticatedActivity.java
+++ b/app/src/main/java/fr/free/nrw/commons/auth/AuthenticatedActivity.java
@@ -28,9 +28,7 @@ public abstract class AuthenticatedActivity extends NavigationBaseActivity {
                 .subscribeOn(Schedulers.io())
                 .doOnError(Timber::e)
                 .observeOn(AndroidSchedulers.mainThread())
-                .subscribe(
-                        cookie -> onAuthCookieAcquired(cookie),
-                        throwable -> onAuthFailure());
+                .subscribe(this::onAuthCookieAcquired, throwable -> onAuthFailure());
     }
 
     private void addAccount(AccountManager accountManager) {
diff --git a/app/src/main/java/fr/free/nrw/commons/category/CategorizationFragment.java b/app/src/main/java/fr/free/nrw/commons/category/CategorizationFragment.java
index b281a6c1b..f654dca7d 100644
--- a/app/src/main/java/fr/free/nrw/commons/category/CategorizationFragment.java
+++ b/app/src/main/java/fr/free/nrw/commons/category/CategorizationFragment.java
@@ -97,6 +97,7 @@ public class CategorizationFragment extends Fragment {
         categoriesCache = new HashMap<>();
         if (savedInstanceState != null) {
             items.addAll(savedInstanceState.getParcelableArrayList("currentCategories"));
+            //noinspection unchecked
             categoriesCache.putAll((HashMap<String, ArrayList<String>>) savedInstanceState
                     .getSerializable("categoriesCache"));
         }
@@ -201,9 +202,7 @@ public class CategorizationFragment extends Fragment {
                 .distinct()
                 .observeOn(AndroidSchedulers.mainThread())
                 .subscribe(
-                        s -> categoriesAdapter.add(s),
-                        throwable -> Timber.e(throwable),
-                        () -> {
+                        s -> categoriesAdapter.add(s), Timber::e, () -> {
                             categoriesAdapter.notifyDataSetChanged();
                             categoriesSearchInProgress.setVisibility(View.GONE);
 
diff --git a/app/src/main/java/fr/free/nrw/commons/mwapi/ApacheHttpClientMediaWikiApi.java b/app/src/main/java/fr/free/nrw/commons/mwapi/ApacheHttpClientMediaWikiApi.java
index 7af42b0fe..47a91acfb 100644
--- a/app/src/main/java/fr/free/nrw/commons/mwapi/ApacheHttpClientMediaWikiApi.java
+++ b/app/src/main/java/fr/free/nrw/commons/mwapi/ApacheHttpClientMediaWikiApi.java
@@ -30,6 +30,7 @@ import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 import java.util.Locale;
+import java.util.concurrent.Callable;
 
 import fr.free.nrw.commons.BuildConfig;
 import fr.free.nrw.commons.PageTitle;
@@ -235,8 +236,7 @@ public class ApacheHttpClientMediaWikiApi implements MediaWikiApi {
             }
 
             return categories;
-        })
-                .flatMapObservable(list -> Observable.fromIterable(list));
+        }).flatMapObservable(Observable::fromIterable);
     }
 
     @Override
@@ -265,15 +265,14 @@ public class ApacheHttpClientMediaWikiApi implements MediaWikiApi {
             }
 
             return categories;
-        })
-                .flatMapObservable(list -> Observable.fromIterable(list));
+        }).flatMapObservable(Observable::fromIterable);
     }
 
     @Override
     @NonNull
     public Observable<String> searchTitles(String title, int searchCatsLimit) {
-        return Single.fromCallable(() -> {
-            ArrayList<ApiResult> categoryNodes = null;
+        return Single.fromCallable((Callable<List<String>>) () -> {
+            ArrayList<ApiResult> categoryNodes;
 
             try {
                 categoryNodes = api.action("query")
@@ -287,7 +286,7 @@ public class ApacheHttpClientMediaWikiApi implements MediaWikiApi {
                         .getNodes("/api/query/search/p/@title");
             } catch (IOException e) {
                 Timber.e("Failed to obtain searchTitles", e);
-                return new ArrayList();
+                return Collections.emptyList();
             }
 
             if (categoryNodes == null) {
@@ -302,8 +301,7 @@ public class ApacheHttpClientMediaWikiApi implements MediaWikiApi {
             }
 
             return titleCategories;
-        })
-                .flatMapObservable(list -> Observable.fromIterable(list));
+        }).flatMapObservable(Observable::fromIterable);
     }
 
     @Override