Improve arXiv fetcher

Author: stefan-kolb

Diff for: src/main/java/org/jabref/logic/importer/fetcher/ArXiv.java

@@ -74,10 +74,8 @@ public Optional<URL> findFullText(BibEntry entry) throws IOException {
                                                           .filter(Optional::isPresent)
                                                           .map(Optional::get)
                                                           .findFirst();
+            pdfUrl.ifPresent(url -> LOGGER.info("Fulltext PDF found @ arXiv."));
 
-            if (pdfUrl.isPresent()) {
-                LOGGER.info("Fulltext PDF found @ arXiv.");
-            }
             return pdfUrl;
         } catch (FetcherException e) {
             LOGGER.warn("arXiv API request failed", e);

Diff for: src/test/java/org/jabref/logic/importer/fetcher/ArXivTest.java

@@ -23,7 +23,6 @@
 
 @FetcherTest
 class ArXivTest {
-
     private ArXiv finder;
     private BibEntry entry;
     private BibEntry sliceTheoremPaper;
@@ -121,6 +120,16 @@ void findFullTextByDOINotAvailableInCatalog() throws IOException {
         assertEquals(Optional.empty(), finder.findFullText(entry));
     }
 
+    @Test
+    void findFullTextEntityWithoutDoi() throws IOException {
+        assertEquals(Optional.empty(), finder.findFullText(entry));
+    }
+
+    @Test
+    void findFullTextTrustLevel() {
+        assertEquals(TrustLevel.PREPRINT, finder.getTrustLevel());
+    }
+
     @Test
     void searchEntryByPartOfTitle() throws Exception {
         assertEquals(Collections.singletonList(sliceTheoremPaper),