fix: resolve checkstyle violations and improve code quality

Author: ungerts

jabgui/src/main/java/org/jabref/gui/commonfxcontrols/SaveOrderConfigPanel.java

@@ -1,7 +1,6 @@
 package org.jabref.gui.commonfxcontrols;
 
 import java.util.List;
-import java.util.stream.Collectors;
 
 import javax.swing.undo.UndoManager;
 

jabgui/src/main/java/org/jabref/gui/preview/PreviewPreferences.java

@@ -68,7 +68,7 @@ public void setLayoutCyclePosition(int position) {
     }
 
     public PreviewLayout getSelectedPreviewLayout() {
-        if (layoutCycle.size() == 0
+        if (layoutCycle.isEmpty()
                 || layoutCyclePosition.getValue() < 0
                 || layoutCyclePosition.getValue() >= layoutCycle.size()) {
             return getCustomPreviewLayout();

jablib/src/main/java/org/jabref/logic/importer/fetcher/DoiResolution.java

@@ -10,7 +10,6 @@
 import java.util.Locale;
 import java.util.Objects;
 import java.util.Optional;
-import java.util.stream.Collectors;
 
 import org.jabref.logic.importer.FulltextFetcher;
 import org.jabref.logic.importer.ImportFormatPreferences;
@@ -150,7 +149,7 @@ private Optional<URL> citationMetaTag(Document html) {
         if (citationPdfUrl.isPresent()) {
             try {
                 return Optional.of(URLUtil.create(citationPdfUrl.get()));
-            } catch (MalformedURLException e) {
+            } catch (MalformedURLException _) {
                 return Optional.empty();
             }
         }
@@ -167,7 +166,7 @@ private Optional<URL> findEmbeddedLink(Document html, URL base) {
             try {
                 URL url = base.toURI().resolve(pdfUrl.get()).toURL();
                 return Optional.of(url);
-            } catch (MalformedURLException | URISyntaxException e) {
+            } catch (MalformedURLException | URISyntaxException _) {
                 return Optional.empty();
             }
         }

jablib/src/main/java/org/jabref/logic/integrity/FileChecker.java

@@ -4,7 +4,6 @@
 import java.nio.file.Path;
 import java.util.List;
 import java.util.Optional;
-import java.util.stream.Collectors;
 
 import org.jabref.logic.FilePreferences;
 import org.jabref.logic.importer.util.FileFieldParser;

jablib/src/main/java/org/jabref/logic/l10n/Encodings.java

@@ -15,7 +15,6 @@ private Encodings() {
     static {
         ENCODINGS_DISPLAYNAMES = ENCODINGS_LIST.stream().map(Charset::displayName).distinct().toArray(String[]::new);
         ENCODINGS = ENCODINGS_LIST.toArray(new Charset[0]);
-
     }
 
     public static List<Charset> getCharsets() {

jablib/src/main/java/org/jabref/model/entry/BibEntry.java

@@ -18,7 +18,6 @@
 import java.util.function.BiFunction;
 import java.util.function.Predicate;
 import java.util.stream.Collectors;
-import java.util.stream.Stream;
 
 import javafx.beans.Observable;
 import javafx.beans.property.ObjectProperty;