Fix enum class cast error in TreeSet by specifying an explicit comparator

src/main/java/org/jabref/gui/collab/EntryChangeViewModel.java

@@ -1,6 +1,7 @@
 package org.jabref.gui.collab;
 
 import java.util.ArrayList;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
@@ -44,7 +45,7 @@ public EntryChangeViewModel(BibEntry memEntry, BibEntry tmpEntry, BibEntry diskE
         LOGGER.debug("Modified entry: " + memEntry.getCiteKeyOptional().orElse("<no BibTeX key set>")
                 + "\n Modified locally: " + isModifiedLocally + " Modifications agree: " + modificationsAgree);
 
-        Set<Field> allFields = new TreeSet<>();
+        Set<Field> allFields = new TreeSet<>(Comparator.comparing(Field::getName));
         allFields.addAll(memEntry.getFields());
         allFields.addAll(tmpEntry.getFields());
         allFields.addAll(diskEntry.getFields());

src/main/java/org/jabref/gui/mergeentries/FetchAndMergeEntry.java

@@ -2,6 +2,7 @@
 
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.List;
 import java.util.Optional;
 import java.util.Set;
@@ -92,8 +93,10 @@ private void showMergeDialog(BibEntry originalEntry, BibEntry fetchedEntry, WebF
             NamedCompound ce = new NamedCompound(Localization.lang("Merge entry with %0 information", fetcher.getName()));
 
             // Updated the original entry with the new fields
-            Set<Field> jointFields = new TreeSet<>(mergedEntry.get().getFields());
-            Set<Field> originalFields = new TreeSet<>(originalEntry.getFields());
+            Set<Field> jointFields = new TreeSet<>(Comparator.comparing(Field::getName));
+            jointFields.addAll(mergedEntry.get().getFields());
+            Set<Field> originalFields = new TreeSet<>(Comparator.comparing(Field::getName));
+            originalFields.addAll(originalEntry.getFields());
             boolean edited = false;
 
             // entry type

src/main/java/org/jabref/gui/mergeentries/MergeEntries.java

@@ -4,6 +4,7 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -46,18 +47,18 @@ public class MergeEntries extends BorderPane {
 
     // Headings
     private final List<String> columnHeadings = Arrays.asList(Localization.lang("Field"),
-            Localization.lang("Left entry"),
-            Localization.lang("Left"),
-            Localization.lang("None"),
-            Localization.lang("Right"),
-            Localization.lang("Right entry"));
+                                                              Localization.lang("Left entry"),
+                                                              Localization.lang("Left"),
+                                                              Localization.lang("None"),
+                                                              Localization.lang("Right"),
+                                                              Localization.lang("Right entry"));
     private final Set<Field> identicalFields = new HashSet<>();
     private final Set<Field> differentFields = new HashSet<>();
     private final BibEntry mergedEntry = new BibEntry();
     private final BibEntry leftEntry;
     private final BibEntry rightEntry;
     private final Map<Field, TextFlow> leftTextPanes = new HashMap<>();
-    private final Set<Field> allFields = new TreeSet<>();
+    private final Set<Field> allFields = new TreeSet<>(Comparator.comparing(Field::getName));
     private final Map<Field, TextFlow> rightTextPanes = new HashMap<>();
     private final Map<Field, List<RadioButton>> radioButtons = new HashMap<>();
     private Boolean identicalTypes;
@@ -81,7 +82,6 @@ public MergeEntries(BibEntry entryLeft, BibEntry entryRight, String headingLeft,
         setRightHeaderText(headingRight);
     }
 
-
     /**
      * Constructor taking two entries
      *
@@ -240,8 +240,8 @@ private void setupHeadingRows(GridPane mergePanel) {
     private void fillDiffModes() {
         diffMode.setItems(FXCollections.observableList(Arrays.asList(DiffMode.values())));
         new ViewModelListCellFactory<DiffMode>()
-                .withText(MergeEntries::getDisplayText)
-                .install(diffMode);
+                                                .withText(MergeEntries::getDisplayText)
+                                                .install(diffMode);
         DiffMode diffModePref = Globals.prefs.getAsOptional(JabRefPreferences.MERGE_ENTRIES_DIFF_MODE)
                                              .flatMap(DiffMode::parse)
                                              .orElse(DiffMode.WORD);

src/main/java/org/jabref/model/database/BibDatabase.java

@@ -133,7 +133,7 @@ public ObservableList<BibEntry> getEntries() {
      * @return set of fieldnames, that are visible
      */
     public Set<Field> getAllVisibleFields() {
-        Set<Field> allFields = new TreeSet<>();
+        Set<Field> allFields = new TreeSet<>(Comparator.comparing(Field::getName));
         for (BibEntry e : getEntries()) {
             allFields.addAll(e.getFields());
         }