Add warning for empty BibTeX keys in entry editor (#5025)

* added a warning icon for empty Bibtex keys in entry editor

* fixed formatting issue in FieldCheckers.java

* modified logic and error message in ValidBibtexKeyCheck.checkValue

* Revert "fixed formatting issue in FieldCheckers.java"

This reverts commit 9271562.

* Revert "modified logic and error message in ValidBibtexKeyCheck.checkValue"

This reverts commit bad8856.

* changed status message for empty BibTeX keys to one in localization resources

* ordered class imports alphabetically in FieldCheckers.java

* added entry to CHANGELOG.md for issue #4440

* removed explanation text for issue 4440 fix and updated boolean toggle for empty or null BibTeX keys

CHANGELOG.md

@@ -61,6 +61,7 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 - We changed the title of Group Dialog to "Add subgroup" from "Edit group" when we select Add subgroup option.
 - We enable import button only if entries are selected. [#4755](https://github.com/JabRef/jabref/issues/4755)
 - We made modifications to improve contrast of UI elements. [#4583](https://github.com/JabRef/jabref/issues/4583)
+- We added a warning for empty BibTeX keys in the entry editor. [#4440](https://github.com/JabRef/jabref/issues/4440)
 - We added an option in the settings to set the default action in JabRef when right clicking on any entry in any database and selecting "Open folder". [#4763](https://github.com/JabRef/jabref/issues/4763)
 - The Medline fetcher now normalizes the author names according to the BibTeX-Standard [#4345](https://github.com/JabRef/jabref/issues/4345)
 - We added an option on the Linked File Viewer to rename the attached file of an entry directly on the JabRef. [#4844](https://github.com/JabRef/jabref/issues/4844)
@@ -125,7 +126,6 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 - We fixed an issue where the JabRef Icon in the macOS launchpad was not displayed correctly [#5003](https://github.com/JabRef/jabref/issues/5003)
 
 
-
 ### Removed
 - The feature to "mark entries" was removed and merged with the groups functionality.  For migration, a group is created for every value of the `__markedentry` field and the entry is added to this group.
 - The number column was removed.

src/main/java/org/jabref/logic/integrity/FieldCheckers.java

@@ -6,6 +6,7 @@
 
 import org.jabref.logic.journals.JournalAbbreviationRepository;
 import org.jabref.model.database.BibDatabaseContext;
+import org.jabref.model.entry.BibEntry;
 import org.jabref.model.entry.FieldName;
 import org.jabref.model.entry.InternalBibtexFields;
 import org.jabref.model.metadata.FilePreferences;
@@ -15,7 +16,7 @@
 
 public class FieldCheckers {
 
-    private Multimap<String, ValueChecker> fieldChecker;
+    private final Multimap<String, ValueChecker> fieldChecker;
 
     public FieldCheckers(BibDatabaseContext databaseContext, FilePreferences filePreferences, JournalAbbreviationRepository abbreviationRepository, boolean enforceLegalKey) {
         fieldChecker = getAllMap(databaseContext, filePreferences, abbreviationRepository, enforceLegalKey);
@@ -49,6 +50,7 @@ private static Multimap<String, ValueChecker> getAllMap(BibDatabaseContext datab
         fieldCheckers.put(FieldName.URL, new UrlChecker());
         fieldCheckers.put(FieldName.YEAR, new YearChecker());
         fieldCheckers.put(FieldName.KEY, new ValidBibtexKeyChecker(enforceLegalKey));
+        fieldCheckers.put(BibEntry.KEY_FIELD, new ValidBibtexKeyChecker(enforceLegalKey));
 
         if (databaseContext.isBiblatexMode()) {
             fieldCheckers.put(FieldName.DATE, new DateChecker());

src/main/java/org/jabref/logic/integrity/ValidBibtexKeyChecker.java

@@ -4,6 +4,7 @@
 
 import org.jabref.logic.bibtexkeypattern.BibtexKeyGenerator;
 import org.jabref.logic.l10n.Localization;
+import org.jabref.model.strings.StringUtil;
 
 /**
  * Makes sure the key is legal
@@ -18,7 +19,12 @@ public ValidBibtexKeyChecker(boolean enforceLegalKey) {
 
     @Override
     public Optional<String> checkValue(String value) {
+        if (StringUtil.isNullOrEmpty(value)) {
+            return Optional.of(Localization.lang("empty BibTeX key"));
+        }
+
         String cleaned = BibtexKeyGenerator.cleanKey(value, enforceLegalKey);
+
         if (cleaned.equals(value)) {
             return Optional.empty();
         } else {