src/java.base/share/classes/java/util/Comparators.java

@@ -60,10 +60,11 @@
 
     /**
      * Null-friendly comparators
      */
     static final class NullComparator<T> implements Comparator<T>, Serializable {
+        @java.io.Serial
         private static final long serialVersionUID = -7569533591570686392L;
         private final boolean nullFirst;
         // if null, non-null Ts are considered equal
         private final Comparator<T> real;