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

@@ -64,10 +64,11 @@
     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
+        @SuppressWarnings("serial") // Not statically typed as Serializable
         private final Comparator<T> real;
 
         @SuppressWarnings("unchecked")
         NullComparator(boolean nullFirst, Comparator<? super T> real) {
             this.nullFirst = nullFirst;