import org.infinispan.functional.EntryView.*;
import org.infinispan.functional.FunctionalMap.*;

WriteOnlyMap<String, String> writeOnlyMap = ...

// Force a function to be Serializable
Consumer<WriteEntryView<String>> function =
   (Consumer<WriteEntryView<String>> & Serializable) wv -> wv.set("one");

CompletableFuture<Void> writeFuture = writeOnlyMap.eval("key1", function);