public final B entityTag(BiFunction<String, HttpFileAttributes, String> entityTagFunction) {
        this.entityTagFunction = requireNonNull(entityTagFunction, "entityTagFunction");
        return self();
    }
public final B entityTag ( BiFunction < String , HttpFileAttributes , String > entityTagFunction ) { this . entityTagFunction = requireNonNull ( entityTagFunction , "entityTagFunction" ) ; return self ( ) ; }
Sets the function which generates the entity tag that's used for setting the {@code "etag"} header
automatically.

@param entityTagFunction the entity tag function that generates the entity tag, or {@code null}
to disable setting the {@code "etag"} header.