question: @ Override public void write ( Kryo kryo , Output output ) { int size = index . size ( ) ; output . writeInt ( size + 1 , true ) ; for ( int i = 0 ; i < size ; ++ i ) { String s = index . get ( i ) ; if ( s == null ) { throw new RuntimeException ( "Cannot serialize null string at index " + Integer . toString ( i ) ) ; } output . writeString ( s ) ; } }
target:  Custom serializer. Follows kryo's StringArraySerializer example.