How to disable pretty-printing(white space/newline) in XStream?

This is how I create XStream instance for XML:

XStream xstream = new XStream();

This is for JSON:

private final XStream xstream = new XStream(new JsonHierarchicalStreamDriver() {
        public HierarchicalStreamWriter createWriter(Writer writer) {
            return new JsonWriter(writer, JsonWriter.DROP_ROOT_MODE);
        }
    });

Both of them are pretty-printing (indenting) the output.

How can I ask the XStream to disable the pretty printing?