gogoshell own commands piping

I'm trying to use gogo-shell to add some console commands For example I'm create commands add and show

public void add(CommandSession commandSession, int i) {
    List<Integer> il = commandSession.get("list");
    if (il == null) {
        il = new ArrayList<Integer>();
        il.add(i);
        commandSession.put("list",il)
    } else {
        il.add(i)
    }
}
public void show(CommandSession commandSession) {
    List<Integer> il = commandSession.get("list");
    il.foreach(System.out::println);
}

and when i use them like

add 1 | add 2 | add 3 | add 4 | show

I'm geting something like

null pointer Exception

or

1
3
4
2

I think this happens because pipes (add) runs in parallel. So how can I write command where piping will be sequential.

Pipelines in gogo (like in bash) expect to consume data from standard input and produce data on standard output. Each element in the pipeline runs concurrently, as a separate thread.

The 'add' command in your example does not consume or produce data on standard in/out and is thus not suitable to run in a pipeline.

If you just want the commands to run sequentially, then use the ';' command separator:

g! add 1; add 2; add 3; add 4; show