Cannot create DataStream inside a flatMap() function

I want to create a DataStream inside a flatMap function. However I when I run dataStream.print(), it shows nothing. Here is my code:

public class test {

    static StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();

    public static void main(String[] args) throws Exception {

        String s = "test message";

        DataStream<String> data = env.fromElements(s);
        data.flatMap(new customFlatMap()).print();

        env.execute();
    }

    private static class customFlatMap extends RichFlatMapFunction<String, String> {
        @Override
        public void flatMap(String s, Collector<String> collector) {
            DataStream<String> newStream = env.fromElements(s);
            System.out.println("-----");
            newStream.print();
            System.out.println("-----");

            collector.collect(s);
        }
    }
}

When I run my code, it prints the following result:

-----
-----
8> test message

Nothing shows between two dashes.

Can someone help me explain this? Thank you in advance!

You need to create a side output, which can't be done if you're using a FlatMap function.