Java completion handler

Main.java

/**
 * Created by sanf0rd on 21/06/17.
 */
public class Main {
    public static void main(String[] args) {
        Wrapper wrapper = new Wrapper();

        wrapper.doStuff(result -> System.out.println(result));

    }
}

interface MyListener {
    void onCompletion(String any);
}

Wrapper.java

public class Wrapper {
    public void doStuff(MyListener listener) {
        //make the assync call, you might need to save the listener's state in a field.

        String result = "Now i have the result";
        listener.onCompletion(result);
    }
}