I am using a data-related API interface which has a key void callback function that is automatically invoked to mark the end of some IO operations. I want to make the class Callable<String> and use Future<String> result.

I struggle to figure how to make the Callable return a String. Making a String returnResult(){return this.result} function to call inside doesn’t do.

Please advise.

It looks something like:

public class MyCallable implements someAPIWrapper, Callable<String> {
    
    String result;

    @Override
    public void endOfJobCallback() { //predefined API callback marking end of work
      /*
      usually read the data and write to a file, but not my case.
      how to return this.result string from here?
      */
    }

    @Override
    public String call() throws Exception {
      //some logic stuff
      //make API call to request a bunch of data
      //receiving messages and appending to the *result* variable
      //end of all messages, so the ending callback is automatically entered
    }

}

class Main {
    public static void main(String[] args){
      MyCallable callable = new MyCallable();
      ExecutorService executor = Executors.newFixedThreadPool(2);
      Future<String> future = executor.submit(callable);
      String result = future.get(); //blocking until result ready
    }
}