How can i make my method return a callback?

Question

Im trying to code for an trading bot using the java binance api. what i`d like to do is use to async client from the library to create a method to fetch data/candlesticks and then return it.

My problem is that the async client returns the response to a callback and i have now idea how to handle it, make my method return the data like the example below:

    public List<Candlestick> asyncGetCandles(){
    //get the data
        return response /**List<Candlestick>response*/
    }

This is what i got so far:

    public void asyncGetCandles() throws ParseException, IOException {

    BinanceApiClientFactory factory = BinanceApiClientFactory.newInstance();
    BinanceApiAsyncRestClient asyncClient = factory.newAsyncRestClient();

    long start = dateTime.startOfListing();
    long now = System.currentTimeMillis();

    BinanceApiCallback<List<Candlestick>> callback = response -> {
        System.out.println(response);//prints 1000 candles
    };


    asyncClient.getCandlestickBars("BTCUSDT", CandlestickInterval.HOURLY, 1000, start, now, callback);

    /**How to return the response here?*/
}

Any help on how to do this would be really appreciated!

Answer 1

It should look something like this:

package stackoverflow;

import java.io.IOException;
import java.text.ParseException;
import java.util.List;

public class ReturnAsyncCallback {
    interface BinanceApiCallback<T> { // either this usecase with interface
        BinanceApiCallback<T> run(String response);
    }
    abstract class BinanceApiCallback2<T> { // or this usecase with abstract base class
        abstract BinanceApiCallback<T> run(String response);
    }
    static class Candlestick {

    }

    public BinanceApiCallback<List<Candlestick>> asyncGetCandles() throws ParseException, IOException {

        final BinanceApiClientFactory factory = BinanceApiClientFactory.newInstance();
        final BinanceApiAsyncRestClient asyncClient = factory.newAsyncRestClient();

        final long start = dateTime.startOfListing();
        final long now = System.currentTimeMillis();

        final BinanceApiCallback<List<Candlestick>> callback = (response) -> {
            System.out.println(response);//prints 1000 candles
        };


        asyncClient.getCandlestickBars("BTCUSDT", CandlestickInterval.HOURLY, 1000, start, now, callback);

        /**How to return the response here?*/
        return callback;
    }

}

Note, that there are TWO ways for lambda expressions. Both need a class with exactly ONE abstract (unimplemented) method:

1. Use an interface with only 1 (abstract) method, can have as many default methods as it wants.
2. Use an abstract base class with exactly ONE abstract method.

Additional notes for my example:

1. The inner classes BinanceApiCallback and BinanceApiCallback2 and Candlestick are only there for the http://sscce.org/

Hints for further questions:

1. Your code was NOT an http://sscce.org/, so it makes it a lot harder for us to answer.