Java async serial port communication request response

I am trying to create a class which sends and receives commands via serial port. Sadly the whenComplete is never called. I also tried to log the response and in the callback it is correct but when I try to get the variable in execute, it is completely different and looks like some random bytes. I tried to use the Latch to get the callback data. Is there any better way?

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class Device {
    private static final String port = "/dev/ttyS0";
    private static final int baud = 9600;
    private SerialHelper2 serialPort;
    private final int timeout = 500;
    private CountDownLatch responseLatch;
    private String response;

    public Device() {
    }

    public boolean open() {
        this.serialPort = new SerialHelper2(port, baud) {
            @Override
            protected void onDataReceived(final byte[] data) {
                Device.this.response = new String(data).trim();
                Serial.out.println(Device.this.response); // Output is correct
                Device.this.responseLatch.countDown();
            }
        };

        try {
            this.serialPort.open();
            return this.serialPort.isOpen();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return false;
    }

    public CompletableFuture<String> execute(String cmd) {
        // reset latch
        Device.this.responseLatch = new CountDownLatch(1);
        return CompletableFuture.supplyAsync(() -> {
            Device.this.serialPort.sendHex(cmd);
            try {
                if (responseLatch.await(Device.this.timeout, TimeUnit.MILLISECONDS)) {
                    Serial.out.println(Device.this.response); // Complete garbage. Looks like some wrong memory access?
                    return Device.this.response;
                } else { // Timeout exceeded
                    throw new TimeoutException(String.format("Command '%s' timed out after %d ms", cmd, Device.this.timeout));
                }
            } catch (Exception e) {
                throw new IllegalStateException(e);
            }
        });
    }

    public CompletableFuture<Boolean> init() {
        return this.execute("REQ")
                .thenApply(res -> res.equalsIgnoreCase("RES"));
    }


}

Somewhere I call:

device.init()
    .whenComplete((result, ex) -> {
        if (null != ex) {
            Serial.out.println("Failed")
        } else {
            Serial.out.println("Device initialization " + (result ? "succeeded" : "failed"));
        }
    });