MainActivity.java

package me.aflak.libraries;

import android.hardware.usb.UsbDevice;
import android.os.Handler;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.text.method.ScrollingMovementMethod;
import android.widget.TextView;

import me.aflak.arduino.Arduino;
import me.aflak.arduino.ArduinoListener;

public class MainActivity extends AppCompatActivity implements ArduinoListener {
    private Arduino arduino;
    private TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textView = findViewById(R.id.textView);
        textView.setMovementMethod(new ScrollingMovementMethod());
        arduino = new Arduino(this);
        display("Please plug an Arduino via OTG.\nOn some devices you will have to enable OTG Storage in the phone's settings.\n\n");
    }

    @Override
    protected void onStart() {
        super.onStart();
        arduino.setArduinoListener(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        arduino.unsetArduinoListener();
        arduino.close();
    }

    @Override
    public void onArduinoAttached(UsbDevice device) {
        display("Arduino attached!");
        arduino.open(device);
    }

    @Override
    public void onArduinoDetached() {
        display("Arduino detached");
    }

    @Override
    public void onArduinoMessage(byte[] bytes) {
        display("> "+new String(bytes));
    }

    @Override
    public void onArduinoOpened() {
        String str = "Hello World !";
        arduino.send(str.getBytes());
    }

    @Override
    public void onUsbPermissionDenied() {
        display("Permission denied... New attempt in 3 sec");
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                arduino.reopen();
            }
        }, 3000);
    }

    public void display(final String message){
        runOnUiThread(new Runnable() {
            @Override
            public void run() {
                textView.append(message+"\n");
            }
        });
    }
}