MainActivity

public class MainActivity extends AppCompatActivity {
    public static final int CAMERA_PERMISSION_REQUEST = 666;
    private boolean isZXing;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        isZXing = true;
        Button buttonZxing = findViewById(R.id.btn_start_qr_zxing);
        buttonZxing.setOnClickListener(v -> {
            isZXing = true;
            checkCameraPermission();
        });
        Button buttonZbar = findViewById(R.id.btn_start_qr_zbar);
        buttonZbar.setOnClickListener(v -> {
            isZXing = false;
            checkCameraPermission();
        });
    }

    private void checkCameraPermission() {
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.READ_CONTACTS)
                != PackageManager.PERMISSION_GRANTED) {

            if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                    Manifest.permission.CAMERA)) {
                Toast.makeText(this, "Suck the cock! We need your fuck'n camera!", Toast.LENGTH_SHORT).show();
            } else {
                ActivityCompat.requestPermissions(this,
                        new String[]{Manifest.permission.CAMERA},
                        CAMERA_PERMISSION_REQUEST);
            }
        } else {
            startQr(isZXing ? QRActivityZxing.class : QRActivityZbar.class);
        }
    }

    private void startQr(Class activityClass) {
        startActivity(new Intent(this, activityClass));
    }


    @Override
    public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
        switch (requestCode) {
            case CAMERA_PERMISSION_REQUEST: {
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    startQr(isZXing ? QRActivityZxing.class : QRActivityZbar.class);
                } else {
                    checkCameraPermission();
                }
            }
        }
    }
}

ZxingActivity

public class QRActivityZbar extends AppCompatActivity implements ZBarScannerView.ResultHandler{

    private ZBarScannerView mScannerView;

    @Override
    public void onCreate(Bundle state) {
        super.onCreate(state);
        mScannerView = new ZBarScannerView(this);    // Programmatically initialize the scanner view
        setContentView(mScannerView);                // Set the scanner view as the content view
    }

    @Override
    public void onResume() {
        super.onResume();
        mScannerView.setResultHandler(this); // Register ourselves as a handler for scan results.
        mScannerView.startCamera();          // Start camera on resume
    }

    @Override
    public void onPause() {
        super.onPause();
        mScannerView.stopCamera();           // Stop camera on pause
    }

    @Override
    public void handleResult(Result rawResult) {
        // Do something with the result here
        Log.v("TAG", rawResult.getContents()); // Prints scan results
        Log.v("TAG", rawResult.getBarcodeFormat().getName()); // Prints the scan format (qrcode, pdf417 etc.)

        // If you would like to resume scanning, call this method below:
        mScannerView.resumeCameraPreview(this);
    }
}