There are too many libraries available to read qr code like Zbar and zxing library. Here we are showing how to use Zxing lib to scan qr code :

First of all add dependency in app gradle file :

  compile 'me.dm7.barcodescanner:zxing:1.8.4'

Then on click of button start an activity to read qr code :

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M && checkSelfPermission(Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
        requestPermissions(new String[]{Manifest.permission.CAMERA}, THE_PERMISSIONS_REQUEST_CAMERA);
        } else {
        Intent intent = new Intent(AadhaarView.this, ScannerActivity.class);
        startActivityForResult(intent, THE_QR_SCAN_REQUEST);
        }

Now scanner activity will scan the qr code and will return data back to Main sctivity :

public class ScannerActivity extends AppCompatActivity implements ZXingScannerView.ResultHandler{

    private ZXingScannerView mScannerView;

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

    }

    and in onActivityResult of Main activity we can read result :

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode == THE_QR_SCAN_REQUEST) {

            Toast.makeText(getApplicationContext()," result :"+data.getStringExtra("result"),Toast.LENGTH_LONG).show();
        }
    }
    private void setupScanner(){
        mScannerView = new ZXingScannerView(ScannerActivity.this);
        setContentView(mScannerView);
        mScannerView.setResultHandler(ScannerActivity.this);
        mScannerView.startCamera();
    }


    @Override
    public void handleResult(Result result) {
        Intent returnIntent = new Intent();
        returnIntent.putExtra("result",result.getText().toString());
        setResult(Activity.RESULT_OK,returnIntent);
        finish();
    }

}