Android, How to read QR code in my application?

try {

    Intent intent = new Intent("com.google.zxing.client.android.SCAN");
    intent.putExtra("SCAN_MODE", "QR_CODE_MODE"); // "PRODUCT_MODE for bar codes

    startActivityForResult(intent, 0);

} catch (Exception e) {

    Uri marketUri = Uri.parse("market://details?id=com.google.zxing.client.android");
    Intent marketIntent = new Intent(Intent.ACTION_VIEW,marketUri);
    startActivity(marketIntent);

}

and in onActivityResult():

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {           
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == 0) {

        if (resultCode == RESULT_OK) {
            String contents = data.getStringExtra("SCAN_RESULT");
        }
        if(resultCode == RESULT_CANCELED){
            //handle cancel
        }
    }
}

if user doesn’t have any qr reader, what will happen to the application? if it crashes, may i ask user to download for example QrDroid and after that use it?

Interestingly, Google now introduced Mobile Vision APIs, they are integrated in play services itself.

In your Gradle file just add:

compile 'com.google.android.gms:play-services-vision:11.4.0'

Taken from this QR code tutorial.

UPDATE 2020:

Now QR code scanning is also a part of ML Kit, so you can bundle the model inside the app and use it by integrating the following gradle dependency:

dependencies {
  // ...
  // Use this dependency to bundle the model with your app
  implementation 'com.google.mlkit:barcode-scanning:16.0.3'
}

Or you can use the following gradle dependency to dynamically download the models from Google Play Services:

dependencies {
  // ...
  // Use this dependency to use the dynamically downloaded model in Google Play Services
  implementation 'com.google.android.gms:play-services-mlkit-barcode-scanning:16.1.2'
}

Taken from this link.

Zxing is an excellent library to perform Qr code scanning and generation.
The following implementation uses Zxing library to scan the QR code image Don’t forget to add following dependency in the build.gradle

implementation 'me.dm7.barcodescanner:zxing:1.9'

Code scanner activity:

    public class QrCodeScanner extends AppCompatActivity implements ZXingScannerView.ResultHandler {
        private ZXingScannerView mScannerView;

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

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

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

        @Override
        public void handleResult(Result rawResult) {
            // Do something with the result here
            // Prints scan results
            Logger.verbose("result", rawResult.getText());
            // Prints the scan format (qrcode, pdf417 etc.)
            Logger.verbose("result", rawResult.getBarcodeFormat().toString());
            //If you would like to resume scanning, call this method below:
            //mScannerView.resumeCameraPreview(this);
            Intent intent = new Intent();
            intent.putExtra(AppConstants.KEY_QR_CODE, rawResult.getText());
            setResult(RESULT_OK, intent);
            finish();
        }
    }