Pergntar se o usuário deseja instalar o ZXing

MainActivity.java

public class MainActivity extends Activity {

  protected static final String ZXING_MARKET = "market://search?q=pname:com.google.zxing.client.android";
	protected static final String ZXING_DIRECT = "https://zxing.googlecode.com/files/BarcodeScanner3.1.apk";

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

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.activity_main, menu);
		return true;
	}

	/**
	 * Método que será chamado no clique do botão
	 * 
	 * @param view
	 */
	public void lerQR(View view) {
		Intent intent = new Intent("com.google.zxing.client.android.SCAN");
		intent.putExtra("SCAN_MODE", "QR_CODE_MODE");

		try {
			startActivityForResult(intent, 0);

		} catch (ActivityNotFoundException e) {
			mostrarMensagem();
		}
	}

	/**
	 * Pergnta se o usuário deseja instalar o ZXing
	 */
	private void mostrarMensagem() {
		new AlertDialog.Builder(this)
				.setTitle("Instalar barcode scanner?")
				.setMessage("Para escanear QR code você precisa instalar o ZXing barcode scanner.")
				.setIcon(android.R.drawable.ic_dialog_alert)
				.setPositiveButton("Instalar",
						new DialogInterface.OnClickListener() {
							public void onClick(DialogInterface dialog, int whichButton) {
								Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(ZXING_MARKET));
								try {
									startActivity(intent);
								} catch (ActivityNotFoundException e) { // Se não tiver o Play Store
									intent = new Intent(Intent.ACTION_VIEW, Uri.parse(ZXING_DIRECT));
									startActivity(intent);
								}
							}
						})
				.setNegativeButton("Cancelar", null).show();

	}

}