Crea tu lector de códigos QR

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

 <Button
        android:id="@+id/btnScan"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"        
        android:text="@string/txt_scan" />

    <TextView
        android:id="@+id/tvResult"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
// import zxing 
import com.google.zxing.integration.android.IntentIntegrator;
import com.google.zxing.integration.android.IntentResult;

public class MainActivity extends Activity {

 private Button btnScan;
 private TextView tvResult;

 @Override 
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  //------------------------
  btnScan = (Button) findViewById(R.id.btnScan);
  tvResult = (TextView) findViewById(R.id.tvResult);
  //evento cuando se presiona el boton
  btnScan.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
           //se inicia el barcode Scanner
           IntentIntegrator.initiateScan(MainActivity.this); 
          }
  });
  //------------------------ 
 }//end: onCreate

 /**
 * Cuando termina de leer el código QR muestra el resultado en el textview 
 * @param requestCode The integer request code originally supplied to startActivityForResult(), allowing you to identify who this result came from.
 * @param resultCode The integer result code returned by the child activity through its setResult().
 * @param data An Intent, which can return result data to the caller (various data can be attached to Intent "extras").
 * */
 public void onActivityResult(int requestCode, int resultCode, Intent data) {
     IntentResult scanResult = IntentIntegrator.parseActivityResult(requestCode, resultCode, data);
     if ( scanResult != null ) {         
         String contentData = "SCAN_RESULT: " + data.getStringExtra("SCAN_RESULT") + "\n";
         contentData += "SCAN_RESULT_FORMAT: " + data.getStringExtra("SCAN_RESULT_FORMAT") + "\n";
         byte[] rawBytes = data.getByteArrayExtra("SCAN_RESULT_BYTES");
         contentData += "SCAN_RESULT_BYTES: " + ( rawBytes == null ? 0 : rawBytes.length ) + "\n";
         contentData += "SCAN_RESULT_ERROR_CORRECTION_LEVEL: " + data.getStringExtra("SCAN_RESULT_ERROR_CORRECTION_LEVEL");         
         tvResult.setText(contentData);         
     }
 }

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

}