ZXing QR代碼生成器應用程序崩潰與NPE

Question

在發佈此之前，我已經檢查了其他相關帖子stackoverflow中，但似乎沒有真正解決我的類似問題。

這是我的錯誤：

06-29 21:02:38.274: E/AndroidRuntime(2261): FATAL EXCEPTION: main 
06-29 21:02:38.274: E/AndroidRuntime(2261): Process: org.example.helloandroid, PID: 2261 
06-29 21:02:38.274: E/AndroidRuntime(2261): java.lang.RuntimeException: Unable to start activity  ComponentInfo{org.example.helloandroid/org.example.helloandroid.QRGenActivity}: java.lang.NullPointerException 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at  android.app.ActivityThread.access$800(ActivityThread.java:135) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.os.Handler.dispatchMessage(Handler.java:102) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.os.Looper.loop(Looper.java:136) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.ActivityThread.main(ActivityThread.java:5017) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at java.lang.reflect.Method.invokeNative(Native Method) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at java.lang.reflect.Method.invoke(Method.java:515) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at dalvik.system.NativeStart.main(Native Method) 
06-29 21:02:38.274: E/AndroidRuntime(2261): Caused by: java.lang.NullPointerException 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at org.example.helloandroid.QRGenActivity.onCreate(QRGenActivity.java:81) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.Activity.performCreate(Activity.java:5231) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159) 
06-29 21:02:38.274: E/AndroidRuntime(2261):  ... 11 more 

這是我的QRGenerator代碼應生成QR碼，但它拋出一個NPE。

public class QRGenActivity extends Activity { 
@Override 
protected void onCreate(Bundle savedInstanceState) { 


    super.onCreate(savedInstanceState); 


    Intent intent = getIntent(); 
    String emailaddress = intent.getStringExtra("emailValue"); 
    String password = intent.getStringExtra("password"); 

    ImageView view = (ImageView) findViewById(R.id.qr); 
    Log.d("EmailValue", emailaddress); 
    Log.d("password", password); 


    String qrInputText = emailaddress+"_"+password; 
    Log.v("QR TAG", qrInputText); 

    WindowManager manager = (WindowManager) getSystemService(WINDOW_SERVICE); 
    Display display = manager.getDefaultDisplay(); 

    Point point = new Point(); 
    display.getSize(point); 

    int width = point.x; 
    int height = point.y; 

    int smallerDimension = width < height ? width: height; 
    smallerDimension*=3/4; 

    com.google.zxing.Writer writer = new QRCodeWriter(); 
    String finalData = Uri.encode(qrInputText,"utf-8"); 

    try { 
     BitMatrix bm = writer.encode(finalData, BarcodeFormat.QR_CODE, 150, 150); 
     Bitmap ImageBitmap = Bitmap.createBitmap(150, 150, Config.ARGB_8888); 

     for(int i=0;i<150;i++){ 
      for(int j=0;j<150;j++){ 
       ImageBitmap.setPixel(i, j, bm.get(i, j) ? Color.BLACK: Color.WHITE); 
      } 
     } 

     if(ImageBitmap!=null){ 
Log.d("QRCreated", "QR Is created "+R.id.qr); 

      view.setImageBitmap(ImageBitmap); 
     } 
     else{ 
      Toast.makeText(getApplicationContext(), "Some error occurred in generating QRCode", Toast.LENGTH_LONG).show(); 
     } 
    } catch (WriterException e) { 
     // TODO Auto-generated catch block 
     e.printStackTrace(); 
    } 



    Handler handler = new Handler(); 
setContentView(R.layout.display_qr); 

getActionBar().setDisplayHomeAsUpEnabled(true); 
} 

Answer 1

你把

ImageView view = (ImageView) findViewById(R.id.qr); 

setContentView(R.layout.display_qr); 

這將無法正常工作之前，因爲findViewById()搜索與指定ID的視圖的視圖層次，你有沒有在那個時候添加了任何視圖。因此，它將返回null。這可能是導致事故的原因。

您應該在super.onCreate()之後移動setContentView()。