關於選擇圖像的NPE

Question

我想將自定義對話框的區域設置爲所選圖像。如果我設置整個應用程序的背景圖像，下面的代碼將與一些重新安排一起工作。出於某種原因，當我將它移動到設定自定義對話框我得到以下錯誤的一個區域：

錯誤：

11-03 18:19:05.216: E/AndroidRuntime(10428): FATAL EXCEPTION: main 
11-03 18:19:05.216: E/AndroidRuntime(10428): java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=100, result=-1, data=Intent { dat=content://media/external/images/media/700 }} to activity {com.example.pictures/com.example.pictures.MainActivity}: java.lang.NullPointerException 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread.deliverResults(ActivityThread.java:2536) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread.handleSendResult(ActivityThread.java:2578) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread.access$2000(ActivityThread.java:117) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:965) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.os.Handler.dispatchMessage(Handler.java:99) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.os.Looper.loop(Looper.java:130) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread.main(ActivityThread.java:3691) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at java.lang.reflect.Method.invokeNative(Native Method) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at java.lang.reflect.Method.invoke(Method.java:507) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:907) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:665) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at dalvik.system.NativeStart.main(Native Method) 
11-03 18:19:05.216: E/AndroidRuntime(10428): Caused by: java.lang.NullPointerException 
11-03 18:19:05.216: E/AndroidRuntime(10428): at com.example.pictures.MainActivity.onActivityResult(MainActivity.java:133) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.Activity.dispatchActivityResult(Activity.java:3950) 
11-03 18:19:05.216: E/AndroidRuntime(10428): at android.app.ActivityThread.deliverResults(ActivityThread.java:2532) 
11-03 18:19:05.216: E/AndroidRuntime(10428): ... 11 more 

缺少什麼我在這裏？我讀過這可以通過設置意圖來解決 - 但我已經在做這個。任何幫助，將不勝感激。

在此先感謝

CODE：

public class MainActivity extends Activity { 
    private static final int SELECT_PHOTO = 100; 
    private Bitmap chosenBitmap; 
    final Context context = this; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     final DrawingView drawing = new DrawingView(this); 

     Button button = (Button) findViewById(R.id.buttonShowCustomDialog); 

     // add button listener 
     button.setOnClickListener(new OnClickListener() { 

      @Override 
      public void onClick(View arg0) { 
       final Dialog dialog = new Dialog(context); 
      // custom dialog 
      //dialog = new Dialog(context); 
      dialog.setContentView(R.layout.custom); 
      dialog.setTitle("Title..."); 

      // set the custom dialog components - text, image and button 
      TextView text = (TextView) dialog.findViewById(R.id.text); 
      text.setText("Android custom dialog example!"); 
      ImageView image = (ImageView) dialog.findViewById(R.id.image); 
      image.setImageResource(R.drawable.ic_launcher); 

      Button dialogButton = (Button) dialog.findViewById(R.id.dialogButtonOK); 
      // if button is clicked, close the custom dialog 
      dialogButton.setOnClickListener(new OnClickListener() { 
       @Override 
       public void onClick(View v) { 
        dialog.dismiss(); 
       } 
      }); 


      Button captureButton = (Button) dialog.findViewById(R.id.selectImage); 
      captureButton.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View v) { 
        Intent photoPickerIntent = new Intent(Intent.ACTION_PICK); 
        photoPickerIntent.setType("image/*"); 
        startActivityForResult(photoPickerIntent, SELECT_PHOTO); 
       } 
      }); 
      dialog.show(); 
      } 
     }); 
    } 

    @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; 
    } 

    @Override 
    protected void onActivityResult(int requestCode, int resultCode, Intent imageReturnedIntent) { 
     super.onActivityResult(requestCode, resultCode, imageReturnedIntent); 
     final Dialog dialog = new Dialog(context); 

     switch(requestCode) { 
     case SELECT_PHOTO: 
      if(resultCode == RESULT_OK){ 
       Uri selectedImage = imageReturnedIntent.getData(); 
       InputStream imageStream = null; 
       try { 
        imageStream = getContentResolver().openInputStream(selectedImage); 
       } catch (FileNotFoundException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
       } 
       Bitmap yourSelectedImage = BitmapFactory.decodeStream(imageStream); 
       ImageView imageview= (ImageView)dialog.findViewById(R.id.imageview); 
       chosenBitmap = yourSelectedImage; 
       Drawable res = new BitmapDrawable(yourSelectedImage); 
       imageview.setImageDrawable(res); 
      } 
     } 
    } 

} 

Answer 1

當你執行該代碼：

InputStream imageStream = null; 
try { 
    imageStream = getContentResolver().openInputStream(selectedImage); 
} catch (FileNotFoundException e) { 
    // TODO Auto-generated catch block 
    e.printStackTrace(); 
} 

，你實際上有一個例外，該代碼繼續，你的imageStream仍空值。

你應該改變的代碼，以配合這一點，檢查空：

if (imageStream != null) { 
    Bitmap yourSelectedImage = BitmapFactory.decodeStream(imageStream); 
    ImageView imageview= (ImageView)dialog.findViewById(R.id.imageview); 
    chosenBitmap = yourSelectedImage; 
    Drawable res = new BitmapDrawable(yourSelectedImage); 
    imageview.setImageDrawable(res); 
} 

或處理一些其他適當的方式這種情況。

雖然這可能不是導致NPE的問題。

Answer 2

hichris123問正確的問題：什麼是133行？這是因爲該行的堆棧跟蹤：

11-03 18:19:05.216: E/AndroidRuntime(10428): at com.example.pictures.MainActivity.onActivityResult(MainActivity.java:133)

NullPointerExceptions的真的很簡單，和一點點對細節的關注將解決這些問題。你不能在null上調用方法，或者引用它的字段。

如果，例如，線133是這樣一個：

  Uri selectedImage = imageReturnedIntent.getData(); 

那麼唯一可能的空實體將成爲imageReturnedIntent - 你要麼需要處理的地方是空不同的情況下，或找出爲什麼它的爲空，並防止它爲空。在任何一種情況下，理解爲什麼它爲空都是很好的。

但要記住的是：使用堆棧跟蹤;它會告訴你究竟哪一行遇到null。知道該行通常直接指向空實體;那麼你有責任找出如何處理null。