Android的 - NullPointerException在ViewPager

Question

我想創建對話框的片段，但是當我調用setAdapter方法我有一個Java NullPointerException，並且我找不到該錯誤。

Java代碼：

public class MainActivity extends FragmentActivity { 

Context myContext; 

private MyAdapter mAdapter; 
private ViewPager mPager; 

String TAG = "Dialog"; 

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

      myContext = this; 

    findViewById(R.id.button1).setOnClickListener(new OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      final Dialog dialog = new Dialog(myContext); 

      dialog.setContentView(R.layout.layout_dialog); 

      mAdapter = new MyAdapter(getSupportFragmentManager()); 

      mPager = (ViewPager) dialog.findViewById(R.id.myViewPager); 
      **mPager.setAdapter(mAdapter);** //Line 39, the error is on this line. 

      dialog.show(); 
     } 
    }); 
} 

public class MyAdapter extends FragmentPagerAdapter { 
    int counts; 

    public MyAdapter(FragmentManager fm) { 
     super(fm); 
    } 

    @Override 
    public int getCount() { 
     return 3; 
    } 

    @Override 
    public Fragment getItem(int position) { 
     switch (position) { 
     case 0: 
      return new ImageFragment(R.drawable.icecream); 

     case 1: 
      return new ImageFragment(R.drawable.cupcake); 

     case 2: 
      return new ImageFragment(R.drawable.donut); 

     default: 
      return null; 
     } 

    } 
} 
} 

片段類：

public class ImageFragment extends Fragment { 

private final int imageResourceId; 

public ImageFragment(int imageResourceId) { 
    this.imageResourceId = imageResourceId; 
} 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
} 

@Override 
public void onActivityCreated(Bundle savedInstanceState) { 
    super.onActivityCreated(savedInstanceState); 

} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
     Bundle savedInstanceState) { 
    View view = inflater.inflate(R.layout.activity_dialog_item, container, 
      false); 
    ImageView imageView = (ImageView) view.findViewById(R.id.imageView1); 
    imageView.setImageResource(imageResourceId); 
    return view; 
} 

}

activity_main XML代碼：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
xmlns:tools="http://schemas.android.com/tools" 
android:layout_width="match_parent" 
android:layout_height="match_parent"> 

<Button 
    android:id="@+id/button1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_alignParentRight="true" 
    android:layout_marginRight="20dip" 
    android:layout_marginTop="30dip" 
    android:text="Show Dialog" /> 

    </RelativeLayout> 

layout_dialog.xml代碼：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" 
android:orientation="vertical" > 

<android.support.v4.view.ViewPager 
    android:id="@+id/myViewPager" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" /> 

    </LinearLayout> 

activity_dialog_item.xml代碼：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
android:layout_width="match_parent" 
android:layout_height="match_parent" > 

<ImageView 
    android:id="@+id/imageView1" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:layout_centerInParent="true" 
    android:src="@drawable/cupcake" /> 

    </RelativeLayout> 

錯誤：

03-06 13:58:22.546: E/AndroidRuntime(818): FATAL EXCEPTION: main 
03-06 13:58:22.546: E/AndroidRuntime(818): java.lang.NullPointerException 
03-06 13:58:22.546: E/AndroidRuntime(818): at com.example.viewpager.MainActivity$1.onClick(MainActivity.java:39) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.view.View.performClick(View.java:2485) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.view.View$PerformClick.run(View.java:9080) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.os.Handler.handleCallback(Handler.java:587) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.os.Handler.dispatchMessage(Handler.java:92) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.os.Looper.loop(Looper.java:123) 
03-06 13:58:22.546: E/AndroidRuntime(818): at android.app.ActivityThread.main(ActivityThread.java:3683) 
03-06 13:58:22.546: E/AndroidRuntime(818): at java.lang.reflect.Method.invokeNative(Native Method) 
03-06 13:58:22.546: E/AndroidRuntime(818): at java.lang.reflect.Method.invoke(Method.java:507) 
03-06 13:58:22.546: E/AndroidRuntime(818): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 
03-06 13:58:22.546: E/AndroidRuntime(818): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 
03-06 13:58:22.546: E/AndroidRuntime(818): at dalvik.system.NativeStart.main(Native Method) 

Answer 1

編輯：

通過從資源inflatting的視圖中未設置對話框的圖

LayoutInflater inflatter= (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
View v= inflatter.inflate(R.layout.layout_dialog); 
dialog.setContentView(v); 

---

你不會得到，直到該的onCreate的背景下，

Context myContext = this; // this throws you NPE, 

嘗試做這樣的

Context mycontext; 

和的onCreate

mycontext=this;