DialogFragment给NullPointerException

Question

我试图通过谷歌here给出的代码，我得到一个NullPointerException，因为getActivity()在mDialogFragment返回null。我不明白为什么。有人可以解释这个错误吗？

错误日志

07-11 21:12:59.240: E/AndroidRuntime(1968): FATAL EXCEPTION: main 
07-11 21:12:59.240: E/AndroidRuntime(1968): java.lang.NullPointerException 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:142) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.app.AlertDialog$Builder.<init>(AlertDialog.java:359) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at com.vinit.android.market.licensing.mDialogFragment.onCreateDialog(mDialogFragment.java:18) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at com.vinit.android.market.licensing.MainActivity$1.onClick(MainActivity.java:90) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.view.View.performClick(View.java:3511) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.view.View$PerformClick.run(View.java:14105) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.os.Handler.handleCallback(Handler.java:605) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.os.Handler.dispatchMessage(Handler.java:92) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.os.Looper.loop(Looper.java:137) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at android.app.ActivityThread.main(ActivityThread.java:4424) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at java.lang.reflect.Method.invokeNative(Native Method) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at java.lang.reflect.Method.invoke(Method.java:511) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
07-11 21:12:59.240: E/AndroidRuntime(1968): at dalvik.system.NativeStart.main(Native Method) 

MainActivity.java

public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
    mStatusText = (TextView) findViewById(R.id.status_text); 
    mCheckLicenseButton = (Button) findViewById(R.id.check_license_button); 
    mCheckLicenseButton.setOnClickListener(new View.OnClickListener() { 
     public void onClick(View view) {     
     (new mDialogFragment()).onCreateDialog(null).show(); //this line giving error 
     } 
    }); 

} 

mDialogFragment.java

public class mDialogFragment extends DialogFragment{ 

@Override 
public Dialog onCreateDialog(Bundle savedInstanceState) { 
    // TODO Auto-generated method stub 
    AlertDialog.Builder builder=new AlertDialog.Builder(getActivity()); //this line giving error 
    builder.setMessage(R.string.Testing).setPositiveButton(R.string.Buy, new DialogInterface.OnClickListener() { 

     @Override 
     public void onClick(DialogInterface dialog, int which) { 
      // TODO Auto-generated method stub 

     } 
    }).setNegativeButton(R.string.quit_button,new DialogInterface.OnClickListener() { 

     @Override 
     public void onClick(DialogInterface dialog, int which) { 
      // TODO Auto-generated method stub 

     } 
    }); 
    return builder.create(); 
}} 

Answer 1

更改

(new mDialogFragment()).onCreateDialog(null).show(); //this line giving error 

到

mDialogFragment dialog = new mDialogFragment(); 
dialog.show(getFragmentManager(), "mDialogFragment"); 

Answer 2

取决于如何使用它，这可以解决您的问题就在这里。

Fragment has not attached to Activity yet

为了解决你所要做的是等待，直到建议的活动已准备就绪，或通过上下文自己，当适用于您的对话吧。