android - 空指针异常 - onPrepareDialog

Question

我正在制作一个应用程序，我在其中使用对话框来接受、存储和显示数据，它对我来说很有效，但是每当我重新启动我的应用程序时，都会出现NPE

  @Override
    public void onPrepareDialog(int id, Dialog dialog) {
        switch (id) {
        case DIALOG_LOGIN:
        final AlertDialog alertDialog = (AlertDialog) dialog;
            .........
            .........
            // check for parent email  
             if(GetEventsActivity.parental.equals("no"))  // throwing error
            {   
                // hide
                parental_email.setVisibility(View.GONE);
            }
            else
            {
                 // show
                parental_email.setVisibility(View.VISIBLE);
            }
            ........
         }
    }

我在列表项中显示按钮，每当用户点击该按钮时，我都会显示对话框。

        btnData.setOnClickListener(new View.OnClickListener() {
            @SuppressWarnings("deprecation")
            public void onClick(View v) {
                showDialog(DIALOG_LOGIN);
            }
        });

和日志说:

W/dalvikvm(26689): threadid=1: thread exiting with uncaught exception (group=0x41a4d2a0)
E/AndroidRuntime(26689): FATAL EXCEPTION: main
E/AndroidRuntime(26689): java.lang.NullPointerException
E/AndroidRuntime(26689):    at app.mac.Camera.UploadActivity.onPrepareDialog(UploadActivity.java:469)
E/AndroidRuntime(26689):    at android.app.Activity.onPrepareDialog(Activity.java:3071)
E/AndroidRuntime(26689):    at android.app.Activity.showDialog(Activity.java:3134)
E/AndroidRuntime(26689):    at android.app.Activity.showDialog(Activity.java:3085)
E/AndroidRuntime(26689):    at app.mac.Camera.UploadActivity$ImageAdapter$2.onClick(UploadActivity.java:286)
E/AndroidRuntime(26689):    at android.view.View.performClick(View.java:4223)
E/AndroidRuntime(26689):    at android.view.View$PerformClick.run(View.java:17275)
E/AndroidRuntime(26689):    at android.os.Handler.handleCallback(Handler.java:615)
E/AndroidRuntime(26689):    at android.os.Handler.dispatchMessage(Handler.java:92)
E/AndroidRuntime(26689):    at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(26689):    at android.app.ActivityThread.main(ActivityThread.java:4921)
E/AndroidRuntime(26689):    at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(26689):    at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(26689):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1036)
E/AndroidRuntime(26689):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:803)
E/AndroidRuntime(26689):    at dalvik.system.NativeStart.main(Native Method)
I/Process(26689): Sending signal. PID: 26689 SIG: 9

Answer 1

这意味着

GetEventsActivity.parental

是 null。另一方面，我建议始终将常量字符串与可以更改的字符串进行比较

if ("no".equals(GetEventsActivity.parental))

这样你至少可以避免 NPE，因为第一个测试将是

if (! (GetEventsActivity.parental instanceof String))
 return false;

和null instanceof String总是false