EmPubLite 应用程序在教程 #13 上崩溃

Question

所以，我正在阅读教程 #12，一切正常。然后我尝试进入教程 #13 保存/恢复上次读取位置，然后应用程序崩溃：

04-10 15:59:26.510: E/Trace(704): error opening trace file: No such file or directory (2)
04-10 15:59:27.491: D/AndroidRuntime(704): Shutting down VM
04-10 15:59:27.491: W/dalvikvm(704): threadid=1: thread exiting with uncaught exception (group=0x40a13300)
04-10 15:59:27.500: E/AndroidRuntime(704): FATAL EXCEPTION: main
04-10 15:59:27.500: E/AndroidRuntime(704): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.commonsware.empublite/com.commonsware.empublite.EmPubLiteActivity}: java.lang.NullPointerException
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread.access$600(ActivityThread.java:130)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.os.Handler.dispatchMessage(Handler.java:99)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.os.Looper.loop(Looper.java:137)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread.main(ActivityThread.java:4745)
04-10 15:59:27.500: E/AndroidRuntime(704):  at java.lang.reflect.Method.invokeNative(Native Method)
04-10 15:59:27.500: E/AndroidRuntime(704):  at java.lang.reflect.Method.invoke(Method.java:511)
04-10 15:59:27.500: E/AndroidRuntime(704):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
04-10 15:59:27.500: E/AndroidRuntime(704):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-10 15:59:27.500: E/AndroidRuntime(704):  at dalvik.system.NativeStart.main(Native Method)
04-10 15:59:27.500: E/AndroidRuntime(704): Caused by: java.lang.NullPointerException
04-10 15:59:27.500: E/AndroidRuntime(704):  at com.commonsware.empublite.ModelFragment.executeAsyncTask(ModelFragment.java:51)
04-10 15:59:27.500: E/AndroidRuntime(704):  at com.commonsware.empublite.ModelFragment.deliverModel(ModelFragment.java:38)
04-10 15:59:27.500: E/AndroidRuntime(704):  at com.commonsware.empublite.ModelFragment.onActivityCreated(ModelFragment.java:29)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:891)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1080)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:622)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1416)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:505)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1163)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.Activity.performStart(Activity.java:5018)
04-10 15:59:27.500: E/AndroidRuntime(704):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2032)
04-10 15:59:27.500: E/AndroidRuntime(704):  ... 11 more

我试图注释掉我所做的代码，直到它工作的最后一点，但这似乎不起作用。我也不确定如何阅读此堆栈跟踪。我应该看什么来解决这个问题？

谢谢。

Answer 1

我也不确定如何阅读此堆栈跟踪。我应该看什么来解决这个问题？

ModelFragment.java 的第 51 行，在 executeAsyncTask() 方法中是您崩溃的地方（请参阅最后一个堆栈跟踪节，以 Caused by: java.lang.NullPointerException 开头）。

您的行号和those in the answers 没有完全对齐。但是，executeAsyncTask() 可以有 NullPointerException 的唯一方法是，如果您传入的 AsyncTask 是 null，这应该是不可能的，因为您在 @987654329 中调用 executeAsyncTask() 之前立即定义了这些任务@ 方法。也许您错过了deliverModel() 中的一行。