Android中的空指针异常。

我对Android开发非常陌生，并且一直在关注关于Android编程的书

我一直收到这个空指针异常的错误，我不明白这个错误指的是什么

当我按下应用程序中的任何东西时，应用程序崩溃

这里是错误

01-17 17:10:54.794      811-811/com.bignerdranch.android.criminalintent E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.NullPointerException
            at com.bignerdranch.android.criminalintent.CrimeFragment.onCreateView(CrimeFragment.java:81)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:927)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1467)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:472)
            at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:163)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1068)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:914)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1436)
            at android.view.View.measure(View.java:15848)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at android.view.View.measure(View.java:15848)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
            at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302)
            at android.view.View.measure(View.java:15848)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2189)
            at android.view.View.measure(View.java:15848)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1905)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1104)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1284)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1004)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5481)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
            at android.view.Choreographer.doCallbacks(Choreographer.java:562)
            at android.view.Choreographer.doFrame(Choreographer.java:532)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
            at android.os.Handler.handleCallback(Handler.java:730)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5103)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:525)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

这是我得到错误的代码

---

在我出错的地方，任何帮助都会很好。谢谢。

您需要在Crime classCrime.java的构造函数中创建一个新的日期对象

public Crime(){
mId = UUID.randomUUID();
mDate = new Date();
}

你可能会错过台词

mDate = new Date();

---

为易发生异常的代码添加try-catch块。在使用之前，请使用nCrime检查null。还要检查CrimeLab为什么给出空值。

NPE在第81行。猜测mTitleField或mCrime为空。请查看CrimeFragment.java中的第81行。我认为这是mCrime，可能您没有放入任何数据，因为您正在初始化类。

mDate = new Date();