我正在尝试为正在制作的Android应用程序创建可滑动的标签布局。我的第一个标签正常运行，但是现在我添加了第二个标签，当我打开应用程序时，它崩溃并显示以下错误：
2019-11-01 20:26:17.457 26813-26813/com.example.nhlstats E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.nhlstats, PID: 26813
kotlin.KotlinNullPointerException
at com.example.nhlstats.ui.fragments.StandingsFragment.<init>(StandingsFragment.kt:24)
at com.example.nhlstats.ui.adapters.HomePagerAdapter.getItem(HomePagerAdapter.kt:15)
at androidx.fragment.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:170)
at androidx.viewpager.widget.ViewPager.addNewItem(ViewPager.java:1010)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1224)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1092)
at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1622)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:24545)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:995)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:143)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24545)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.DecorView.onMeasure(DecorView.java:742)
at android.view.View.measure(View.java:24545)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3006)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1833)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2122)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1721)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7598)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966)
at android.view.Choreographer.doCallbacks(Choreographer.java:790)
at android.view.Choreographer.doFrame(Choreographer.java:725)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:951)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
我在片段的两种方法中都设置了断点，但是它在碰到任何一个之前就崩溃了。我不确定基于该错误输出可能为null，尤其是如果它甚至没有进入片段以达到这些断点的情况下。
我的片段类在这里：
class StandingsFragment: Fragment() {
private lateinit var binding: FragmentStandingsBinding
private val rowHeaderList: List<RowHeader> = emptyList()
private val columnHeaderList: List<ColumnHeader> = emptyList()
private val cellList: List<List<Cell>> = emptyList()
private val tableView: TableView = TableView(requireContext())
private val tableViewAdapter: StandingsTableViewAdapter by lazy { StandingsTableViewAdapter(requireContext())}
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
binding = DataBindingUtil.inflate(inflater, R.layout.fragment_standings, container, false)
binding.lifecycleOwner = this
return binding.root
}
override fun onActivityCreated(savedInstanceState: Bundle?) {
super.onActivityCreated(savedInstanceState)
tableView.adapter = tableViewAdapter
tableViewAdapter.setAllItems(columnHeaderList, rowHeaderList, cellList)
}
}
这是我的适配器类：
class HomePagerAdapter(fm: FragmentManager) : FragmentPagerAdapter(fm){
override fun getItem(position: Int): Fragment {
when (position) {
0 -> return ScoreboardFragment()
1 -> return StandingsFragment()
}
return ScoreboardFragment()
}
override fun getCount(): Int {
return 2
}
override fun getPageTitle(position: Int): CharSequence? {
when (position) {
0 -> return "Scores"
1 -> return "Standings"
}
return "Scores"
}
}
谢谢。
答案 0 :(得分：3)
发生这种情况是因为您在Fragment的{{1}}可用之前就对其进行访问，而您正在用Context对其进行拆包。这会引发!!。
KotlinNullPointerException要解决此问题，可以将其设置为惰性，以便仅在实际使用时（例如，在private val tableView: TableView = TableView(context!!)
private val tableViewAdapter: StandingsTableViewAdapter = StandingsTableViewAdapter(context!!)
中）对其进行评估：
onViewCreated请记住，您可以使用private val tableView: TableView by lazy { TableView(requireContext()) }
private val tableViewAdapter: StandingsTableViewAdapter by lazy { StandingsTableViewAdapter(requireContext()) }
代替requireContext()，这会引发更明显的异常。