java - fragment 之间通信时出现空指针异常，android studio 无法识别 fragment id，因为它需要 id 类型的资源？

Question

所以我在尝试在 fragment 之间传递数据时遇到了空指针异常。该错误发生在我的主要 Activity 中，它告诉我当对我的 fragment 之一调用 updateText 方法时(并且定义了此方法和我的 fragment 类)，它返回一个空引用。以下是完整的错误消息文本:

03-01 17:25:13.062    2011-2011/edu.sbu.cs.android E/AndroidRuntime﹕         FATAL EXCEPTION: main
Process: edu.sbu.cs.android, PID: 2011
java.lang.NullPointerException: Attempt to invoke virtual method 'void edu.sbu.cs.android.NMR.core.SpectraFragment.updateText(java.lang.String)' on a null object reference
        at edu.sbu.cs.android.NMR.core.MainActivity.onSelected(MainActivity.java:31)
        at edu.sbu.cs.android.NMR.core.HomeFragment$1.onItemClick(HomeFragment.java:66)
        at android.widget.AdapterView.performItemClick(AdapterView.java:305)
        at android.widget.AbsListView.performItemClick(AbsListView.java:1146)
        at android.widget.AbsListView$PerformClick.run(AbsListView.java:3053)
        at android.widget.AbsListView.onTouchUp(AbsListView.java:3872)
        at android.widget.AbsListView.onTouchEvent(AbsListView.java:3637)
        at android.view.View.dispatchTouchEvent(View.java:8471)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2399)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2092)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2405)
        at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2106)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2369)
        at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1719)
        at android.app.Activity.dispatchTouchEvent(Activity.java:2742)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2330)
        at android.view.View.dispatchPointerEvent(View.java:8666)
        at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4123)
        at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3989)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544)
        at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3597)
        at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3563)
        at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3680)
        at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3571)
        at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3737)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544)
        at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3597)
        at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3563)
        at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3571)
        at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3544)
        at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:5807)
        at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:5781)
        at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:5752)
        at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:5897)
        at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
        at android.os.MessageQueue.nativePollOnce(Native Method)
        at android.os.MessageQueue.next(MessageQueue.java:143)
        at android.os.Looper.loop(Looper.java:122)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.in

这是我遇到错误的java代码

MainActivity 类:

@Override
public void onSelected(String text){
    SpectraFragment frag = (SpectraFragment)
            getSupportFragmentManager().findFragmentById(R.layout.fragment_spectra);
    frag.updateText(text);

这是我尝试在之间传递数据的两个 fragment 的相关代码:

fragment A(主页):

public interface OnSelectedListener {
        public void onSelected(String text);
  }

OnSelectedListener mListener;
  @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
      this.getActivity();
      try {
            mListener = (OnSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnSelectedListener");
        }
    }

@Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_home, container, false);
    gridview = (GridView)rootView.findViewById(R.id.gridView1);
    gridview.setAdapter(new ImageAdapter(gridview.getContext()));
    gridview.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                                int position, long id) {
            ((MainActivity) getActivity()).setProblem(position);
            switch (position) {
                case 0:                        mListener.onSelected("file:///android_asset/nmr0.html");
                    FragmentManager fragMan;
                    fragMan = getFragmentManager();
                    Fragment frg= fragMan.findFragmentByTag("fragment_spectra");
                    Fragment frg2 = fragMan.findFragmentByTag("fragment_questions");
                    final FragmentTransaction ft = fragMan.beginTransaction();
                    ft.detach(frg);
                    ft.attach(frg);
                    ft.detach(frg2);
                    ft.attach(frg2);
                    ft.commit();

                case 1:
                    mListener.onSelected("file:///android_asset/nmr1.html");
fragMan = getFragmentManager();
                    frg=      fragMan.findFragmentByTag("fragment_spectra");
                    frg2 = fragMan.findFragmentByTag("fragment_questions");
                    final FragmentTransaction ft2 = fragMan.beginTransaction();
                    ft2.detach(frg);
                    ft2.attach(frg);
                    ft2.detach(frg2);
                    ft2.attach(frg2);
                    ft2.commit();
 default:
                        SpectraFragment.w.loadUrl("file:///android_asset/nmr0.html");
            }

fragment B(光谱):

public void updateText(String text){
    setStr(text);
    FragmentManager fragMan;
    fragMan = getFragmentManager();
    Fragment frg= fragMan.findFragmentByTag("fragment_spectra");
    final FragmentTransaction ft = fragMan.beginTransaction();
   // ft.detach(frg);
    ft.attach(frg);
    ft.commit();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_spectra, container, false);;
    if (getStr() != null) {
        w.loadUrl(getStr());
    }
    else
    {
        w.loadUrl("file:///android_asset/nmr0.html");
    }
        w.getSettings().setBuiltInZoomControls(true);
    w.getSettings().setDisplayZoomControls(false);
    return rootView;

}

private void setStr (String text){
    str = text;
}

private String getStr (){
    return str;
}

在 android studio 编辑器中，它告诉我当我在接收 fragment 中调用 updateText 方法时，它会给出消息“预期类型 id 的资源”我知道这与 R 类有关，目前我的 fragment 类定义为r.layout.fragment_a。问题是否出在这个引用文献中？我没有在 fragment 之间正确传递数据吗？我知道这与 android.support.v4.app 和 andoid.app 不兼容无关，因为我已经删除了所有这些引用并让应用程序可以使用它，感谢 Stack Overflow。我拥有所有必需的方法用在正确的地方。我做错了什么？

编辑:这是我的相关 xml 文件:

MainActivity 的 XML:

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
<edu.sbu.cs.android.NMR.core.NonSwipeableViewPager xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginTop="45dp">

</edu.sbu.cs.android.NMR.core.NonSwipeableViewPager>

    <edu.sbu.cs.android.NMR.adapter.SlidingTabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="top"
        android:background= "#000000"
        android:textColor="#ffffff"
        />

</RelativeLayout>

HomeFragment 的 XML:

 <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#949494" >

<GridView 
    android:id="@+id/gridView1"
    android:background="#949494" 
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:columnWidth="35dp"
    android:numColumns="5"
    android:verticalSpacing="5dp"
    android:horizontalSpacing="5dp"
    android:stretchMode="columnWidth"
    android:gravity="center"
    android:listSelector="#FFF">


</GridView>

</RelativeLayout>

SpectraFragment 的 XML:

 <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#949494">

   <WebView
        android:id="@+id/imgView"
        android:background="#949494" 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true" />
</RelativeLayout>

编辑:查看 View 层次结构，如果我正确理解它，看起来我应该引用 android:id 内容来获取 SpectraFragment 的容器，因为它包含我的选项卡和 fragment 。是这样吗？

Answer 1

我认为问题是因为您试图通过布局获取 fragment 。因此，frag 没有获取 fragment 的值并且为 null，因此您的对象引用为 null。您应该通过容器的 ID 获取 fragment 。

问题代码:

@Override
public void onSelected(String text){
    SpectraFragment frag = (SpectraFragment)
        getSupportFragmentManager().findFragmentById(R.layout.fragment_spectra);
    frag.updateText(text);
}

修复:

SpectraFragment frag = (SpectraFragment)
        getSupportFragmentManager().findFragmentById(ID_OF_CONTAINER);