尝试在Fragment中的ImageButton上设置单击侦听器时Android应用程序崩溃[复制]

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• What is a NullPointerException, and how do I fix it? 12个答案

试图在片段（SessionsFragment.java）中的ImageButton上设置onClickListener，并且我不断获得空指针异常 . 尝试了所有不同的解决方案，仍然得到相同的错误 . 该按钮位于ExpandableListView中的项目中，因此我添加了list_item.xml . 看到一些评论说它是空的，因为它不在主要活动或类似的东西 . 无论如何，我怎么能克服这个？

SessionsFragment.java

public class SessionsFragment extends Fragment {

ExpandableListAdapter expandableListAdapter;
ExpandableListView expandableListView;
List<String> listDataHeader;
HashMap<String, List<String>> listDataChild;
ImageButton addButton;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_sessions, container, false);

    // Get the ListView
    expandableListView = view.findViewById(R.id.sessionsExpandableListView);

    // Prepare the list data
    prepareListData();

    expandableListAdapter = new ExpandableListAdapter(getContext(), listDataHeader, listDataChild);

    // Set the list adapter and expand lists by default
    expandableListView.setAdapter(expandableListAdapter);
    expandableListView.expandGroup(0);

    // Get button values
    addButton = view.findViewById(R.id.addButton);
    addButton.setOnClickListener(new ImageButton.OnClickListener() {
        @Override
        public void onClick(View view) {
            Toast.makeText(getActivity(), "Session added", Toast.LENGTH_SHORT).show();
        }
    });

    return view;
}

list_item.xml

<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="55dp">

<TextView
    android:id="@+id/listItem"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:textSize="16sp"
    android:paddingTop="5dp"
    android:paddingBottom="5dp" />
    <!--android:paddingLeft="?android:attr/expandableListPreferredChildPaddingLeft"/>-->

<ImageButton
    android:id="@+id/addButton"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:src="@drawable/ic_add_black_24dp"
    android:layout_alignParentRight="true" />

logcat的

04-25 15：12：04.053 11894-11901 /？ E / zygote64：向调试器发送回复失败：断管04-25 15：12：04.578 11894-11894 /？ E / AndroidRuntime：FATAL EXCEPTION：main进程：com.example.android.engineeringweek，PID：11894 java.lang.RuntimeException：无法启动活动ComponentInfo {com.example.android.engineeringweek / com.example.android.engineeringweek.SessionsActivity }：java.lang.NullPointerException：尝试在android.app.ActivityThread.performLaunchActivity（ActivityThread.java）上的空对象引用上调用虚方法'void android.widget.ImageButton.setOnClickListener（android.view.View $ OnClickListener）' . 2946）在android.app.A活动中的android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:3046）android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1688）的android.app.ActivityThread.-wrap11（未知来源：0） android.os.Handler.dispatchMessage（Handler.java:105）位于android.app.Looper.loop（Looper.java:164）的android.app.ActivityThread.main（ActivityThread.java:6809），位于java.lang.reflect .Method.invoke（Native Method）在com.android.internal.os.Zygote $ MethodAndArgsCaller.run（Zygote.java:24 0）at com.android.internal.os.ZygoteInit.main（ZygoteInit.java:767）引起：java.lang.NullPointerException：尝试调用虚方法'void android.widget.ImageButton.setOnClickListener（android.view.View） $ OnClickListener）'在android上的android.support.android.engineeringweek.SessionsFragment.onCreateView（SessionsFragment.java:47）中的空对象引用，位于android.support.v4.app.Fragment.performCreateView（Fragment.java:2354） . 在android.support.v4.app.FragmentManagerImpl.moveToState（FragmentManager）的android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState（FragmentManager.java:1740）上的support.v4.app.FragmentManagerImpl.moveToState（FragmentManager.java:1419） . java：1809）在android.support.v4.app.BackStackRecord.executeOps（BackStackRecord.java:799）的android.support.v4.app.FragmentManagerImpl.executeOps（FragmentManager.java:2580）android.support.v4.app在android.support.v4.app.FragmentManagerImpl.re上的.FragmentManagerImpl.executeOpsTogether（FragmentManager.java:2367） android.support上的android.support.v4.app.FragmentManagerImpl.execPendingActions（FragmentManager.java:2229）中的moveRedundantOperationsAndExecute（FragmentManager.java:2322）位于android.support的android.support.v4.app.FragmentManagerImpl.dispatchStateChange（FragmentManager.java:3221） .s4.app.FragmentManagerImpl.dispatchActivityCreated（FragmentManager.java:3171）位于android.support.v4.app.FragmentController.dispatchActivityCreated（FragmentController.java:192）的android.support.v4.app.FragmentActivity.onStart（FragmentActivity.java） ：560）在Android.app.A.StactActivityStart（AppCompatActivity.java:177）的android.app.Instrumentation.callActivityOnStart（Instrumentation.java:1412）android.app.Activity.performStart（Activity.java：）上的android.support.v7.app.AppCompatActivity.onStart（AppCompatActivity.java:177） 7015）在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2909）

Edit

ExpandableListAdapter.java

public class ExpandableListAdapter extends BaseExpandableListAdapter {
    private Context _context;
    private List<String> _listDataHeader;
    private HashMap<String, List<String>> _listDataChild;

    public ExpandableListAdapter(Context context, List<String> listDataHeader, HashMap<String, List<String>> listDataChild) {
        this._context = context;
        this._listDataHeader = listDataHeader;
        this._listDataChild = listDataChild;
    }

    @Override
    public int getGroupCount() {
        return this._listDataHeader.size();
    }

    @Override
    public Object getGroup(int groupPosition) {
        return this._listDataHeader.get(groupPosition);
    }

    @Override
    public long getGroupId(int groupPosition) {
        return groupPosition;
    }

    @Override
    public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
        String headerTitle = (String) getGroup(groupPosition);
        if(convertView == null) {
            LayoutInflater layoutInflater = (LayoutInflater) this._context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = layoutInflater.inflate(R.layout.list_group, null);
        }

        TextView listHeader = convertView.findViewById(R.id.listHeader);
        listHeader.setTypeface(null, Typeface.BOLD);
        listHeader.setText(headerTitle);

        return convertView;
    }

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition)).get(childPosition);
    }

    @Override
    public long getChildId(int groupPosition, int childPosition) {
        return childPosition;
    }

    @Override
    public View getChildView(int groupPosition, final int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
        final String childText = (String) getChild(groupPosition, childPosition);

        if(convertView == null) {
            LayoutInflater layoutInflater = (LayoutInflater) this._context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            convertView = layoutInflater.inflate(R.layout.list_item, null);
        }

        TextView textListChild = convertView.findViewById(R.id.listItem);

        textListChild.setText(childText);

        return convertView;
    }

    @Override
    public int getChildrenCount(int groupPosition) {
        return this._listDataChild.get(this._listDataHeader.get(groupPosition)).size();
    }

    @Override
    public boolean isChildSelectable(int i, int i1) {
        return true;
    }

    @Override
    public boolean hasStableIds() {
        return false;
    }
}