Error in my Fragment: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference [duplicate]

问题

I try to create a ListView which showes my Pojos called "Baustelle". I wrote a Adapter and a ViewHolder as I learned it in school but the difference is that we only did it for the MainActivity in my school. Now i tried to use a Fragment for it but I always get this Error:

"Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference"

This is my Fragment("Baustellenverwaltung") which also contains the Adapter and the ViewHolder

public class Bautellenverwaltung extends Fragment {


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        List<Baustelle> list = new LinkedList<>();
        list.add(new Baustelle(1,"Town 1",false, 0));
        list.add(new Baustelle(2,"City 1234",false, 0));

        ListView lv = (ListView)getView().findViewById(R.id.baustellenverwaltung_listview);
        lv.setAdapter(new MyListAdapter(getActivity(),R.layout.list_item,list));
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_bautellenverwaltung, container, false);
    }

    private class MyListAdapter extends ArrayAdapter<Baustelle> {
        private int layout;

        public MyListAdapter( Context context, int resource, List<Baustelle> objects) {
            super(context, resource, objects);
            layout = resource;
        }

        @NonNull
        @Override
        public View getView(final int position, @Nullable View convertView, @NonNull ViewGroup parent) {
            if(convertView == null){
                LayoutInflater inflater = LayoutInflater.from(getContext());
                convertView = inflater.inflate(layout,parent,false);
                ViewHolder viewHolder = new ViewHolder();
                viewHolder.detail.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Detail Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });
                viewHolder.loschen.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Loschen Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });
                viewHolder.mangel_detail.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Mängel Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });

                convertView.setTag(viewHolder);
            }
            else{
                ViewHolder viewHolder = (ViewHolder)convertView.getTag();
                viewHolder.id.setText(getItem(position).getId());
                viewHolder.name.setText(getItem(position).getName());
                viewHolder.mangel.setText(getItem(position).getAnzMangel());
            }

            return convertView;
        }
    }

    public class ViewHolder{
        private TextView id;
        private TextView name;
        private TextView mangel;
        private Button detail;
        private Button loschen;
        private Button mangel_detail;

        public ViewHolder(){
            id = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_id);
            name = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_name);
            mangel = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_mangel);
            detail = (Button)getView().findViewById(R.id.baustellenverwaltung_button_deatil);
            loschen = (Button)getView().findViewById(R.id.baustellenverwaltung_button_loschen);
            mangel_detail = (Button)getView().findViewById(R.id.baustellenverwaltung_button_mangel);
        }

    }
}

This is the .xml File which is inflated by the Fragment

<?xml version="1.0" encoding="utf-8"?>
<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=".presentation.Bautellenverwaltung">

    <!-- TODO: Update blank fragment layout -->


    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:id="@+id/baustellenverwaltung_listview"/>

</RelativeLayout>

That is the list_item.xml which ist the layout for one list item

<?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">

    <TextView
        android:id="@+id/baustellenverwaltung_textview_id"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentStart="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="20sp"
        android:layout_marginTop="10sp"
        android:text="TextView" />

    <TextView
        android:id="@+id/baustellenverwaltung_textview_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/baustellenverwaltung_textview_id"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="20sp"
        android:layout_marginTop="10sp"
        android:text="TextView" />

    <TextView
        android:id="@+id/baustellenverwaltung_textview_mangel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/baustellenverwaltung_textview_name"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="20sp"
        android:layout_marginTop="10sp"
        android:text="TextView" />

    <CheckBox
        android:id="@+id/baustellenverwaltung_checkbox_abgenommen"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toRightOf="@+id/baustellenverwaltung_textview_mangel"
        android:layout_marginLeft="20sp"
        android:layout_marginTop="5sp"
        android:text="CheckBox" />

    <Button
        android:id="@+id/baustellenverwaltung_button_deatil"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toRightOf="@+id/baustellenverwaltung_checkbox_abgenommen"
        android:layout_marginLeft="20sp"
        android:text="Detail" />
    <Button
        android:id="@+id/baustellenverwaltung_button_loschen"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toRightOf="@+id/baustellenverwaltung_button_deatil"
        android:layout_marginLeft="20sp"
        android:text="Löschen" />

    <Button
        android:id="@+id/baustellenverwaltung_button_mangel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_toRightOf="@+id/baustellenverwaltung_button_loschen"
        android:layout_marginLeft="20sp"
        android:text="Mängel" />

</RelativeLayout>

My Pojo "Baustelle"

public class Baustelle {
    private int id;
    private String name;
    private boolean checked;
    private int anzMangel;

    public Baustelle(int id, String name, boolean checked, int anzMangel) {
        this.id = id;
        this.name = name;
        this.checked = checked;
        this.anzMangel = anzMangel;
    }

    public int getAnzMangel() {
        return anzMangel;
    }

    public void setAnzMangel(int anzMangel) {
        this.anzMangel = anzMangel;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public boolean isChecked() {
        return checked;
    }

    public void setChecked(boolean checked) {
        this.checked = checked;
    }
}

StacktTrace

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                          at at.htlpinkafeld.pfeifferdipl.presentation.Bautellenverwaltung.onCreateView(Bautellenverwaltung.java:84)
                          at android.support.v4.app.Fragment.performCreateView(Fragment.java:2354)
                          at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1419)
                          at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1740)
                          at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1809)
                          at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:799)
                          at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2580)
                          at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2367)
                          at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2322)
                          at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:2199)
                          at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:651)
                          at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:167)
                          at android.support.v4.view.ViewPager.populate(ViewPager.java:1236)
                          at android.support.v4.view.ViewPager.populate(ViewPager.java:1084)
                          at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1614)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                          at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                          at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                          at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                          at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                          at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                          at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                          at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                          at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                          at com.android.internal.policy.DecorView.onMeasure(DecorView.java:687)
                          at android.view.View.measure(View.java:19734)
                          at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2271)
                          at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1358)
                          at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1607)
                          at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1246)
                          at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6301)
                          at android.view.Choreographer$CallbackRecord.run(Choreographer.java:871)
                          at android.view.Choreographer.doCallbacks(Choreographer.java:683)
                          at android.view.Choreographer.doFrame(Choreographer.java:619)
                          at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:857)
                          at android.os.Handler.handleCallback(Handler.java:751)
                          at android.os.Handler.dispatchMessage(Handler.java:95)
                          at android.os.Looper.loop(Looper.java:154)
                          at android.app.ActivityThread.main(ActivityThread.java:6077)
                          at java.lang.reflect.Method.invoke(Native Method)
                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)

回答1:

Your view hasn't been inflated yet in onCreate on Fragments, try to move your code to onCreateView:

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view =  inflater.inflate(R.layout.fragment_bautellenverwaltung, container, false);


        List<Baustelle> list = new LinkedList<>();
        list.add(new Baustelle(1,"Town 1",false, 0));
        list.add(new Baustelle(2,"City 1234",false, 0));

        ListView lv = view.findViewById(R.id.baustellenverwaltung_listview);
        lv.setAdapter(new MyListAdapter(getActivity(),R.layout.list_item,list));

        return view;
    }

You also need to change your view holder to receive the inflated view:

ViewHolder viewHolder = new ViewHolder(convertView);

public class ViewHolder{
        private TextView id;
        private TextView name;
        private TextView mangel;
        private Button detail;
        private Button loschen;
        private Button mangel_detail;

        public ViewHolder(View view){
            id = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_id);
            name = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_name);
            mangel = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_mangel);
            detail = (Button)view.findViewById(R.id.baustellenverwaltung_button_deatil);
            loschen = (Button)view.findViewById(R.id.baustellenverwaltung_button_loschen);
            mangel_detail = (Button)view.findViewById(R.id.baustellenverwaltung_button_mangel);
        }

Finally, you're not setting the data according to your pojos for when the view holder is recently created, only when it is recycled:

public View getView(final int position, @Nullable View convertView, @NonNull ViewGroup parent) {
            ViewHolder viewHolder;
            if(convertView == null){
                LayoutInflater inflater = LayoutInflater.from(getContext());
                convertView = inflater.inflate(layout,parent,false);
                viewHolder = new ViewHolder(convertView);
                viewHolder.detail.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Detail Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });
                viewHolder.loschen.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Loschen Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });
                viewHolder.mangel_detail.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(),"Mängel Button from" + getItem(position).getName()+" pressed",Toast.LENGTH_SHORT).show();
                    }
                });

                convertView.setTag(viewHolder);
            }
            else{
                viewHolder = (ViewHolder)convertView.getTag();
            }
            viewHolder.id.setText(getItem(position).getId());
            viewHolder.name.setText(getItem(position).getName());
            viewHolder.mangel.setText(getItem(position).getAnzMangel());

            return convertView;
        }

回答2:

onCreate() will be invoked before onCreateView() So View will be null in that time

Move this lines to onCreateView()

ListView lv = (ListView)YourView.findViewById(R.id.baustellenverwaltung_listview);
lv.setAdapter(new MyListAdapter(getActivity(),R.layout.list_item,list));

回答3:

Pass your View in this change this-:

public ViewHolder(){
            id = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_id);
            name = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_name);
            mangel = (TextView)getView().findViewById(R.id.baustellenverwaltung_textview_mangel);
            detail = (Button)getView().findViewById(R.id.baustellenverwaltung_button_deatil);
            loschen = (Button)getView().findViewById(R.id.baustellenverwaltung_button_loschen);
            mangel_detail = (Button)getView().findViewById(R.id.baustellenverwaltung_button_mangel);
        }

To-:

public ViewHolder(View view){
super(view);                
id = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_id);
                name = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_name);
                mangel = (TextView)view.findViewById(R.id.baustellenverwaltung_textview_mangel);
                detail = (Button)view.findViewById(R.id.baustellenverwaltung_button_deatil);
                loschen = (Button)view.findViewById(R.id.baustellenverwaltung_button_loschen);
                mangel_detail = (Button)view.findViewById(R.id.baustellenverwaltung_button_mangel);
            }

来源：https://stackoverflow.com/questions/50583639/error-in-my-fragment-attempt-to-invoke-virtual-method-android-view-view-androi

标签

android

listview

android-fragments

nullpointerexception

findviewbyid