Android: NullPointerException when adding an Adapter to a ListView

Question

I am creating a ViewPager in activity_table.xml.

This ViewPager has two pages which both share the same layout: list.xml.
This layout contains a ListView which I am trying to create an Adapter for.

The app compiles correctly but crashes when it tries to add the said Adapter to the ListView with a

NullPointerException when attempting to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference.

I have heard that I have to "load" a layout before I can use the Views it contains, but I how do I do that?

I have tried setContentView(R.layout.list), but it throws the same error...

Table.java

    SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
    ViewPager viewPager = (ViewPager) findViewById(R.id.container);
    viewPager.setAdapter(sectionsPagerAdapter);

    String[] tmp = new String[mainTable.length-2];
    ListView listView = (ListView)findViewById(R.id.listView);
    ListAdapter listAdapter = new ListAdapter(this,R.layout.list_item,tmp);
    listView.setAdapter(listAdapter);

public static class TableFragment extends Fragment {
    private static final String ARG_SECTION_NUMBER = "section number";

    public TableFragment() {
    }
    public static TableFragment newInstance(int sectionNumber) {
        TableFragment fragment = new TableFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER,sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.list,container,false);
        TextView title = (TextView) rootView.findViewById(R.id.title);

        if(getArguments().getInt(ARG_SECTION_NUMBER) == 1)
            title.setText("Heute");
        else if(getArguments().getInt(ARG_SECTION_NUMBER) == 2)
            title.setText("Morgen");

        return rootView;
    }
}

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return TableFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        return 2;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "today";
            case 1:
                return "tomorrow";
        }
        return null;
    }
}


public class ListAdapter extends ArrayAdapter<String> {
    public ListAdapter(Context context, int textViewResourceId) {
        super(context,textViewResourceId);
    }
    public ListAdapter(Context context, int resource, String[] items) {
        super(context,resource,items);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;

        if(v == null) {
            LayoutInflater vi;
            vi = LayoutInflater.from(getContext());
            v = vi.inflate(R.layout.list_item, null);
        }

        String p = getItem(position);

        if(p!=null) {
            TextView grade = (TextView) v.findViewById(R.id.grade);
            TextView hour = (TextView) v.findViewById(R.id.hour);
            TextView course = (TextView) v.findViewById(R.id.course);
            TextView teacher = (TextView) v.findViewById(R.id.teacher);
            TextView room = (TextView) v.findViewById(R.id.room);
            TextView description = (TextView) v.findViewById(R.id.description);

            try {
                grade.setText(tableData[position][1]);
                hour.setText(tableData[position][2]);
                course.setText(tableData[position][3]);
                teacher.setText(tableData[position][4]);
                room.setText(tableData[position][5]);
                description.setText(tableData[position][6]);
            } catch (ArrayIndexOutOfBoundsException e) {
                return v;
            }
        }

        return v;
    }
}

activity_table.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/main_content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context="de.fuchstim.vertretungsplan.Table">

<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/appbar_padding_top"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|enterAlways"
        app:popupTheme="@style/AppTheme.PopupOverlay">

    </android.support.v7.widget.Toolbar>
</android.support.design.widget.AppBarLayout>

<android.support.v4.view.ViewPager
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"/>


    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="@dimen/fab_margin"
        android:layout_gravity="bottom|right"
        app:srcCompat="@drawable/reload_arrow_white" />


</android.support.design.widget.CoordinatorLayout>

list.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
    android:text="Heute (25.10.16)"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/title"
    android:textAppearance="@android:style/TextAppearance.DeviceDefault.DialogWindowTitle"
    android:textAlignment="center" />

<GridLayout android:layout_gravity="start"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <TextView
        android:id="@+id/grade"
        android:layout_height="wrap_content"
        android:text="Klasse"
        android:layout_row="0"
        android:layout_column="0"
        android:layout_width="0dp"
        android:textSize="14sp"
        android:layout_columnWeight="40"
        android:paddingStart="10dp" />

    <TextView
        android:text="Stunde"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/hour"
        android:layout_row="0"
        android:layout_column="1"
        android:layout_columnWeight="15"/>

    <TextView
        android:text="Lehrer"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/teacher"
        android:layout_row="0"
        android:layout_column="3"
        android:layout_columnWeight="15"/>

    <TextView
        android:text="Kurs"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/course"
        android:layout_row="0"
        android:layout_column="2"
        android:layout_columnWeight="15"/>

    <TextView
        android:text="Raum"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:id="@+id/room"
        android:layout_row="0"
        android:layout_column="4"
        android:layout_columnWeight="15"/>

</GridLayout>

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

</LinearLayout>

setContentView needs a layout. Lets see a little of your code — Xjasz, Oct 26 '16 at 19:04
Please post the relevant code. I left my crystal ball in the other pants. — Robert, Oct 26 '16 at 19:07
Where is the Activity code? I do not see a `setContentView` anywhere — OneCricketeer, Oct 26 '16 at 19:40

score 0 · Accepted Answer · answered Oct 26 '16 at 19:41

0

I have tried setContentView(R.layout.list), but it throws the same error...

First, you need to use setContentView(R.layout.activity_table) instead.

Second, you cannot find the ListView from the Activity, since it is contained within the Fragment layout.

See here? Your ListView is in the same layout as this title element, so you need to find the ListView here...

View rootView = inflater.inflate(R.layout.list,container,false);
TextView title = (TextView) rootView.findViewById(R.id.title);

Move that ListView code from the Activity to the Fragment. And use getActivity() for the Adapter within the Fragment instead of this to get the Context.

That is where the @+id/listView value exists. That layout is searched with rootView.findViewById.

answered Oct 26 '16 at 19:41

OneCricketeer

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That works, thank you very much ^.^ – rocket_doge_ Oct 26 '16 at 19:52
One more question; what do I have to change so that the content of the TextViews is being changed by the ListAdapter? Doesnt seem to work atm... – rocket_doge_ Oct 26 '16 at 20:21
I don't know what you mean because you have not shown what the value of `tableData` is. If you have another question, please ask it by clicking the [Ask Question](//stackoverflow.com/questions/ask) button. – OneCricketeer Oct 26 '16 at 20:24

DiName · Answer 2 · 2016-10-26T20:04:56.610

Since, you did not get null pointer at this location :

ViewPager viewPager = (ViewPager) findViewById(R.id.container);
viewPager.setAdapter(sectionsPagerAdapter);

So, this means you are using :

setContentView(R.layout.activity_table);

Because viewpager is only in above layout, and since listview is not in this layout xml file, you get null pointer on :

ListView listView = (ListView)findViewById(R.id.listView);
ListAdapter listAdapter = new ListAdapter(this,R.layout.list_item,tmp);
listView.setAdapter(listAdapter);

because listview in null, not present in activity_table xml file.

You need to set adapter in OnCreateView() :

@Override
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.list,container,false);
    TextView title = (TextView) rootView.findViewById(R.id.title);

    // Add below ...
    String[] tmp = new String[mainTable.length-2];
    ListView listView = (ListView)rootView.findViewById(R.id.listView);
    ListAdapter listAdapter = new ListAdapter(getActivity(),R.layout.list_item,tmp);
    listView.setAdapter(listAdapter);

    // ends here ...

    if(getArguments().getInt(ARG_SECTION_NUMBER) == 1)
        title.setText("Heute");
    else if(getArguments().getInt(ARG_SECTION_NUMBER) == 2)
        title.setText("Morgen");

    return rootView;
}

Hope this helps !

`new ListAdapter(this` should be `new ListAdapter(getActivity()` — OneCricketeer, Oct 26 '16 at 20:00
Well, the app is not crashing anymore, at least, but the TextViews are not being changed by the ListAdapter anymore... What do I have to chage to make that work as well? — rocket_doge_, Oct 26 '16 at 20:16

Android: NullPointerException when adding an Adapter to a ListView

2 Answers2