listviewが表示されない

Question

javaソース

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button newCreate;
    private ListView listitem;
    private List<DtoItem> items;
    private static final String[] texts = {
            "abc ", "bcd", "cde", "def", "efg",
            "fgh"
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        newCreate = (Button)findViewById(R.id.button);
        newCreate.setOnClickListener(this);

        items = DaoItem.findAll(getApplicationContext());

       // ListView listView = new ListView(this);
       // setContentView(listitem);

        ArrayAdapter<String> arrayAdapter =
                new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, texts);
        
        listitem.setAdapter(arrayAdapter);

xmlソース

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="com.example.minamigani.todolist.MainActivity" tools:layout_editor_absoluteY="81dp"
        android:id="@+id/constrainlayout">

    
    <ListView
            android:id="@+id/todolistview"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_weight="1" tools:layout_editor_absoluteY="100dp" tools:layout_editor_absoluteX="60dp"/>


    <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:visibility="visible" tools:layout_editor_absoluteY="464dp" tools:layout_editor_absoluteX="0dp">

        <Button
                android:text="@string/NewCreate"
                android:layout_width="wrap_content"
                android:layout_height="47dp"
                android:id="@+id/button"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintHorizontal_bias="0.895" app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintTop_toTopOf="parent" app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintVertical_bias="0.034"/>

    </LinearLayout>

</android.support.constraint.ConstraintLayout>

実行すると

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'

と言われました。何が違うのでしょうか

Answer 1

初歩的なミス

  listitem = (ListView)findViewById(R.id.todolistview);

viewの設定をしていなかった。。。