Java 活动崩溃，因为ListAdapter等于null

我为我的应用程序创建了此适配器：

public class DeadlineAdapter extends ArrayAdapter<Deadline> {
    private Activity context;
    public List<Deadline> deadlineList;

    public DeadlineAdapter(Activity context, List<Deadline> deadlineList){
        super(context, R.layout.item_deadline, deadlineList);
        this.context = context;
        this.deadlineList = deadlineList;
    }

    @NonNull
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        LayoutInflater inflater = context.getLayoutInflater();
        View listView = inflater.inflate(R.layout.item_deadline, parent, false);

        TextView displayDate = listView.findViewById(R.id.display_deadline_date_tv);
        TextView displayName = listView.findViewById(R.id.display_deadline_name_tv);
        Deadline deadline = deadlineList.get(position);
        displayDate.setText(deadline.getDate());
        displayName.setText(deadline.getDeadlineName());

        return listView;
    }
}

但每次我尝试运行应用程序时，我的应用程序都会崩溃。日志上说：

原因：java.lang.NullPointerException:尝试对空对象引用调用虚拟方法“void android.widget.ListView.setAdapterandroid.widget.ListAdapter”

---

如何解决这个问题？

检查我的示例。我只使用字符串作为列表中的元素

请记住，您应该使用视图保持器模式使用传递到此方法的循环视图作为第二个参数，以便更平滑地滚动并节省内存

activity_main.xml

ActivityMain.java

和适配器-DeadlineAdapter.java：

和列表项目-项目截止日期：

预期产出：

---

请注意，它不会告诉您适配器为空，但ListView似乎为空，正如您在日志中看到的，无论您在哪个对象上调用setAdapter，都是空的。您没有在此类中调用该方法，因此错误源自其他地方。这与问题无关。.这里有一个工作示例，可以由@MoritzL改编

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <ListView
        android:id="@+id/list_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

public class ActivityMain extends AppCompatActivity {

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

        setupUi();
    }

    private void setupUi() {
        List<String> elements = new ArrayList<>(Arrays.asList(
                "Red",
                "Blue",
                "Green",
                "Black"
        ));

        DeadlineAdapter adapter = new DeadlineAdapter(this, elements);

        ListView listView = findViewById(R.id.list_view);
        listView.setAdapter(adapter);
    }
}

public class DeadlineAdapter extends ArrayAdapter<String> {

    private Activity context;
    private List<String> deadlineList;

    public DeadlineAdapter(Activity context, List<String> deadlineList) {
        super(context, R.layout.item_deadline, deadlineList);
        this.context = context;
        this.deadlineList = deadlineList;
    }

    @NonNull
    public View getView(int position, View convertView, @NotNull ViewGroup parent) {

        LayoutInflater inflater = context.getLayoutInflater();
        View listView = inflater.inflate(R.layout.item_deadline, parent, false);

        TextView positionTv = listView.findViewById(R.id.row_id);
        TextView textTv = listView.findViewById(R.id.row_text);

        positionTv.setText(String.format("%s.", position + 1));
        textTv.setText(deadlineList.get(position));

        return listView;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="30dp">

    <TextView
        android:id="@+id/row_id"
        android:layout_width="100dp"
        android:layout_height="wrap_content" />

    <TextView
        android:id="@+id/row_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</LinearLayout>