活动崩溃，因为 ListAdapter 等于 null

Question

我为我的应用程序创建了这个适配器：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;    }}但是每次我尝试运行该应用程序时，我的应用程序都会崩溃。日志显示以下内容：原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)'我该如何解决这个问题？

Answer 1

检查我的例子。我使用String的只是列表中的元素。

请记住，您应该使用View Holder 模式（使用传递给此方法的回收视图作为第二个参数）以实现更平滑的滚动和节省内存。

activity_main.xml

<?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>

ActivityMain.java

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);

    }

}

和适配器 - DeadlineAdapter.java：

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;

    }

}

和项目列表 - item_deadline：

<?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>

预期输出：

检查我的例子。我使用String的只是列表中的元素。

请记住，您应该使用View Holder 模式（使用传递给此方法的回收视图作为第二个参数）以实现更平滑的滚动和节省内存。

activity_main.xml

<?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>

ActivityMain.java

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);

    }

}

和适配器 - DeadlineAdapter.java：

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;

    }

}

和项目列表 - item_deadline：

<?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>

预期输出：