使用自定义适配器和方向接收 ListView 程序的空指针异常？

Question

我正在制作一个应用程序，其中 ArrayList 在纵向和横向方向上由两个 ListViews 表示。为此，我一直在使用一个自定义适配器来扩充我的 listView。但是每当我为 landscape ListView 实例化自定义适配器时，我都会收到 NullPointer Exception。有什么建议么？抛出异常的行：

listLand.setAdapter(customAdapter);

主要活动

public class MainActivity extends AppCompatActivity {
    ListView listPort;
    ListView listLand;
    ArrayList<Flick> flicks;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        listLand = findViewById(R.id.listLand);
        listPort = findViewById(R.id.listPort);

        flicks = new ArrayList<>();
        flicks.add(new Flick("Wizard of Oz", "Metro-Goldwyn-Mayer", "VARIOUS","1939", "", 98));
        flicks.add(new Flick("Star Wars", "LucasFilm", "George Lucas", "1977","",93));
        flicks.add(new Flick("Psycho","Paramount", "Alfred Hitchcock", "1960", "", 97));
        flicks.add(new Flick("King Kong", "RKO Pictures", "Merian C. Cooper & Ernest B. Schoedsack", "1933", "", 98));
        flicks.add(new Flick("2001: A Space Odyssey", "Metro-Goldwyn-Mayer", "Stanley Kubrick", "1968", "", 93));
        flicks.add(new Flick("Citizen Kane", "RKO Pictures", "Orson Welles", "1941", "", 100));
        flicks.add(new Flick("Snow White and the Seven Dwarfs", "Walt Disney Pictures", "VARIOUS", "1937", "",98));
        flicks.add(new Flick("Casablanca", "Warner Bros.", "Michael Curtiz", "1942", "", 97));
        flicks.add(new Flick("The Godfather", "Paramount", "Francis Ford Coppola", "1972", "", 98));
        flicks.add(new Flick("Jaws", "Universal", "Steven Spielberg", "1975", "", 97));

        CustomAdapter customAdapter = new CustomAdapter(this, R.layout.custom_layout, flicks);
        listPort.setAdapter(customAdapter);
        listLand.setAdapter(customAdapter);
    }
    public class CustomAdapter extends ArrayAdapter<Flick>
    {
        Context context;
        int resource;
        List<Flick> list;

        public CustomAdapter(@NonNull Context context, int resource, @NonNull List<Flick> objects) {
            super(context, resource, objects);
            this.context=context;
            this.resource=resource;
            list=objects;
        }

        @NonNull
        @Override
        public View getView(int position, @Nullable View convertView, @NonNull ViewGroup parent) {
            LayoutInflater layoutInflater=(LayoutInflater)context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            View adapterView=layoutInflater.inflate(resource,null);


            TextView title = adapterView.findViewById(R.id.title);
            ImageView imageView=adapterView.findViewById(R.id.imageView);
            Button up=adapterView.findViewById(R.id.upButton);
            Button down=adapterView.findViewById(R.id.downButton);

            title.setText(list.get(position).getTitle());

            switch(list.get(position).getStudio())
            {
                case "Warner Bros.":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "Paramount":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "Universal":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "LucasFilm":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "RKO Pictures":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "Walt Disney Pictures":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
                case "Metro-Goldwyn-Mayer":
                    imageView.setImageResource(R.drawable.warnerbros);
                    break;
            }

            return adapterView;
        }
    }
}

Answer 1

猜测：您有两种不同的布局 - 一种用于纵向，一种用于横向。根据设备方向，只有其中一个将通过 findViewById 加载。请记住，当您旋转设备时，将重新创建 activity（并调用 onCreate）。在设置适配器之前，您应该检查 listPort 和 listLand 是否有 null。您没有同时使用两者。

像这样：

if (listPort != null) {
    listPort.setAdapter(customAdapter); // Portrait mode
} else {
    listLand.setAdapter(customAdapter); // Landscape mode
}

其中一个应该是非空的，前提是其他一切正常。

如果正如我猜测的那样，您有一个 values/activity_main.xml 和一个 values-land/activity_main.xml，您可以通过在两个地方使用相同的列表 ID 来让自己的生活更轻松。例如，

public class MainActivity extends AppCompatActivity {
    ListView list;
    ArrayList<Flick> flicks;

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

        list = findViewById(R.id.list);
        flicks = new ArrayList<>();
        flicks.add(new Flick("Wizard of Oz", "Metro-Goldwyn-Mayer", "VARIOUS","1939", "", 98));
        ...
        flicks.add(new Flick("Jaws", "Universal", "Steven Spielberg", "1975", "", 97));

        CustomAdapter customAdapter = new CustomAdapter(this, R.layout.custom_layout, flicks);
        listt.setAdapter(customAdapter);
    }
    ...
}

使用自定义适配器和方向接收 ListView 程序的空指针异常？

Receiving nullpointer exception for ListView program using custom adapter and orientation?

java

android

listview

nullpointerexception

orientation