正在接收使用自定义适配器和方向的listview程序的nullpointer异常？

这个问题在这里已经有答案了：

什么是nullpointerexception，如何修复它(12个答案）
findviewbyid返回null（29个答案）
两年前关门了。
我正在申请一个 ArrayList 由两个 ListViews 在纵向和横向上。为此，我使用了一个自定义适配器来扩展我的列表视图。但我一直收到一封信 NullPointer Exception 每当我为我的 landscape ListView . 有什么建议吗？引发异常的行：

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;
        }
    }
}

猜测一下：你有两种不同的布局——一种是纵向的，另一种是横向的。只有一个会通过 findViewById ，取决于设备方向。请记住，活动是重新创建的（并且 onCreate 调用）当您旋转设备时。你应该检查一下 listPort 以及 listLand 为了 null 设置适配器之前。你不能同时使用两者。
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，前提是其他一切都正常工作。
我猜，如果你有 values/activity_main.xml 还有一个 values-land/activity_main.xml ，您可以通过在两个位置使用相同的列表id使自己的生活更轻松。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);
    }
    ...
}

正在接收使用自定义适配器和方向的listview程序的nullpointer异常？

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