回收器视图中的空对象引用

Question

我创建了按目录显示图片的 recyclerView，但是我在适配器中收到关于 getItemCount 的错误。错误说我的参考是空的。

我该如何解决这个问题？

其实我并没有想到要为此做些什么。

我的适配器（获取项目计数给我错误）

   public class Adaptor extends RecyclerView.Adapter<Adaptor.ViewHolder> {
        private Context context;
        private List<model> list;

        public Adaptor(Context context, List<model> list) {
            this.context = context;
            this.list = list;
        }

        @NonNull
        @Override
        public Adaptor.ViewHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
            View view= LayoutInflater.from(context).inflate(R.layout.imageview,viewGroup,false);
            return new ViewHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull Adaptor.ViewHolder viewHolder, int i) {
            model model=list.get(i);
            Uri uri= Uri.parse(model.getPath());
            File file=new File(String.valueOf(uri));
                Bitmap bitmap= BitmapFactory.decodeFile(file.getAbsolutePath());
                viewHolder.img.setImageBitmap(bitmap);
        }

        @Override
        public int getItemCount() {
            return list.size();
        }
        public class ViewHolder extends RecyclerView.ViewHolder {
            ImageView img;
            public ViewHolder(@NonNull View itemView) {
                super(itemView);
                img=itemView.findViewById(R.id.img_view);
            }
        }

我的数据库

public class database extends SQLiteOpenHelper {
    public database(@Nullable Context context) {
        super(context, "add_image", null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL("CREATE TABLE IF NOT EXISTS ADD_IMAGE(_ID INTEGER PRIMARY KEY AUTOINCREMENT,_PICTURES varchar )");


    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {

    }
    public long insert(String directory){
        SQLiteDatabase db=this.getWritableDatabase();
        ContentValues cv=new ContentValues();
        cv.put("_PICTURES",directory);
        return db.insert("ADD_IMAGE",null,cv);

    }
    public Cursor cursor(){
        SQLiteDatabase DB=this.getReadableDatabase();
        return DB.rawQuery(" select * from ADD_IMAGE",null);
    }
}

我的模型

public class model {
    String path;

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }
}

我的activity

  database database=new database(this);
    SQLiteDatabase sqLiteDatabase;
    RecyclerView rec_view;
    List<model> list;
    Context context;
Button btn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        context = this;
        Adaptor adaptor = new Adaptor(context, list);
        rec_view = findViewById(R.id.rec_image);
        rec_view.setAdapter(adaptor);
        rec_view.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));
        btn = findViewById(R.id.loadimage);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(Intent.ACTION_PICK,
                        android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
                startActivityForResult(intent, 0);
            }
        });


        Cursor cursor = database.cursor();
        for (cursor.moveToFirst(); !cursor.isAfterLast(); cursor.moveToNext()) {
            model model = new model();
            model.setPath(cursor.getString(1));
            list.add(model);
        }
    }
    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
    if (requestCode==RESULT_OK){
        Uri tartgeturi=data.getData();
        String directory=String.valueOf(tartgeturi);
        database.insert(directory);
        }



    }

我希望我的错误得到修正

Answer 1

在你的activity,

而不是

List<model> list;

使用

List<model> list = new ArrayList(); // to initialize the list as empty

Answer 2

1) 首先，你必须在使用它之前初始化你的list。（这就是你得到异常的原因）

2) 还有 set layout manager 到 recycler view 之前 set adapter 如下。

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

        // list initialization
        list = new ArrayList(); 

        Adaptor adaptor = new Adaptor(context, list);
        rec_view = findViewById(R.id.rec_image);

        // alwasy set layout manager before set adapter
        rec_view.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false)); 

        rec_view.setAdapter(adaptor);

        btn = findViewById(R.id.loadimage);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(Intent.ACTION_PICK,
                        android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);
                startActivityForResult(intent, 0);
            }
        });


        Cursor cursor = database.cursor();
        for (cursor.moveToFirst(); !cursor.isAfterLast(); cursor.moveToNext()) {
            model model = new model();
            model.setPath(cursor.getString(1));
            list.add(model);
            // notify your adapter that your data set is changed by adding folling line.
            adapter.notifyDataSetChanged();
        }
    }

希望对你有帮助。

Answer 3

替换此

 @Override
    public int getItemCount() {
        return list.size();
    }

有了这个

 @Override
    public int getItemCount() {
        return list == null ? 0 : list.size();
    }

回收器视图中的空对象引用

null object reference in recycler view

java

android

image

gallery

android-studio