java - 当我打开模块时它崩溃

Question

我正在创建 android 项目，当我打开 foodlist 模块时，它应该显示带有描述的数据图像，但它崩溃了，logcat 显示

由以下原因引起:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“android.database.Cursor com.example.pointofsale.DatabaseHelper.getItemsdata(java.lang.String)”
在 com.example.pointofsale.FoodList.onCreate(FoodList.java:31)

如何解决这个问题？

public class FoodList extends AppCompatActivity {

    GridView gridView;
    ArrayList<Food> list;
    FoodListAdapter adapter = null;

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

        gridView = (GridView) findViewById(R.id.foodgridviewID);
        list = new ArrayList<>();
        adapter = new FoodListAdapter(this, R.layout.food_items,list);
        gridView.setAdapter(adapter);

        //get data from database

        Cursor cursor = Add_Items.databaseHelper.getItemsdata("SELECT * FROM ITEMS");
        list.clear();
        while (cursor.moveToNext())
        {
            int id  = cursor.getInt(0);
            String name = cursor.getString(1);
            String price = cursor.getString(2);
            byte[] image = cursor.getBlob(3);

            list.add(new Food(name, price, image, id));
        }

        adapter.notifyDataSetChanged();
        if (list.size() == 0)
        {
            Toast.makeText(this,"No record found", Toast.LENGTH_SHORT).show();
        }
    }
}

//数据库

public Cursor getItemsdata (String sql)
{
    SQLiteDatabase db = getReadableDatabase();
    return db.rawQuery(sql, null);
}

Answer 1

在您的代码中，Add_Items.databaseHelper 为 null。

确保您初始化了 Add_Items.databaseHelper 的实例。