Question

我基本上是在Android Studio（完整的初学者）上制作预算应用程序，使用显示所有购买内容的ListView等。对于ListView，我使用了ArrayList，其中Purchase是带有购买名称，价格和日期的类。

为了在关闭应用程序时保持对应用程序的购买，我使用了一个类来编写数据并在打开时读取数据。当我的ArrayList为空时，该应用程序运行良好，但是当我的列表不为空时，该应用程序立即崩溃，并使用NPE崩溃“ FATAL EXCEPTION：main ...”：

无法启动活动ComponentInfo {com.rhstudio.budgetrescue / com.rhstudio.budgetrescue.SecondActivity}：java.lang.NullPointerException：尝试在null上调用接口方法'int java.util.List.size（）'对象参考原因：java.lang.NullPointerException：尝试在android.widget上的android.widget.ArrayAdapter.getCount（ArrayAdapter.java:344）上的空对象引用上调用接口方法'int java.util.List.size（）' .com.rhstudio.budgetrescue.SecondActivity.onCreate（SecondActivity.java:44）上的.ListView.setAdapter（ListView.java:493）

line44是这个：

itemsListView.setAdapter(adapter);

下面没有完整的代码，但我认为多数

任何帮助将不胜感激

public class SecondActivity extends AppCompatActivity implements CustomPopUp.CustomPopUpListener, AdapterView.OnItemClickListener {
    private SharedPreferences sharedPref = null;
    private ListView itemsListView;
    private static ArrayList<Purchase> items;
    private PurchaseListAdapter adapter;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        itemsListView = (ListView) findViewById(R.id.items_list);

        sharedPref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());

        items = FileHelper.readData(this);
        adapter = new PurchaseListAdapter(this, R.layout.adapter_view_layout, items);
        itemsListView.setAdapter(adapter);

        itemsListView.setOnItemClickListener(this);
    }


    public void applyChanges(String name, String currentDate, Float value) {
        Purchase purchase = new Purchase(name, currentDate, value);
        items.add(purchase);
        FileHelper.writeData(items, this);
        adapter = new PurchaseListAdapter(this, R.layout.adapter_view_layout, items);
        itemsListView.setAdapter(adapter);
    }


    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        items.remove(position);
        adapter.notifyDataSetChanged();
        FileHelper.writeData(items, this);
        Toast.makeText(this, "Delete", Toast.LENGTH_SHORT).show();
    }
}




public class FileHelper {
    public static final String FILENAME = "listinfo.dat";

    public static void writeData(ArrayList<Purchase> items, Context context) {
        try {
            FileOutputStream fos = context.openFileOutput(FILENAME, Context.MODE_PRIVATE);
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(items);
            oos.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static ArrayList<Purchase> readData(Context context) {
        ArrayList<Purchase> itemsList = null;
        try {
            FileInputStream fis = context.openFileInput(FILENAME);
            ObjectInputStream ois = new ObjectInputStream(fis);
            itemsList = (ArrayList<Purchase>) ois.readObject();
        } catch (FileNotFoundException e) {
            itemsList = new ArrayList<>();

            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return itemsList;
    }
}


public class PurchaseListAdapter extends ArrayAdapter<Purchase> {
private static final String TAG ="PurchaseListAdapter";

private Context mContext;
private int mResource;

public PurchaseListAdapter(Context context, int resource, ArrayList<Purchase> items) {
    super(context, resource, items);
    mContext = context;
    mResource = resource;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    String name = getItem(position).getName();
    String currentDate = getItem(position).getCurrentDate();
    Float value = getItem(position).getValue();
    String sValue = Float.toString(value);

    Purchase purchase = new Purchase(name, currentDate, value);

    LayoutInflater inflater = LayoutInflater.from(mContext);
    convertView = inflater.inflate(mResource, parent, false);

    TextView tvName = (TextView) convertView.findViewById(R.id.textView25);
    TextView tvCurrentDate = (TextView) convertView.findViewById(R.id.textView26);
    TextView tvValue = (TextView) convertView.findViewById(R.id.textView27);

    tvName.setText(name);
    tvCurrentDate.setText(currentDate);
    tvValue.setText(sValue + "€");

    return convertView;
}

}

当我的ArrayList不为空时，我的应用程序不断崩溃

0 个答案: