需要重新启动应用程序以显示添加到 recyclerView 的第一个项目。后来一切正常。

Question

[我创建了一个小应用程序，其中只包含我面临的问题。下方视频link

问题：我有一个 recyclerView 和一个向 recyclerView 添加项目的按钮。在应用程序首次启动时，recyclerView 是空的。如果我点击按钮一次、两次、三次，什么也没有发生。现在，如果我关闭应用程序并重新启动它，我可以看到这些项目已添加到 recyclerView。现在，如果我再次点击该按钮，项目将按预期添加到 recyclerView 中。这不是那么简单，所以我添加了一个 video 来显示确切的问题。

当我添加第一项时，我在 data.add(0,i); 处收到 nullPointerExeption。为了防止我把它放在 try-catch 里面。

这是一些代码： MainActivity.java

public class MainActivity extends ActionBarActivity {

private RecyclerView recyclerView;
private InfoTodayAdapter adapter;
FileOutputStream fos;
FileInputStream fis;
List<InformationToday> data = new ArrayList<>();

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

    recyclerView = (RecyclerView) findViewById(R.id.recyclerview_today);
    try {
        adapter = new InfoTodayAdapter(this,getData());
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }
    recyclerView.setAdapter(adapter);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
}

public List<InformationToday> getData() throws IOException, ClassNotFoundException {
    fis = openFileInput("arrayListToday");
    ObjectInputStream ois = new ObjectInputStream(fis);
    data = (List<InformationToday>) ois.readObject();
    ois.close();
    fis.close();
    return data;
}

public void addWater(View view) throws IOException {
    InformationToday i = new InformationToday();
    i.volume = "100";
    try {    // I get a nullPointerException if I dont include this try.
        data.add(0,i);
        adapter.notifyItemInserted(0);
    }catch (Exception e){

    }
    fos = openFileOutput("arrayListToday", Context.MODE_PRIVATE);
    ObjectOutputStream oos = new ObjectOutputStream(fos);
    oos.writeObject(data);
    oos.close();
    fos.close();

}
}

InfoTodayAdapter.java（我猜不需要）

public class InfoTodayAdapter extends RecyclerView.Adapter<InfoTodayAdapter.MyViewHolder> {

private LayoutInflater inflater;
List<InformationToday> data2 = Collections.emptyList();

public InfoTodayAdapter(Context context,List<InformationToday> data){
    inflater = LayoutInflater.from(context);
    this.data2 = data;
}

@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = inflater.inflate(R.layout.custom_row_today, parent, false);
    MyViewHolder holder = new MyViewHolder(view);
    return holder;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
    InformationToday current = data2.get(position);
    String s = current.volume;
    holder.volume.setText(s);
}

@Override
public int getItemCount() {
    return data2.size();
}

class MyViewHolder extends RecyclerView.ViewHolder{
    TextView volume;
    public MyViewHolder(View itemView) {
        super(itemView);
        volume = (TextView) itemView.findViewById(R.id.volume);
    }
}
}

Answer 1

经过几个小时的变通，我终于找到了答案。当应用程序首次启动时，它会创建一个 IOexception，因为带有 data 对象的文件不存在。

我所做的只是，在我发现错误的地方，我添加了遗漏的东西。如，刚刚进行了以下更改：

try {
        adapter = new InfoTodayAdapter(this,getData());
    } catch (IOException e) {
        e.printStackTrace();
        data = new ArrayList<>(); // Added this line
        adapter = new InfoTodayAdapter(this,data); // Added this line as well
    }

瞧！而已。

需要重新启动应用程序以显示添加到 recyclerView 的第一个项目。后来一切正常。

Need to restart the app to show the first item added to recyclerView. Later it all works fine.

java

android

nullpointerexception

android-recyclerview