尝试通过列表视图显示数据时应用程序崩溃

Question

我正在尝试使用 retrofit2 获取数据并使用作为自定义适配器参数传递的列表显示这些数据。当我在 onResponse() 方法中将数据存储在列表中时，在 onResponse() 方法中列表具有一些值。但是在 oncreate() 方法中它给了我 null。虽然，我将 List 声明为全局的。当我运行应用程序有时不显示任何内容，有时应用程序崩溃。我知道这听起来很疯狂。但它发生了。所以，我想知道，我的代码有什么问题？如何在列表视图中显示数据？

如果我的问题模式有问题，请原谅我，这是我在这个网站上的第一个问题。

MainActivity`

public class LaboratoryValues extends AppCompatActivity {

    public List<Data> productList = null;
    List<Data>arrayList = null;
    int size;
    String st;

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

        //productList = new ArrayList<Data>();
        getInvestigation();
        for(int i =0; i < size; i++){

            st = arrayList.get(i).getName();


        }
        System.out.println("Name : "+st);//here print Name : null

        ListView lview = (ListView) findViewById(R.id.listview);
        ListviewAdapter adapter = new ListviewAdapter(this, arrayList);
        lview.setAdapter(adapter);



    }


    private void getInvestigation() {

       /* final ProgressDialog progressDialog = new ProgressDialog(MainActivity.this);
        progressDialog.setCancelable(false); // set cancelable to false
        progressDialog.setMessage("Please Wait"); // set body
        progressDialog.show(); // show progress dialog*/


        ApiInterface apiService =
                Api.getClient(ApiInterface.BASE_URL).create(ApiInterface.class);


        Call<Investigation> investigationCall = apiService.getInvestigation();
        investigationCall.enqueue(new Callback<Investigation>() {
            @Override
            public void onResponse(Call<Investigation> call, Response<Investigation> response) {

                arrayList = response.body().getData();
              //productList.addAll(arrayList);
                size = response.body().getData().size();
                for (int i = 0; i < size; i++) {

                    System.out.println("Name : " + arrayList.get(i).getName());//here printing Name is ok


                }

            }
            @Override
            public void onFailure(Call<Investigation> call, Throwable t) {
                Toast.makeText(getApplicationContext(),"data list is empty",Toast.LENGTH_LONG).show();

            }
        });




    }


}

自定义适配器 (listviewAdapter)

public class ListviewAdapter extends BaseAdapter {


        public List<Data> productList;
        Activity activity;
        //Context mContext;

        public ListviewAdapter(Activity activity, List<Data> productList) {
            super();
            this.activity = activity;
            this.productList = productList;
        }

        @Override
        public int getCount() {
            return productList.size();
        }

        @Override
        public Object getItem(int position) {
            return productList.get(position);
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        private class ViewHolder {
            TextView name;
            TextView normal_finding;
            TextView increased;
            TextView decreased;
            TextView others;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {

            ViewHolder holder;
            LayoutInflater inflater = activity.getLayoutInflater();


            if (convertView == null) {
                convertView = inflater.inflate(R.layout.listview_row, null);
                holder = new ViewHolder();
             holder.name = convertView.findViewById(R.id.name);
             holder.normal_finding =convertView.findViewById(R.id.normal_finding);
             holder.increased = convertView.findViewById(R.id.increased);
             holder.decreased = convertView.findViewById(R.id.decreased);
             holder.others =convertView.findViewById(R.id.others);
             convertView.setTag(holder);
            } else {
                holder = (ViewHolder) convertView.getTag();
            }

            Data item = productList.get(position) ;
            holder.name.setText(item.getName());
            System.out.println("holderName : "+item.getName() );
            holder.normal_finding.setText(item.getNormal_finding());
            System.out.println("holderName : "+item.getNormal_finding() );
            holder.increased.setText(item.getIncreased());
            holder.decreased.setText(item.getDecreased());
            holder.others.setText(item.getOthers());

            return convertView;


        }

    }

Answer 1

它不起作用是完全正常的。将您的方法 getInvistigation() 放在循环之前并不意味着您的请求响应已完成调用网络服务会创建另一个等待服务器发送响应的线程，有时响应需要时间取决于服务器和您的互联网连接的延迟。

你只需要在获取数据后将处理（循环和适配器）放在 getInvistagion 中。

尝试通过列表视图显示数据时应用程序崩溃

App get crashed when trying to display Data through listview

android

list

listview

retrofit2

custom-adapter