ListView.setAdapter(android.widget.ListAdapter)' 在空对象引用上

Question

我想要一个列表视图。但我遇到了错误。

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.photoapp/com.example.photoapp.ViewAdmins}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

这是我的代码。

 public class ViewAdmins extends AppCompatActivity {

    SyncAdminsDatabaseHelper mSyncAdminsDatabaseHelper;
    private ListView ymListView;

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

        ymListView = (ListView) findViewById(R.id.listviewadmins);
        mSyncAdminsDatabaseHelper = new SyncAdminsDatabaseHelper(this);

        //call method populateListView();
        populateListView();

    }
    private void populateListView(){

        //get the data to append in the list view
        Cursor data = mSyncAdminsDatabaseHelper.getData();

        if(data.getCount() == 0){
            toastMessage("The database is empty.");
        }else{

            //Create an Array list
            ArrayList<String> listData = new ArrayList<>();

            //Store the result in array using while loop
            while (data.moveToNext()){
                //get the value from the database in column 1
                //than add to array list
                listData.add(data.getString(0));

            }
            System.out.println(listData);
//        //create the list adapter and set the adapter
            ListAdapter adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1,listData);
           ymListView.setAdapter(adapter);


        }


    }

我想将应用表中的数据显示到列表视图。但由于某种原因，我的列表视图没有正确呈现数据。

我的问题是如何从 SQLite 表中获取数据并显示在列表视图中。

以及如何在列表视图中显示多个字段。

Activity_view_admins.xml 代码

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ViewAdmins">


    <ListView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/listviewadmins">

    </ListView>
</LinearLayout>

Answer 1

这是我几年前所做的，但不是从 SQL lite 读取，而是从我的 web 服务返回读取。但它应该让您了解如何填充列表视图。

   public class MainActivity extends AppCompatActivity {
    private static final String url = "http://10.0.2.2/android_app/urltomywebservice";
    private ProgressDialog dialog;
    private List<Item> array = new ArrayList<Item>();
    private ListView listView;
    private Adapter adapter;

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

        listView = (ListView) findViewById(R.id.list_item);
        adapter=new Adapter(this,array);
        listView.setAdapter(adapter);

        dialog=new ProgressDialog(this);
        dialog.setMessage("Loading...");
        dialog.show();

        //Creat volley request obj
        JsonArrayRequest jsonArrayRequest=new JsonArrayRequest(url, new Response.Listener<JSONArray>() {
            @Override
            public void onResponse(JSONArray response) {
                hideDialog();
                //parsing json
                for(int i=0;i<response.length();i++){
                    try{
                        JSONObject obj=response.getJSONObject(i);
                        Item item=new Item();
                        item.setTitle(obj.getString("Title"));
                        item.setImage(obj.getString("Image"));
                        item.setRate(obj.getInt("rating"));
                        item.setPrice(obj.getInt("Price"));



                        //add to array
                        array.add(item);
                    }catch(JSONException ex){
                        ex.printStackTrace();
                    }
                }
                adapter.notifyDataSetChanged();
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {

            }
        });
        AppController.getmInstance().addToRequesQueue(jsonArrayRequest);
    }
    public void hideDialog(){
        if(dialog !=null){
            dialog.dismiss();
            dialog=null;
        }
    }

    }

这里添加到列表视图的数组是我的模型用于设置和获取所需数据的列表。如果适合您，您可以创建类似的东西。

private List<Item> array = new ArrayList<Item>();