Question

我有一些数据存储在sqlite数据库的不同表中。例如，我从表名main_category获取Tutorial标题，从表教程获得教程描述，我想在expandable listview中显示它们。但我得到nullpointer异常。这是我从sqlite数据库获取值的代码。

 public List<String> getTutNames() {
    List<String> list = new ArrayList<>();
    Cursor cursor = database.rawQuery("Select cat_name From main_category;", null);
    cursor.moveToFirst();
    while (!cursor.isAfterLast()) {
        list.add(cursor.getString(0));
        cursor.moveToNext();
    }
    cursor.close();
    return list;
}

public List<String> getTutDes() {
    List<String> list = new ArrayList<>();
    Cursor cursor = database.rawQuery("Select tut_detail From tutorials;", null);
    cursor.moveToFirst();
    while (!cursor.isAfterLast()) {
        list.add(cursor.getString(0));
        cursor.moveToNext();
    }
    cursor.close();
    return list;
}

这是我将其放入可扩展列表视图中的代码。

public class Main2Activity extends AppCompatActivity {

ExpandableListAdapter listAdapter;
ExpandableListView elv;
List<String> listDataHeader;
List<String> listChild;
HashMap<String, List<String>> listDataChild;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    elv = (ExpandableListView) findViewById(R.id.elv);

    DatabaseAccess db = DatabaseAccess.getInstance(this);
    db.open();
    listDataHeader = db.getTutNames();
    listChild = db.getTutDes();
    db.close();

    for (int i=0; i<listDataHeader.size();i++) {
        listDataChild.put(listDataHeader.get(i), listChild);
    }

    listAdapter = new ExpandableListAdapter(this, listDataHeader, listDataChild);
    elv.setAdapter(listAdapter);

}

}

Answer 1

试试这个！可能有帮助

在检索数据时检查logcat。

public List<String> getTutNames() {
    List<String> list = new ArrayList<>();
    Cursor cursor = database.rawQuery("Select cat_name From main_category;", null);
    if(cursor.getCount() > 0){
        while (cursor.moveToNext()){
            list.add(cursor.getString(0));
        }
    }
    cursor.moveToFirst();
    return list;
}

public List<String> getTutDes() {
    List<String> list = new ArrayList<>();
    Cursor cursor = database.rawQuery("Select tut_detail From tutorials;", null);
    if(cursor.getCount() > 0) {
        while (cursor.moveToNext()) {
            list.add(cursor.getString(0));
        }
    }
    return list;
}

Answer 2

也许您的查询数据库需要一些时间。您可以在使用时添加synchronized to method和check list null。

从sqlite数据库获取数据并放入可扩展列表？

2 个答案: