0

这是从数据库适配器类中的函数获取所有类别的代码：

public List<CategoryClass> getAllCategory()
    {
        List<CategoryClass> getCategoryList = new ArrayList<CategoryClass>();
        dataAdapter = dataAdapter.openToWrite();
        Cursor cursor = db.rawQuery("SELECT * FROM " + tb_Name,null);
        if (cursor.moveToFirst()) {
            do {
                CategoryClass categoryInstance = new CategoryClass();
                categoryInstance.setCategory(cursor.getString(1));

                // Adding contact to list
                getCategoryList.add(categoryInstance);
                cursor.close();
            }
            while (cursor.moveToNext());
        }
        Log.d("retrieving all categories","count < 0");

        return getCategoryList;


    }

现在需要在活动启动时在列表视图中显示值我的应用程序应该将 db 的所有值显示到列表视图中，这里是代码：

public class Categories extends Activity

{
    DatabaseAdapter dataAdapterInstance;

    @Override
    public void onCreate(Bundle savedInstanceState)

    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.categories_list);
        ListView list = (ListView) findViewById(R.id.listView);
        dataAdapterInstance = new DatabaseAdapter(this);
        dataAdapterInstance = dataAdapterInstance.openToWrite();
        List<CategoryClass> category = dataAdapterInstance.getAllCategory();

        for (CategoryClass cn : category) {

            cn.getCategory();
            category.add(cn);
            ArrayAdapter<CategoryClass> arrayAdapter = new ArrayAdapter<CategoryClass>(this,android.R.layout.simple_list_item_1,category);


            list.setAdapter(arrayAdapter);
            arrayAdapter.notifyDataSetChanged();

         }
}
}

但是在尝试单击按钮并打开包含上述代码的此活动时出现空指针异常：请让我知道如何解决此问题 这里是堆栈跟踪：

06-11 20:46:09.531: W/System.err(17223): java.lang.NullPointerException
06-11 20:46:09.531: W/System.err(17223):    at com.example.todolist.DatabaseAdapter.getAllCategory(DatabaseAdapter.java:88)
06-11 20:46:09.531: W/System.err(17223):    at com.example.todolist.Categories$1.onClick(Categories.java:40)
06-11 20:46:09.531: W/System.err(17223):    at android.view.View.performClick(View.java:3558)
06-11 20:46:09.531: W/System.err(17223):    at android.view.View$PerformClick.run(View.java:14152)
06-11 20:46:09.531: W/System.err(17223):    at android.os.Handler.handleCallback(Handler.java:605)
06-11 20:46:09.531: W/System.err(17223):    at android.os.Handler.dispatchMessage(Handler.java:92)
06-11 20:46:09.531: W/System.err(17223):    at android.os.Looper.loop(Looper.java:137)
06-11 20:46:09.531: W/System.err(17223):    at android.app.ActivityThread.main(ActivityThread.java:4514)
06-11 20:46:09.531: W/System.err(17223):    at java.lang.reflect.Method.invokeNative(Native Method)
06-11 20:46:09.531: W/System.err(17223):    at java.lang.reflect.Method.invoke(Method.java:511)
06-11 20:46:09.531: W/System.err(17223):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:790)
06-11 20:46:09.531: W/System.err(17223):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
06-11 20:46:09.531: W/System.err(17223):    at dalvik.system.NativeStart.main(Native Method)
4

2 回答 2

1 
public List<CategoryClass> getAllCategory()
{
      List<CategoryClass> getCategoryList = new ArrayList<CategoryClass>();
      dataAdapter = dataAdapter.openToWrite();
      Cursor cursor = db.rawQuery("SELECT * FROM " + tb_Name,null);
      if(cursor!=null){
             cursor.moveToFirst(); 
             while(!cursor.isAfterLast()){
                 CategoryClass categoryInstance = new CategoryClass();
                 categoryInstance.setCategory(cursor.getString(1));

            // Adding contact to list
                 getCategoryList.add(categoryInstance);

                 cursor.moveToNext();
              }
           }

       Log.d("retrieving all categories","count < 0");

       return getCategoryList;
 }
于 2013-06-11T11:15:09.200 回答
0

请发布您的 logcat 错误...而且我认为您不需要 foreach 循环，您可以直接将列表传递给适配器..

DatabaseAdapter dataAdapterInstance;

@Override
public void onCreate(Bundle savedInstanceState)

{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.categories_list);
    ListView list = (ListView) findViewById(R.id.listView);
    dataAdapterInstance = new DatabaseAdapter(this);
    dataAdapterInstance = dataAdapterInstance.openToWrite();
    List<CategoryClass> category = dataAdapterInstance.getAllCategory();
    ArrayAdapter<CategoryClass> arrayAdapter = new ArrayAdapter<CategoryClass>(this,android.R.layout.simple_list_item_1,category);
    arrayAdapter.notifyDataSetChanged();
    list.setAdapter(arrayAdapter);}
于 2013-06-11T10:27:23.863 回答