Question

我是新手并且在我的SQLite DB更新后尝试允许我的ListView刷新。修改onResume()方法后，我没有收到编译错误。我正在使用SimpleCursorAdapter重新查询，但它不起作用。收到的错误来自logcat，位于下方。请指教......示例帮助最好。

logcat的：

  02-19 21:31:49.933: E/AndroidRuntime(714): java.lang.RuntimeException: Unable to resume activity {com.loginplus.home/com.loginplus.home.LoginList}: java.lang.NullPointerException
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2444)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2472)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1986)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.access$600(ActivityThread.java:123)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.os.Handler.dispatchMessage(Handler.java:99)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.os.Looper.loop(Looper.java:137)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.main(ActivityThread.java:4424)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at java.lang.reflect.Method.invokeNative(Native Method)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at java.lang.reflect.Method.invoke(Method.java:511)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at dalvik.system.NativeStart.main(Native Method)
  02-19 21:31:49.933: E/AndroidRuntime(714): Caused by: java.lang.NullPointerException
  02-19 21:31:49.933: E/AndroidRuntime(714):    at com.loginplus.home.LoginList.onResume(LoginList.java:101)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1154)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.Activity.performResume(Activity.java:4539)
  02-19 21:31:49.933: E/AndroidRuntime(714):    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2434)

的活动：

     public class LoginList extends Activity implements OnClickListener,    OnItemClickListener {

private ListView loginList;
private Button webLogin;

private ListAdapter loginListAdapter;

private ArrayList<LoginDetails> loginArrayList;

List<String> arrayList = new ArrayList<String>();

@Override 
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    arrayList = populateList();
    loginListAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, populateList());
    setContentView(R.layout.login_listview);


    loginList = (ListView)
   findViewById(R.id.loginlist);
    loginList.setOnItemClickListener(this);

    webLogin = (Button)
   findViewById(R.id.button3);
    webLogin.setOnClickListener(this);


}

@Override
public void onClick (View v) {
    Intent webLoginIntent = new Intent (this, LoginPlusActivity.class);
    startActivity(webLoginIntent);

}

public List<String> populateList (){

    List<String> webNameList = new ArrayList<String>();

    dataStore openHelperClass = new dataStore (this);

    SQLiteDatabase sqliteDatabase = openHelperClass.getReadableDatabase();

    Cursor cursor = sqliteDatabase.query(dataStore.TABLE_NAME_INFOTABLE, null, null, null, null, null, dataStore.COLUMN_NAME_SITE, null);

    startManagingCursor(cursor);


    while (cursor.moveToNext()){
    String sName = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_SITE));
    String wUrl = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_ADDRESS));
    String uName = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_USERNAME));
    String pWord = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_PASSWORD));
    String lNotes = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_NOTES));

    LoginDetails lpDetails = new LoginDetails();

        lpDetails.setsName(sName);
        lpDetails.setwUrl(wUrl);
        lpDetails.setuName(uName);
        lpDetails.setpWord(pWord);
        lpDetails.setlNotes(lNotes);

        loginArrayList.add(lpDetails);
        webNameList.add(sName);
}

sqliteDatabase.close();
return webNameList;
}



@Override
protected void onResume() {
    super.onResume();

    loginArrayList.clear();

    arrayList.clear();

    arrayList = populateList();

    dataStore refreshHelper = new dataStore (this);
    SQLiteDatabase sqliteDatabase = refreshHelper.getWritableDatabase();
    Cursor cursor = sqliteDatabase.query(dataStore.TABLE_NAME_INFOTABLE, null, null, null, null, null, dataStore.COLUMN_NAME_SITE, null);
    String[]columns = new String[] { dataStore.COLUMN_NAME_SITE, dataStore.COLUMN_NAME_ADDRESS, dataStore.COLUMN_NAME_USERNAME, dataStore.COLUMN_NAME_PASSWORD, dataStore.COLUMN_NAME_NOTES };
    int[] to = new int[]{R.id.rusName, R.id.ruwUrl, R.id.ruuName, R.id.rupWord, R.id.ruNotes};
    SimpleCursorAdapter loginListAdapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, cursor, columns, to);
    loginListAdapter.notifyDataSetChanged();

}

@Override
public void onItemClick(AdapterView<?> arg0 , View arg1, int arg2, long arg3) {
    Toast.makeText(getApplicationContext(), "Selected ID :" + arg2, Toast.LENGTH_SHORT).show();

    Intent updateDeleteLoginInfo = new Intent (this, UpdateDeleteLoginList.class);



    LoginDetails clickedObject = loginArrayList.get(arg2);

        Bundle loginBundle = new Bundle();
    loginBundle.putString("clickedWebSite",clickedObject.getsName());
    loginBundle.putString("clickedWebAddress",clickedObject.getwUrl());
    loginBundle.putString("clickedUserName",clickedObject.getuName());
    loginBundle.putString("clickedPassWord",clickedObject.getpWord());
    loginBundle.putString("clickedNotes",clickedObject.getlNotes());

    updateDeleteLoginInfo.putExtras(loginBundle);

    startActivityForResult(updateDeleteLoginInfo, 0);       
       }    
          }

RennoDiniro EditResults：

logcat的：

       02-21 23:40:18.419: E/AndroidRuntime(705): FATAL EXCEPTION: main
       02-21 23:40:18.419: E/AndroidRuntime(705): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.loginplus.home/com.loginplus.home.LoginList}: java.lang.NullPointerException
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread.access$600(ActivityThread.java:123)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.os.Handler.dispatchMessage(Handler.java:99)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.os.Looper.loop(Looper.java:137)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread.main(ActivityThread.java:4424)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at java.lang.reflect.Method.invokeNative(Native Method)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at java.lang.reflect.Method.invoke(Method.java:511)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at dalvik.system.NativeStart.main(Native Method)
       02-21 23:40:18.419: E/AndroidRuntime(705): Caused by: java.lang.NullPointerException
       02-21 23:40:18.419: E/AndroidRuntime(705):   at com.loginplus.home.LoginList.populateList(LoginList.java:88)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at com.loginplus.home.LoginList.onCreate(LoginList.java:37)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.Activity.performCreate(Activity.java:4465)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
       02-21 23:40:18.419: E/AndroidRuntime(705):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920)

活动类：

      public class LoginList extends Activity implements OnClickListener,    OnItemClickListener {

      private ListView loginList;
      private Button webLogin;

      private ListAdapter loginListAdapter;

      private ArrayList<LoginDetails> loginArrayList;

      List<String> arrayList = new ArrayList<String>();

      @Override 
      public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);

      loginListAdapter = new ArrayAdapter<String>(this,   android.R.layout.simple_list_item_1, populateList());
      arrayList = populateList(); 
      setContentView(R.layout.login_listview);


      loginList = (ListView)
      findViewById(R.id.loginlist);
      loginList.setOnItemClickListener(this);

      webLogin = (Button)
      findViewById(R.id.button3);
      webLogin.setOnClickListener(this);
      }

      @Override
      public void onClick (View v) {
      Intent webLoginIntent = new Intent (this, LoginPlusActivity.class);
      startActivity(webLoginIntent);
      }

      public List<String> populateList (){

      List<String> webNameList = new ArrayList<String>();

      dataStore openHelperClass = new dataStore (this);

      SQLiteDatabase sqliteDatabase = openHelperClass.getReadableDatabase();

      Cursor cursor = sqliteDatabase.query(dataStore.TABLE_NAME_INFOTABLE, null, null, null, null, null, dataStore.COLUMN_NAME_SITE, null);

      startManagingCursor(cursor);


      while (cursor.moveToNext()){
      String sName = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_SITE));
      String wUrl = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_ADDRESS));
      String uName = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_USERNAME));
      String pWord = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_PASSWORD));
      String lNotes = cursor.getString(cursor.getColumnIndex(dataStore.COLUMN_NAME_NOTES));

    LoginDetails lpDetails = new LoginDetails();

   lpDetails.setsName(sName);
   lpDetails.setwUrl(wUrl);
   lpDetails.setuName(uName);
   lpDetails.setpWord(pWord);
   lpDetails.setlNotes(lNotes);

   loginArrayList.add(lpDetails);
   webNameList.add(sName);
   }

   sqliteDatabase.close();
   return webNameList;
    }

   @Override
   protected void onResume() {
    super.onResume();

    try{
    loginArrayList = new ArrayList<LoginDetails>();
    arrayList = new ArrayList<String>();
    loginArrayList.clear();
    arrayList.clear();

    arrayList = populateList();

    dataStore refreshHelper = new dataStore (this);
    SQLiteDatabase sqliteDatabase = refreshHelper.getWritableDatabase();
    Cursor cursor = sqliteDatabase.query(dataStore.TABLE_NAME_INFOTABLE, null, null, null, null, null, dataStore.COLUMN_NAME_SITE, null);
    String[]columns = new String[] { dataStore.COLUMN_NAME_SITE, dataStore.COLUMN_NAME_ADDRESS, dataStore.COLUMN_NAME_USERNAME, dataStore.COLUMN_NAME_PASSWORD, dataStore.COLUMN_NAME_NOTES };
     int[] to = new int[]{R.id.rusName, R.id.ruwUrl, R.id.ruuName, R.id.rupWord, R.id.ruNotes};
    SimpleCursorAdapter loginListAdapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, cursor, columns, to);
    loginListAdapter.notifyDataSetChanged();
     }catch(Exception e)
    {
   e.printStackTrace();
     }
    }
    @Override
    public void onItemClick(AdapterView<?> arg0 , View arg1, int arg2, long arg3) {
    Toast.makeText(getApplicationContext(), "Selected ID :" + arg2,    Toast.LENGTH_SHORT).show();

    Intent updateDeleteLoginInfo = new Intent (this, UpdateDeleteLoginList.class);



    LoginDetails clickedObject = loginArrayList.get(arg2);

    Bundle loginBundle = new Bundle();
    loginBundle.putString("clickedWebSite",clickedObject.getsName());
    loginBundle.putString("clickedWebAddress",clickedObject.getwUrl());
    loginBundle.putString("clickedUserName",clickedObject.getuName());
    loginBundle.putString("clickedPassWord",clickedObject.getpWord());
    loginBundle.putString("clickedNotes",clickedObject.getlNotes());

    updateDeleteLoginInfo.putExtras(loginBundle);

    startActivityForResult(updateDeleteLoginInfo, 0);   
   }
   }

Answer 1

使用指向所需数据的光标以及布局信息创建适配器。

SimpleCursorAdapter mAdapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, cursor, columns, to);

在您使用R.layout.simple_list_item_1

的情况下

columns将为null或Particular column data which you get from cursor

to将为android.R.id.text1

例如

  Cursor cursor = getContentResolver().query(People.CONTENT_URI, new String[]{People._ID, People.NAME, People.NUMBER}, null, null, null);
  startManagingCursor(cursor);
   // THE DESIRED COLUMNS TO BE BOUND

   String[] columns = new String[] { People.NAME, People.NUMBER };

  // THE XML DEFINED VIEWS WHICH THE DATA WILL BE BOUND TO
   int[] to = new int[] { R.id.name_entry, R.id.number_entry };

       SimpleCursorAdapter mAdapter = new SimpleCursorAdapter(this,R.layout.list_example_entry, cursor, columns, to);

Answer 2

显然你没有初始化ArrayList。但是..对于您的情况，请考虑使用Loader代替。 ApiDemos中有完整的示例（列在Loader页面的底部）。

Answer 3

注意，您永远不会实例化loginArrayList，因此当您尝试在onResume()中访问它时，它会为空，从而崩溃。在访问之前实例化。

更新22/02/2013：

啊，你忘了在应用程序运行时实例化loginArrayList。

在onCreate()中，将以下代码放在super.OnCreate(...)

之后

loginArrayList = new ArrayList<LoginDetails>();

所以看起来像：

loginArrayList = new ArrayList<LoginDetails>();
loginListAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,populateList());
arrayList = populateList(); 
setContentView(R.layout.login_listview);


//rest of your code...

让您的应用程序运行的快速调整如下，但请注意，要解决此问题，需要对整个暂停和恢复过程中丢失的信息进行更深入的分析。

在onResume()内，执行

try{

// your code

}catch(Exception e)
{
 //Have the printStackTrace to the problems see what's going on without crashing.
 //e.printStackTrace();
}

但这还没有解决问题，

在onResume()。执行以下操作：

首次获取数据时，请将其存储到数据库中。
在onResume（）中重新创建列表并使用存储在数据库中的数据进行更新。

所以开始你

@Override
protected void onResume() {
    super.onResume();


loginArrayList = new ArrayList<LoginDetails>();
arrayList = new ArrayList<String>();

arrayList = populateList();

// any other code you require to be done after the list is populated.
}

祝你好运。

Answer 4

似乎游标为空，修复此问题并使应用程序运行的快速方法是更改：

SimpleCursorAdapter mAdapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, cursor,null,null);
mAdapter.notifyDataSetChanged();

到

if (cursor != null) {
   SimpleCursorAdapter mAdapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_1, cursor,null,null);
   mAdapter.notifyDataSetChanged();
}

否则，您的光标似乎为空，您可能会使用错误的投影进行查询。

Answer 5

您需要在SimpleCursorAdapter构造函数中提供从列名到资源ID的映射 - from和to参数，并为其传递null。

Answer 6

问题是托管游标 -

startManagingCursor(cursor);

尝试自己维护游标，而不是使用不推荐使用的“activity managed”游标。

我在管理光标后2天前使用onResume调用解决了完全相同的问题。

Answer 7

您永远不会初始化loginArrayList（因此它是null）。

无法恢复活动

7 个答案: