如何从Android数据库中获取数据？由光标c = db.query？

这是myDB类 //请看公共光标getplaces（）---我是对还是错？以及如何在主Activity类中编码，如果我想要setListAdapter。感谢您的帮助。如何从Android数据库中获取数据？由光标c = db.query？

 
public class MyDB { 
    protected static final String TAG = "TAG"; 

    private final Context mContext; 
    private SQLiteDatabase mDb; 
    private MyDBhelper mDbHelper; 
public MyDB(Context context){ 
    this.mContext = context; 
    mDbHelper = new MyDBhelper(mContext); 
} 

public MyDB createDatabase() throws SQLException 
{ 
    try 
    { 
     mDbHelper.createDataBase(); 
    } 
    catch (IOException mIOException) 
    { 
     Log.e(TAG, mIOException.toString() + " UnableToCreateDatabase"); 
     throw new Error("UnableToCreateDatabase"); 
    } 
    return this; 
} 

public MyDB open() throws SQLException 
{ 
    try 
    { 
     mDbHelper.openDataBase(); 
     mDbHelper.close(); 
     mDb = mDbHelper.getReadableDatabase(); 
    } 
    catch (SQLException mSQLException) 
    { 
     Log.e(TAG, mSQLException.toString()); 
     throw mSQLException; 
    } 
    return this; 
} 


    public Cursor getplaces() 
    { 
     Cursor c = mDb.query(Constants.DATABASE_NAME, null, null, null, null, null, null); 
     return c; 

    } 

    public void close() 
    { 
     mDbHelper.close(); 
    } 

}

ListplaceActivity类

 
public ListplaceActivity extends ListActivity { 



    private static MyDB mDbHelper; 
    String[] from = new String[] { Constants.TITLE_NAME }; 
    int[] to = new int[] {R.id.place_title}; 


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


     mDbHelper = new MyDB(this); 
     Cursor c = mDbHelper.getplaces(); 
     setListAdapter(new SimpleCursorAdapter(this, 
        R.layout.list_place, c, 
        from, to)); 

     final ListView lv = getListView(); 
lv.setOnItemClickListener(new OnItemClickListener() { 
      public void onItemClick(AdapterView parent, View view,int position, long id) { 

       Intent newActivity = new Intent(view.getContext(), Place.class);  
       startActivity(newActivity); 

      } 
      }); 
    } 

}

在logcat中显示这样之后我点击进入ListplacesActivity页它发生 “意外停止”，并没有表现出任何名单：

 
05-30 21:49:21.543: INFO/Process(17605): Sending signal. PID: 17605 SIG: 9 
05-30 21:49:21.583: INFO/ActivityManager(59): Process com.ctg (pid 17605) has died. 
05-30 21:49:21.593: INFO/WindowManager(59): WIN DEATH: Window{44de90a0 com.ctg/com.ctg.CTG paused=false} 
05-30 21:49:21.674: INFO/ActivityManager(59): Start proc com.ctg for activity com.ctg/.CTG: pid=27207 uid=10028 gids={} 
05-30 21:49:21.733: ERROR/gralloc(59): [unregister] handle 0x5fe9d8 still locked (state=40000001) 
05-30 21:49:22.103: DEBUG/ddm-heap(27207): Got feature list request 
05-30 21:49:22.265: INFO/UsageStats(59): Unexpected resume of com.ctg while already resumed in com.ctg 
05-30 21:49:23.084: DEBUG/dalvikvm(27207): GC freed 801 objects/58520 bytes in 216ms 
05-30 21:49:23.433: DEBUG/dalvikvm(27207): GC freed 93 objects/3632 bytes in 129ms 
05-30 21:49:23.623: WARN/PhoneWindow(27207): Previously focused view reported id 16908298 during save, but can't be found during restore. 
05-30 21:49:23.843: WARN/InputManagerService(59): Got RemoteException sending setActive(false) notification to pid 17605 uid 10028 
05-30 21:49:24.103: INFO/ActivityManager(59): Displayed activity com.ctg/.CTG: 2489 ms (total 33589 ms) 
05-30 21:49:29.013: INFO/ActivityManager(59): Starting activity: Intent { cmp=com.ctg/.AttractionsListActivity } 
05-30 21:49:29.085: DEBUG/AndroidRuntime(27207): Shutting down VM 
05-30 21:49:29.085: WARN/dalvikvm(27207): threadid=3: thread exiting with uncaught exception (group=0x4001b188) 
05-30 21:49:29.094: ERROR/AndroidRuntime(27207): Uncaught handler: thread main exiting due to uncaught exception 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.ctg/com.ctg.AttractionsListActivity}: java.lang.NullPointerException 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread.access$2200(ActivityThread.java:119) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.os.Handler.dispatchMessage(Handler.java:99) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.os.Looper.loop(Looper.java:123) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread.main(ActivityThread.java:4363) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at java.lang.reflect.Method.invokeNative(Native Method) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at java.lang.reflect.Method.invoke(Method.java:521) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at dalvik.system.NativeStart.main(Native Method) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207): Caused by: java.lang.NullPointerException 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at com.ctg.data.MyDB.getplaces(MyDB.java:82) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at com.ctg.AttractionsListActivity.onCreate(AttractionsListActivity.java:32) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459) 
05-30 21:49:29.114: ERROR/AndroidRuntime(27207):  ... 11 more 
05-30 21:49:29.144: INFO/Process(59): Sending signal. PID: 27207 SIG: 3 
05-30 21:49:29.144: INFO/dalvikvm(27207): threadid=7: reacting to signal 3 
05-30 21:49:29.174: INFO/dalvikvm(27207): Wrote stack trace to '/data/anr/traces.txt' 
05-30 21:49:39.039: WARN/ActivityManager(59): Launch timeout has expired, giving up wake lock! 
05-30 21:49:39.057: WARN/ActivityManager(59): Activity idle timeout for HistoryRecord{44eae320 com.ctg/.AttractionsListActivity}

来源

2011-05-30 April Smith

你的问题是什么？你期望看到什么（我猜想有地方XD的列表）？你看到了什么呢？如果崩溃，请粘贴logcat输出。 – Cristian 2011-05-30 14:51:20

我不知道为什么它崩溃..请帮我 – 2011-05-30 14:55:33

它崩溃，因为NullPointerException ...如果你做什么dmon说它会解决你的问题^ _^ – Cristian 2011-05-30 15:00:04

您没有拨打MDbHelper.open()，因此getPlaces()中的变量mDb为空。

来源

2011-05-30 14:56:22 dmon

可以请你修复它在我的代码..我是对开发来说很新颖。 – 2011-05-30 14:59:46

就像我说的，在你的'onCreate'中调用'mDbHelper.getPlaces（）'之前加'mDbHelper.open（）'*。 – dmon 2011-05-30 15:02:07

如果是这样，我已经完成但仍然崩溃 – 2011-05-30 15:06:37

如何从Android数据库中获取数据？由光标c = db.query？

回答

相关问题