在android中的sqlite中求和

我是 java,android和sqlite的新手,我坚持这个.
我有列_id,type,amount(int),category,description.我正在尝试获取特定类别的金额总和并将它们存储在int中.
我知道我需要提供错误,但它只是给我力量关闭.请帮助

public class TransactionReport extends Activity {
private static sqliteDatabase db;
Cursor c;
int amount;
DbCrud localDbCrud = new DbCrud(this);

@Override
protected void onCreate(Bundle savedInstanceState) {
    // Todo Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.report);
    localDbCrud.open();                //localDatabase = localDbHelper.getWritableDatabase();  return this;
    c.movetoFirst();
    do{
        c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;",null);


      }while(c.movetoNext());

    amount = c.getInt(0);
    c.close();
    TextView ltxt = (TextView) findViewById(R.id.amt);
    ltxt.setText(""+amount);
    localDbCrud.close();  //localDatabase.close();


}

}

如果详细解释,因为我是一个完整的菜鸟,我会尝试找到解决方案,但没有找到任何东西会有所帮助.谢谢提前

10-10 03:55:29.828: E/AndroidRuntime(10229): FATAL EXCEPTION: main
10-10 03:55:29.828: E/AndroidRuntime(10229): java.lang.RuntimeException: Unable to      start activity   ComponentInfo{com.hishighness.budgetracker/com.hishighness.budgetracker.TransactionReport}:     java.lang.NullPointerException
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:1651)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at  android.app.ActivityThread.access$1500(ActivityThread.java:117)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.os.Handler.dispatchMessage(Handler.java:99)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.os.Looper.loop(Looper.java:130)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.main(ActivityThread.java:3687)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at java.lang.reflect.Method.invokeNative(Native Method)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at java.lang.reflect.Method.invoke(Method.java:507)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at dalvik.system.NativeStart.main(Native Method)
10-10 03:55:29.828: E/AndroidRuntime(10229): Caused by: java.lang.NullPointerException
10-10 03:55:29.828: E/AndroidRuntime(10229):    at com.hishighness.budgetracker.TransactionReport.onCreate(TransactionReport.java:27)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
10-10 03:55:29.828: E/AndroidRuntime(10229):    at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:1615)
10-10 03:55:29.828: E/AndroidRuntime(10229):    ... 11 more

这是日志.

解决方法

加成
我相信你想使用localDBCrud.rawQuery(),而不是db.rawQuery().

原版的
我猜你有一个NullPointerException.这里：

c.movetoFirst();

在使用db.rawQuery()初始化之前,您尝试使用c.

这里有一些逻辑错误：

c.movetoFirst();
do{
    c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;",null);
  }while(c.movetoNext());

amount = c.getInt(0);
c.close();

试试这个：

c = db.rawQuery("select sum(amount) from transaction_table where category = Salary ;",null);
if(c.movetoFirst())
    amount = c.getInt(0);
else
    amount = -1;
c.close();

@toadsky也做了很好的观察,也听从了他的建议.