Question

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();


}

}

如果详细解释我会是一个完整的菜鸟会有帮助，我确实试图找到解决方案，但没有找到任何。提前谢谢

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。

Answer 1

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

<强>原始
我猜你有一个NullPointerException。这里：

c.moveToFirst();

在使用c初始化之前，您尝试使用db.rawQuery()。

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();

请改为尝试：

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，也听从了他的意见。

Answer 2

您应该将字符串值包装在单引号中。所以代替：

select sum(amount) from transaction_table where category = Salary ;

做的：

select sum(amount) from transaction_table where category = 'Salary';

在android中的sqlite中求和

2 个答案: