error when using the sum function in sqlite
<p>My application want to calculate the total sales in a month. I used sql statements sum. But when I run the application it fails. and I do not know why this error. expect people to help me fix this. I am very thankful.
Code DB_Adapter</p>

<pre><code>public class DB_Adapter {
public static final String DATE="date";
public static final String COST = "cost";
public static final String SALES = "sales";
public static final String NUMBER = "number";
public static final String PROFIT = "profit";
public static final String DB_NAME = "SAIGONFLEAMARKET";
public static final String DB_TABLE = "shipment";
public static final String MONTH="month";
public static final String YEAR="year";
public static final int DB_VERSION = 2;
private final Context mContext;
private SQLiteDatabase mDB;
private DBHelper mDBHelper;

public DB_Adapter(Context c) {
    mContext = c;
}

private static class DBHelper extends SQLiteOpenHelper
{

    public DBHelper(Context context, String name, CursorFactory factory,
            int version) {
        super(context, name, factory, version);
        // TODO Auto-generated constructor stub
    }

    // Khởi tạo một bảng trong database mang tên Saigonfleamarket
    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        try
        {
            db.execSQL("CREATE TABLE shipment (number integer PRIMARY KEY autoincrement, date text, cost float, sales float, profit float, month text, year text );");
        } catch (SQLException ex)
        {
            ex.printStackTrace();
        }
    }

    // Tạo một database mới
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
        Log.i("DBAdapter", "Updating database...");
        db.execSQL("DROP TABLE IF EXISTS shipment");
        onCreate(db);
    }
}

// Mở một kết nối đến database
public DB_Adapter openDB()
{
    mDBHelper = new DBHelper(mContext, DB_NAME, null, DB_VERSION);
    mDB = mDBHelper.getWritableDatabase();
    return this;
}

// sau khi hoàn thành, đóng database lại
public void closeDB()
{
    mDBHelper.close();
}

public Cursor Sum(String _month, String _year)

{
    String sql="select Sum (profit) from shipment where month ='"+_month+"' AND year='"+ _year+"'";
    Cursor mCursor = mDB.rawQuery(sql, null);
    int test = mCursor.getCount();
    if (test != 0)
    {
        mCursor.moveToFirst();
    }

    return mCursor;
}  
}
</code></pre>

<p>log cat error</p>

<pre><code>01-15 13:49:48.840: E/AndroidRuntime(6953): FATAL EXCEPTION: main
01-15 13:49:48.840: E/AndroidRuntime(6953): java.lang.NullPointerException
01-15 13:49:48.840: E/AndroidRuntime(6953):     at Data.DB_Adapter.Sum(DB_Adapter.java:116)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at com.example.xitinshop.Thongtin.onClick(Thongtin.java:40)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.view.View.performClick(View.java:2485)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.view.View$PerformClick.run(View.java:9080)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.os.Handler.handleCallback(Handler.java:587)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.os.Handler.dispatchMessage(Handler.java:92)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.os.Looper.loop(Looper.java:130)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at android.app.ActivityThread.main(ActivityThread.java:3683)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at java.lang.reflect.Method.invokeNative(Native Method)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at java.lang.reflect.Method.invoke(Method.java:507)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-15 13:49:48.840: E/AndroidRuntime(6953):     at dalvik.system.NativeStart.main(Native Method)
</code></pre>
