Java Android SQLite SUM，抛出空指针
所以我正在开发一个应用程序，我刚刚开始学习android版的SQLite，很明显我因为一个问题被困了3天。 每当我试图运行查询来汇总表中月份等于特定月份的字段时，就会抛出null指针Java Android SQLite SUM，抛出空指针,java,android,sqlite,Java,Android,Sqlite,所以我正在开发一个应用程序，我刚刚开始学习android版的SQLite，很明显我因为一个问题被困了3天。 每当我试图运行查询来汇总表中月份等于特定月份的字段时，就会抛出null指针 package com.example.droid; import java.text.SimpleDateFormat; import java.util.Calendar; import com.example.droid.SqDb; import android.accounts.Account; imp
package com.example.droid;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import com.example.droid.SqDb;
import android.accounts.Account;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.text.format.DateFormat;
import android.util.Log;
public class DbCon {
private static SQLiteDatabase database;
private SqDb SqHelper;
private String[] UserColumns =
{
SqDb.ACCOUNT_ID,
SqDb.ACCOUNT_NAME,
SqDb.ACCOUNT_USERNAME,
SqDb.ACCOUNT_PASSWORD,
SqDb.ACCOUNT_AGE,
SqDb.ACCOUNT_CITY,
SqDb.ACCOUNT_COUNTRY
};
private String [] GuestColumns =
{
SqDb.GUEST_ID,
SqDb.GUEST_NAME
};
private String [] TransColumns =
{
SqDb.TRAN_ID,
SqDb.TRAN_DESC,
SqDb.TRAN_TYPE,
SqDb.TRAN_TIME,
SqDb.TRAN_DAY,
SqDb.TRAN_MON,
SqDb.TRAN_YEAR,
SqDb.TRAN_CAT,
SqDb.TRAN_VAL
};
private static DbCon myDCon;
DbCon(Context context)
{
SqHelper = SqDb.getInstance(context);
}
public static DbCon getInstance(Context context)
{
if(myDCon == null)
{
myDCon = new DbCon(context.getApplicationContext());
}
return myDCon;
}
public void open() throws SQLException
{
database = SqHelper.getWritableDatabase();
}
public void close()
{
SqHelper.close();
}
public void createUser(String username, String password, String name, String Age, String City, String Country)
{
ContentValues values = new ContentValues();
values.put(SqDb.ACCOUNT_USERNAME, username);
values.put(SqDb.ACCOUNT_PASSWORD, password);
values.put(SqDb.ACCOUNT_NAME, name);
values.put(SqDb.ACCOUNT_AGE, Age);
values.put(SqDb.ACCOUNT_CITY, City);
values.put(SqDb.ACCOUNT_COUNTRY, Country);
database.insert(SqDb.ACCOUNT, null, values);
}
public void createGuest(String name)
{
ContentValues values = new ContentValues();
values.put(SqDb.GUEST_NAME, name);
database.insert(SqDb.GUEST, null, values);
}
public void AddTask(String desc, String type, String cat, double Value)
{
ContentValues values = new ContentValues();
values.put(SqDb.TRAN_DESC, desc);
values.put(SqDb.TRAN_TYPE, type);
values.put(SqDb.TRAN_CAT, cat);
values.put(SqDb.TRAN_VAL, Value);
SimpleDateFormat df = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
Calendar cal = Calendar.getInstance();
String timedata = df.format(cal.getTime());
String[] whens = timedata.split(" ");
String time = whens[1];
String[] dates = whens[0].split("/");
values.put(SqDb.TRAN_TIME, time);
values.put(SqDb.TRAN_DAY, dates[2]);
values.put(SqDb.TRAN_MON, dates[1]);
values.put(SqDb.TRAN_YEAR, dates[0]);
database.insert(SqDb.TRAN, null, values);
}
Calendar cal = Calendar.getInstance();
public int getMonthInt(){
int month = cal.get(Calendar.MONTH);
return month;
}
public double CountMonthExp()
{
double din = 0;
this.open();
Cursor c = database.rawQuery("SELECT sum("+SqDb.TRAN_VAL+") as"+SqDb.TRAN_VAL+" FROM "+SqDb.TRAN+" WHERE "+SqDb.TRAN_TYPE+"=EXPENDITURE and "+SqDb.TRAN_MON+"=3", null);
if(c.moveToFirst()){
din = c.getDouble(0);
}
else{
din = 0;
}
return din;
}
}
我认为这里至少有两个问题：
SELECT sum(tran_val) astran_val FROM tran WHERE tran_type=EXPENDITURE and tran_mon=3
SELECT sum(tran_val) as tran_val FROM tran WHERE tran_type='EXPENDITURE' and tran_mon=3
我相信您将能够自己更改代码以实现这些调整。在“）之后应该有一个空格作为“谢谢，这为我指明了答案。哦，我的上帝，谢谢，非常感谢！最后我可以继续我的项目了。