SQLitedatabase 更新 Function 找不到行 ID？

Question

I'm trying to update a user's "CashStack" when that user clicks to use their in game currency. So far I've found that the user's ID should be 6 but when I try to use this hard coded value in my database update function I'm getting a null object reference error. Can anyone help me understand why this value is incorrect? I'm working in Android Studio and using SQLite.

Functions being used: public void updateCashStack(int newCashStack){..} (see below), fiveDollar.setOnClickListener(..)

How I'm finding ID number:

int lastID = 0;
for (int i = 0; i < 10; i++) {
    if(userIDs[i] != 0) {
        lastID = userIDs[i];
        Toast.makeText(ChoosePokerStyle.this, "User ID = " + lastID, Toast.LENGTH_LONG).show( );
    }
}

What shows when I run app:

Database Code:

public static final String DATABASE_NAME = "rockfinn.db";
private SQLiteDatabase db;
public static final String TABLE_NAME = "user_data";
public static final String COL1 = "ID";
public static final String COL2 = "NICKNAME";
public static final String COL3 = "CASHSTACK";

public DatabaseHelper(Context context) {super(context, DATABASE_NAME, null, 1);}

@Override
public void onCreate(SQLiteDatabase db) {
    String createTable = "CREATE TABLE " + TABLE_NAME + " (ID INTEGER PRIMARY KEY AUTOINCREMENT, " +
            "NICKNAME TEXT, " +
            "CASHSTACK INT)";
    db.execSQL(createTable);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
}

public boolean addData(String nickname) {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(COL2, nickname);
    contentValues.put(COL3, 1000);

    long result = db.insert(TABLE_NAME, null, contentValues);

    if(result == -1) {
        return false;
    }
    else {
        return true;
    }
}

public void updateCashStack(int newCashStack)
{
    try
    {
        ContentValues values = new ContentValues();
        values.put(COL3, newCashStack);
        String whereClause = "id = ?";
        db.update(TABLE_NAME, values, whereClause, new String[] {"6"});
        //db.setTransactionSuccessful();
    }
    catch (Exception e)
    {
        Log.e("Error in transaction", "Error", e);
    }
    finally
    {
      // db.endTransaction();
    }
}

public Cursor getListContents() {
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor data = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
    return data;
}

}

fiveDollar.setOnClickListener code:

fiveDollar.setOnClickListener(new View.OnClickListener( ) {
    @Override
    public void onClick(View v)
    {
       int five = 5;
       int cashStackAsInt = Integer.parseInt(cashStack);
       cashStackAsInt = cashStackAsInt - five;
       userCashStackPrompt.setText(Integer.toString(cashStackAsInt));
       myDB.updateCashStack(cashStackAsInt);
       Toast.makeText(ChoosePokerStyle.this, "You have bet $5", Toast.LENGTH_LONG).show( );
    }
}

Answer 1

In your table there is no column titled id . It should be ID . Try like below:

whereClause = "ID = ?";

Answer 2

The syntax in SQL is not case sensitive, but in your database define column String COL1 = "ID"; it don't match with String whereClause = "id =?";

You can use String whereClause = COL1 + " =?"; or String whereClause = "ID =?";

Hope this helps!

Answer 3

I'm getting a null object reference error.

The most likely cause is that an in-scope instantiated instance of myDB doesn't exist.

You can check this by placing a breakpoint on the line

db.update(TABLE_NAME, values, whereClause, new String[] {"6"}); 

in DatabaseHelper.java .

Then run in debug mode (eg click green bug)). When debug window appears inspect the variables.

At a guess db is null. If so you need to ensure that myDB has been instantiated and that the call is within the scope of myDB (eg declare myDB at the class level and instantiate it in the onCreate method of an activity or the equivalent for a fragment).

You may wish to refer to Debug your app for more info on Debugging.