[RESOLVED] Properties, Enumeration, & Constructors

Im doing step by step..My instructions are:


The GroceryItem class should contain the following public properties:

1. ScanNumber - Represents the unique serial code for the item on the shelf. This property should be read-only.

2. BrandName - The name as described on the item’s packaging

3. Description - A short description of the item

4. Price - The amount of money it costs to buy the item. Make sure that only positive values can be assigned to this property.

5. Aisle - This should indicate one of the following aisles: Bakery, CannedGoods, Drinks, Deli, DryGoods, FrozenFoods, and Produce.

This is my Grocery Item code:

Code:

---

Public Class GroceryItem
  Public ReadOnly Property ScanNumber As Integer
Get
  Return ScanNumber
End Get
    End Property

    Public Property BrandName As String
 Get
  Return BrandName
 End Get
 Set (value As String)
  BrandName = value
 End Set
End Property

    Public Property Description As String
Get
Return Description
End Get
Set (value As String)
 Description = value
End Set
End Property

    Public Property Price As Integer
Get
Return Price
End Get
Set (value As Integer)
Price = value
End Set
End Property

    Public Property Aisle As String
Get
Return Aisle As String
End Get
Set (value As String)
Aisle = value
End Set
End Property
End Class

---

It says you can use enumeration.

The GroceryItem class should contain the following public constructors:

1. A constructor that accepts and sets only the ScanNumber property. Remember: If a property is read-only, then you’ll need to set the variable, not use the property name.

2. A constructor that accepts and sets the ScanNumber, BrandName and Price properties.

Add a new public class named GroceryBasket to the project.

1. Derive from the generic List class.
2. Ensure that only GroceryItem objects are stored as items.

Add the following variable declaration to the Main module: Friend basket As New GroceryBasket

My Main Code:

Code:

---

Module Main
    Friend blnLoggedIn As Boolean
    Dim arrUsernames() As String = {"Admin", "Clerk", "Manager"}
    Dim arrPasswords() As String = {"P@ssword", "pa$$word", "passw0rd"}
    Dim userIndex As Integer
  Friend basket As New GroceryBasket

---

Test your work.

Note: You can add code to the btnLogin_Click event handler procedure to instantiate the GroceryItem class and add multiple objects to the GroceryBasket variable. Make sure you either remove or comment out this code after testing.

You've got a spurious End Class in the middle of your GroceryItem class. That will certainly mess things up.

Also, while you may be able to get away with assuming the default for a property is Public, it isn't a sound practice, so I agree with Sitten in the other thread that you should explicitly state Public, even if it is the default.

Most likely, the point was not to have you use the default implementation of the properties, anyways, but to spell them out. Therefore, you need variables for each of the properties. Sitten showed that using underscores, but I personally hate underscores because of where they are located on the keyboard (two rows up on the weakest finger). Therefore, I would write one of the properties like this:

Code:

---

Private mBrandName as String

Public Property BrandName As String
 Get
  Return mBrandName
 End Get
 Set (value As String)
  mBrandName = value
 End Set
End Property

---

Much of that gets filled out as soon as you type the Get.

As for constructors, that's just Public Sub New(), with whatever arguments you need. Looks like you need two of them: One that takes one argument and one that takes three.

Quote:

---

Originally Posted by Shaggy Hiker

You've got a spurious End Class in the middle of your GroceryItem class. That will certainly mess things up.

Also, while you may be able to get away with assuming the default for a property is Public, it isn't a sound practice, so I agree with Sitten in the other thread that you should explicitly state Public, even if it is the default.

Most likely, the point was not to have you use the default implementation of the properties, anyways, but to spell them out. Therefore, you need variables for each of the properties. Sitten showed that using underscores, but I personally hate underscores because of where they are located on the keyboard (two rows up on the weakest finger). Therefore, I would write one of the properties like this:

Code:

---

Private mBrandName as String

Public Property BrandName As String
 Get
  Return mBrandName
 End Get
 Set (value As String)
  mBrandName = value
 End Set
End Property

---

Much of that gets filled out as soon as you type the Get.

As for constructors, that's just Public Sub New(), with whatever arguments you need. Looks like you need two of them: One that takes one argument and one that takes three.

---

Where did mBrandName come from??

My own head.

Prior to 2010, that was the only way you could write a property. You had to first declare a variable, then expose that variable via the property, so you had to write it like that. You wrote the variable, then wrote the getter and setter. There were no shortcuts.

Starting with 2010, you could make a shortcut if you wanted a full property (as opposed to ReadOnly or WriteOnly). What you wrote was essentially the shortcut, except that you HAD to have Public/Private/Protected, or Friend as an access qualifier. You STILL had to write out ReadOnly properties, though.

Starting sometime later, perhaps 2013 or 2015, VS was changed such that you didn't have to write out ReadOnly properties, either, which is what you are showing with your properties. There is still a variable hidden behind those properties, you just don't see it anymore because there isn't an explicit Get or Set block. You can't always use that simpler version of properties, though, so you can still write it out the old way, as I showed, because you HAVE to do that if you are doing more with either the Get or Set block.

For example, I might have a class with a Count property. There isn't some variable back there holding the count, the Get block might look like this:

Code:

---

Get
  Return someCollection.Count
End Get

---

The property isn't just returning some value it is holding, it will check how many items are in someCollection, and return that number whenever the property is accessed. Also, there wouldn't be any point to a Set block in such a property, because what would that even mean?

Another example is a class I have where you can set the current time. In the setter, I check to see whether the time being set is the current time, or some other time, and raise various events in either case. In other words, when the user sets the time, the class sends out alerts to other parts of the program to let them know that they need to do something. The default property wouldn't know to do that, so I had to go old-school and write my own Set method.

Quote:

---

Originally Posted by Shaggy Hiker

My own head.

Prior to 2010, that was the only way you could write a property. You had to first declare a variable, then expose that variable via the property, so you had to write it like that. You wrote the variable, then wrote the getter and setter. There were no shortcuts.

Starting with 2010, you could make a shortcut if you wanted a full property (as opposed to ReadOnly or WriteOnly). What you wrote was essentially the shortcut, except that you HAD to have Public/Private/Protected, or Friend as an access qualifier. You STILL had to write out ReadOnly properties, though.

Starting sometime later, perhaps 2013 or 2015, VS was changed such that you didn't have to write out ReadOnly properties, either, which is what you are showing with your properties. There is still a variable hidden behind those properties, you just don't see it anymore because there isn't an explicit Get or Set block. You can't always use that simpler version of properties, though, so you can still write it out the old way, as I showed, because you HAVE to do that if you are doing more with either the Get or Set block.

For example, I might have a class with a Count property. There isn't some variable back there holding the count, the Get block might look like this:

Code:

---

Get
  Return someCollection.Count
End Get

---

The property isn't just returning some value it is holding, it will check how many items are in someCollection, and return that number whenever the property is accessed. Also, there wouldn't be any point to a Set block in such a property, because what would that even mean?

Another example is a class I have where you can set the current time. In the setter, I check to see whether the time being set is the current time, or some other time, and raise various events in either case. In other words, when the user sets the time, the class sends out alerts to other parts of the program to let them know that they need to do something. The default property wouldn't know to do that, so I had to go old-school and write my own Set method.

---

Ok I changed it up top..Is that right so far?

Yes, that'll do. It isn't clear whether you HAVE to write out the properties in the old-school style, but it does seem like the professor is likely looking for that....or not, I'm not sure. This may just be my bias. I almost always write out properties, and have only started using the shorthand way that you had initially in some newer stuff. I always write out ReadOnly properties in all cases. It doesn't hurt anything...unless you are getting graded on it. You'd have to decide whether the professor wants to see the long version of the properties, or doesn't care which you use.

Quote:

---

Originally Posted by Shaggy Hiker

Yes, that'll do. It isn't clear whether you HAVE to write out the properties in the old-school style, but it does seem like the professor is likely looking for that....or not, I'm not sure. This may just be my bias. I almost always write out properties, and have only started using the shorthand way that you had initially in some newer stuff. I always write out ReadOnly properties in all cases. It doesn't hurt anything...unless you are getting graded on it. You'd have to decide whether the professor wants to see the long version of the properties, or doesn't care which you use.

---

Thats why I asked where that came from because the instructions I posted up there are the EXACT instructions from my instructor..It doesnt say anything about doing that..Also Im using VB2017..So maybe I should stick to what hes asking...If I do how should I write it then?

The way you had it originally, except that I do think that adding the Public access qualifier at the beginning is a good idea. I still use VS2010 for lots of stuff, so I often forget that you don't have to write out ReadOnly properties anymore, but in 2017 you do not.

Quote:

---

Originally Posted by Shaggy Hiker

The way you had it originally, except that I do think that adding the Public access qualifier at the beginning is a good idea. I still use VS2010 for lots of stuff, so I often forget that you don't have to write out ReadOnly properties anymore, but in 2017 you do not.

---

Ok so I updated my code up top..Is it right? According to the directions? Thanks!

I thought you were not going to write out the properties?

Quote:

---

Originally Posted by EmilyM1105

Ok so I updated my code up top

---

I would suggest that you don't do that. It will confuse people who come in part way through the thread and it means that noone can see what you've actually changed along the way.

Quote:

---

Originally Posted by Shaggy Hiker

I thought you were not going to write out the properties?

---

So I dont have to write Get and Set statements?

It's entirely up to you, I just thought you felt that the professor would not want you to. There's no advantage to one over the other as far as performance goes. As long as you know that you can do either, that's probably all that matters.

Nevermind im sorry you were right about:

Code:

---

Private mBrandName as String

Public Property BrandName As String
 Get
  Return mBrandName
 End Get
 Set (value As String)
  mBrandName = value
 End Set
End Property

---

Quote:

---

Originally Posted by EmilyM1105

Nevermind im sorry you were right about:

Code:

---

Private mBrandName as String

Public Property BrandName As String
 Get
  Return mBrandName
 End Get
 Set (value As String)
  mBrandName = value
 End Set
End Property

---

---

Does your post above mean that the long way of defining a Property is what your instructor is expecting? If so, then your code at the very top in your first post looks somewhat ok, you just need to create a variable for the rest of the Properties, like you show above with mBrandName. Probably a good idea to just name these variables with the same format "mPropertyName", like mScanNumber, mDescription, etc, making sure the variable is of the same type as the Property (Integer variable for Integer Property, for example). And don't forget that in the other Properties, your Set statement need to be setting this "mPropertyName" variable = value.

So lets look at your Price property. I would suggest changing the type from Integer to Double, since prices can be in dollars and cents, not just whole dollars.

Code:

---

Private mPrice As Double

Public Property Price As Double
  Get
    Return mPrice
  End Get
  Set (value As Double)
    mPrice = value
  End Set
End Property

---

The instructions say to ensure that only positive values can be assigned to the price. When your Property has limitations on what values can be used, the way to test for this is with appropriate logic inside of the Properties Set statement. I'll show you a similar example. Imagine that we had created a Class that stores information about people. And one of the Properties of this Class is the person's Age. You would agree that a person's age can't be negative, and there is also some upper limit to where above that number a person's age doesn't make sense.

Example: Replace an invalid age with the closest valid value:

Code:

---

Private mAge As Integer
Private minAge As Integer = 0
Private maxAge As Integer = 140

Public Property Age As Integer
  Get
    Return mAge
  End Get
  Set (value As Integer)
    If value < minAge Then
      mAge = minAge
    ElseIf value > maxAge Then
      mAge = maxAge
    Else
      mAge = value
    End If
  End Set
End Property

---

There are other ways of handling an invalid property value being passed, like displaying a Messagebox to the user:

Code:

---

  Set (value As Integer)
    If value < minAge OrElse value > maxAge Then
      MessageBox.Show("Invalid Age", "Error")
    Else
      mAge = value
    End If
  End Set

---

or Throwing a custom exception:

Code:

---

  Set (value As Integer)
    If value < minAge OrElse value > maxAge Then
      Throw New AgeException("Invalid Age")
    Else
      mAge = value
    End If
  End Set

---

Notice that in those last two examples, in the case of an invalid age we're not setting mAge to anything. That should demonstrate to you how to add logic to a Set statement to ensure that the value passed to the property is valid, and some different ways of handling it if it isn't valid.

Quote:

---

Originally Posted by OptionBase1

Does your post above mean that the long way of defining a Property is what your instructor is expecting? If so, then your code at the very top in your first post looks somewhat ok, you just need to create a variable for the rest of the Properties, like you show above with mBrandName. Probably a good idea to just name these variables with the same format "mPropertyName", like mScanNumber, mDescription, etc, making sure the variable is of the same type as the Property (Integer variable for Integer Property, for example). And don't forget that in the other Properties, your Set statement need to be setting this "mPropertyName" variable = value.

So lets look at your Price property. I would suggest changing the type from Integer to Double, since prices can be in dollars and cents, not just whole dollars.

Code:

---

Private mPrice As Double

Public Property Price As Double
  Get
    Return mPrice
  End Get
  Set (value As Double)
    mPrice = value
  End Set
End Property

---

The instructions say to ensure that only positive values can be assigned to the price. When your Property has limitations on what values can be used, the way to test for this is with appropriate logic inside of the Properties Set statement. I'll show you a similar example. Imagine that we had created a Class that stores information about people. And one of the Properties of this Class is the person's Age. You would agree that a person's age can't be negative, and there is also some upper limit to where above that number a person's age doesn't make sense.

Example: Replace an invalid age with the closest valid value:

Code:

---

Private mAge As Integer
Private minAge As Integer = 0
Private maxAge As Integer = 140

Public Property Age As Integer
  Get
    Return mAge
  End Get
  Set (value As Integer)
    If value < minAge Then
      mAge = minAge
    ElseIf value > maxAge Then
      mAge = maxAge
    Else
      mAge = value
    End If
  End Set
End Property

---

There are other ways of handling an invalid property value being passed, like displaying a Messagebox to the user:

Code:

---

  Set (value As Integer)
    If value < minAge OrElse value > maxAge Then
      MessageBox.Show("Invalid Age", "Error")
    Else
      mAge = value
    End If
  End Set

---

or Throwing a custom exception:

Code:

---

  Set (value As Integer)
    If value < minAge OrElse value > maxAge Then
      Throw New AgeException("Invalid Age")
    Else
      mAge = value
    End If
  End Set

---

Notice that in those last two examples, in the case of an invalid age we're not setting mAge to anything. That should demonstrate to you how to add logic to a Set statement to ensure that the value passed to the property is valid, and some different ways of handling it if it isn't valid.

---

ok so I have made some changes.. How does this look so far?

This is my Grocery Item code:

Code:

---

Public Class GroceryItem
    Public ReadOnly Property ScanNumber As Integer
        Get
            Return ScanNumber
        End Get
    End Property

    Private mBrandName As String
    Public Property BrandName As String
        Get
            Return mBrandName
        End Get
        Set(value As String)
            mBrandName = value
        End Set
    End Property

    Private mDescription As String
    Public Property Description As String
        Get
            Return mDescription
        End Get
        Set(value As String)
            mDescription = value
        End Set
    End Property

    Private mPrice As Double
    Public Property Price As Double
        Get
            Return mPrice
        End Get
        Set(value As Double)
            If value > 0 Then
                mPrice = value
            End If
        End Set
    End Property

    Private mAisle As String
    Public Property Aisle As String
        Get
            Return mAisle
        End Get
        Set(value As String)
            mAisle = value
        End Set
    End Property
End Class

---

So for price It doesnt tell me anything about a price limit in the instructions?? so how would I know what to use on the part where I say what happens if the price reaches a limit??

The only issue I see is in the ScanNumber property. Although it is read-only, you still want a Private variable to hold what that value is, and then return the contents of that variable.

Code:

---

    Private mScanNumber As Integer
    Public ReadOnly Property ScanNumber As Integer
        Get
            Return mScanNumber
        End Get
    End Property

---

The rest looks good!

Quote:

---

Originally Posted by OptionBase1

The only issue I see is in the ScanNumber property. Although it is read-only, you still want a Private variable to hold what that value is, and then return the contents of that variable.

Code:

---

    Private mScanNumber As Integer
    Public ReadOnly Property ScanNumber As Integer
        Get
            Return mScanNumber
        End Get
    End Property

---

The rest looks good!

---

ok So heres the update lol:

Code:

---

Public Class GroceryItem
    Private mScanNumber As Integer
    Public ReadOnly Property ScanNumber As Integer
        Get
            Return mScanNumber
        End Get
    End Property

    Private mBrandName As String
    Public Property BrandName As String
        Get
            Return mBrandName
        End Get
        Set(value As String)
            mBrandName = value
        End Set
    End Property

    Private mDescription As String
    Public Property Description As String
        Get
            Return mDescription
        End Get
        Set(value As String)
            mDescription = value
        End Set
    End Property

    Private mPrice As Double
    Public Property Price As Double
        Get
            Return mPrice
        End Get
        Set(value As Double)
            If value > 0 Then
                mPrice = value
            End If
        End Set
    End Property

    Private mAisle As String
    Public Property Aisle As String
        Get
            Return mAisle
        End Get
        Set(value As String)
            mAisle = value
        End Set
    End Property
End Class

---

So what number on the instructions up top do you think im on now??...I was confused on this part??

Aisle - This should indicate one of the following aisles: Bakery, CannedGoods, Drinks, Deli, DryGoods, FrozenFoods, and Produce.
It says you can use enumeration?

There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

Code:

---

myGroceryItem.Aisle = "Bakery"

---

Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

Code:

---

myGroceryItem.Aisle = "AisleThatDoesNotExist"

---

but the instructions you posted didn't specify that you need to worry about that.

So I would say you are up to the point where you need to set up the 2 Constructors.

Quote:

---

Originally Posted by OptionBase1

There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

Code:

---

myGroceryItem.Aisle = "Bakery"

---

Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

Code:

---

myGroceryItem.Aisle = "AisleThatDoesNotExist"

---

but the instructions you posted didn't specify that you need to worry about that.

So I would say you are up to the point where you need to set up the 2 Constructors.

---

So would I have to do this for each Aisle?

Code:

---

myGroceryItem.Aisle = "Bakery"

---

Also, I thought that the constructors is what was in my GroceryItem code with Get & Set statements?? So constructors are something different?

Quote:

---

Originally Posted by EmilyM1105

Also, I thought that the constructors is what was in my GroceryItem code with Get & Set statements?? So constructors are something different?

---

The code with the Get an Set are properties.

A constructor in VB is a method named New. When you use the New keyword in code to create an instance of a type, you are invoking a constructor. Every type has a parameterless constructor by default, which would be the equivalent of this:

vb.net Code:

Public Sub New()
End Sub

If you add a constructor yourself then that default constructor is removed, so if you add a constructor with parameters and you want a parameterless constructor too then you must add both.

Quote:

---

Originally Posted by OptionBase1

There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

Code:

---

myGroceryItem.Aisle = "Bakery"

---

Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

Code:

---

myGroceryItem.Aisle = "AisleThatDoesNotExist"

---

but the instructions you posted didn't specify that you need to worry about that.

So I would say you are up to the point where you need to set up the 2 Constructors.

---

Do I write those constructors under my properties??

Like this?

Code:

---

Public Sub New(mScanNumber As Integer, Brandname As String, Price As Double)
Me.mScanNumber = Number
Me.Brandname = Brandname
Me.Price = Number
End Sub

---

Did I do that right?

That's the general idea but your implementation isn't really correct there. For one thing, you're assigning Number twice but you have no Number parameter. If you have properties named 'ScanNumber', 'BrandName' and 'Price' then your constructor parameters should be 'scanNumber', 'brandName' and 'price':

vb.net Code:

Public Sub New(scanNumber As Integer, brandName As String, price As Double)
    Me.ScanNumber = scanNumber
    Me.BrandName = brandName
    Me.Price = price
End Sub

Quote:

---

Originally Posted by jmcilhinney

That's the general idea but your implementation isn't really correct there. For one thing, you're assigning Number twice but you have no Number parameter. If you have properties named 'ScanNumber', 'BrandName' and 'Price' then your constructor parameters should be 'scanNumber', 'brandName' and 'price':

vb.net Code:

Public Sub New(scanNumber As Integer, brandName As String, price As Double)
    Me.ScanNumber = scanNumber
    Me.BrandName = brandName
    Me.Price = price
End Sub

---

ScanNumber is set as read only. So It keeps giving me an error message on the ScanNumber Part saying "it is ReadOnly"

Code:

---

Public Sub New(scanNumber As Integer, brandName As String, price As Double)
    Me.ScanNumber = scanNumber
    Me.BrandName = brandName
    Me.Price = price
End Sub

---

Thats why I put Me.MScanNumber.. when I do that the error goes away.


Next is the instructions....

Add a new public class named GroceryBasket to the project.

1. Derive from the generic List class.
2. Ensure that only GroceryItem objects are stored as items.

Heres my GroceryBasket Code:

Code:

---

Public Class GroceryBasket
    Inherits List(Of GroceryItem)
End Class

---

Is that right?? Sorry I had a hard time understanding this part

Quote:

---

Originally Posted by EmilyM1105

Oh ok I thought it had to be different because ScanNumber is read only??

---

If the property is ReadOnly then yes, you would have to set the backing field instead. I hadn't examined all previous posts so I didn't realise that that was the case.

Quote:

---

Originally Posted by EmilyM1105

Is that all I have to do for the constructor instructions?

---

The constructor needs to contain all the code that needs to be executed when an object is created. Do you need to do anything other than set the property values when you create an object? If so, add code to do that. If not, don't.