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Addition: Why Python3 choose to design new features at cost of compatibility? Is there anything to do with the nature of language?
Why ES6 (7/8…) is able to be compatible with ES5(3) since there is so much difference in grammar?
There aren't differences so much as additions. When new grammar/syntax is added in JavaScript, it's defined carefully so that it would have been a syntax error in earlier versions. That's important, because maintaining backward compatibility with the mind-bogglingly huge amount of JavaScript code out in the wild is an important goal for the committee that moves JavaScript forward (ECMA's TC39). (Backward compatibility isn't 100%, but it's about 99.9999%.) But if something new would have been a syntax error before, then there isn't any working code out there that has the new thing.
For example: Adding async functions. In JavaScript from the beginning, a function declaration looks like this:
function example() {
// ...
}
To create async functions, it became possible to put async in front of function:
async function example() {
// ...
}
That's new grammar. It only works because before it was added, that would have been a syntax error, the same one you get if you do this today:
blah function example() {]
// ...
}
Similarly, with parameter default values, you specify the default value with an = and an expression in the parameter list:
function example(param = "default") {
// ...
}
That was a syntax error until it was added. So were arrow functions, generator functions, class syntax, nullish coalescing, optional chaining, destructuring, and other new grammar/syntax.
So since these would have been syntax errors, they don't exist in working code out in the wild.
In this post there is the description of "subclass" usage in VBA. I'm looking for the next step of it: when I have first subitem added I want to use it and don't know how.
When I write baseItem(1).itemName it doesn't work.
I assume it's just because that baseItem is not a collection or an array, but I don't know any other way.
Welcome to SO!
In vba like many other languages it is a Property. You call them directly and not numerically, but you need to write GET/LET methods unless the class is public (that is not really encapsulation to simply access directly).
baseItem.itemName would be your call.
But ... As I said before, better that you write accessor(s) to your class as methods.
Here is a guy that sets the tone for OOP in VBA (for me):
https://stackoverflow.com/a/45570268/8716187
He is a driver of the Rubberduck project.
I would ask you to ask yourself if you really need a class, I will often use 4-10 dictionaries of keys holding arrays. I could wrap them in a class but why bother? What I need is a searchable and editable ("array"-the dictionary of arrays).
I have very few class modules written, it seems that one can operate without it many times.
-WWC
I assume it's just because that baseItem is not a collection or an
array...
The baseItem itself is not a collection nor an array. It is just an object of type BaseClass. But this object baseItem wrapps a collection so we maybe could say it is almost a collection.
The problem with this object though is, that as it is defined now in the answer you mentioned, it provides no way how the clients can get to this collection. The class BaseClass needs to be modified so it provides access to this inner collection for the client of this class. With access e.g. some public function is meant which will return something from this collection.
Add something like this to the BaseClass:
Public Function getSubItem(index As Variant) As SubClass
Set getSubItem = subClassCollection.Item(index)
End Function
Now objects which will be at runtime created based on this class definition will provide access to the inner collection via this function getSubItem. The code which will use this function will look like this. So it is now almost that what you are trying to achieve.
Dim name As String
name = baseItem.getSubItem(1).itemName
Debug.Print name ' Prints "Something" in output window
But it could be made even exactly to what you are trying to achieve. When exporting the file of BaseClass.cls and adding Attribute Value.VB_UserMemId = 0 to the very beginning of function getSubItem and importing it again to project.
Public Function getSubItem(index As Variant) As SubClass
Attribute Value.VB_UserMemId = 0
Set getSubItem = subClassCollection.Item(index)
End Function
Now you can really write your code exactly that way you wanted. HTH
Dim name As String
name = baseItem(1).itemName
Debug.Print name ' Prints "Something" in output window
For more information about Creating A Default Member In VBA have a look e.g. here.
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Using RestKit and CoreData with Swift
RestKit offers functionality to map JSON directly into NSManagedObject instances.
You use the following code line to init a RKEntityMapping object:
var classMapping :RKEntityMapping = RKEntityMapping(forEntityForName:"className",inManagedObjectStore:managedObjectStore)
This causes the following error:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot initialize an entity mapping for an entity with a nil managed object class: Got nil class for managed object class name 'className'. Maybe you forgot to add the class files to your target?'
The error occurs because of this line in RestKit API:
Class objectClass = NSClassFromString([entity managedObjectClassName]);
Solution:
If your NSManagedObject class is written in Swift, you have to add the following code before your class declaration:
#objc(className)
class className {
...
}
Hope this helps!
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I implemented a Http filter using Filter interface and it works fine in localhost.
The problem is that in a testing environment when two users want to access to the application this filter does not work like always. It mixes the data between two users. I know it because I have lots of logs reporting me the steps every moment. I don't know if there is any problem with the simultaneous access.
Like servlets, filters are application scoped. There's only one instance of a filter class being created during application startup and the very same instance is being reused across all HTTP requests throughout the application's lifetime. Your problem symptoms indicate that you're for some reason assigning request or session scoped variables as an instance variable of the filter instance, such as request parameters or session attributes, or perhaps even the whole request or session object itself.
For example,
public class MyFilter implements Filter {
private String someParam;
public void doFilter(... ...) {
someParam = request.getParameter("someParam");
// ...
}
}
This is not threadsafe! You should be declaring them in the method local scope:
public class MyFilter implements Filter {
public void doFilter(... ...) {
String someParam = request.getParameter("someParam");
// ...
}
}
See also:
How do servlets work? Instantiation, sessions, shared variables and multithreading
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I am doing some VBA programming in Excel and have one workbook where all the datasheets are to be copied from into another sheet. The new sheet will have several header rows, and I would like to keep track of where they are situated so I don't have to find words in them constantly.
Is the simplest thing to use classes and keep them running while the Excel workbook is open? Or will this make it heavy and hard to handle, and I should keep working with subroutines? What are the benefits of using classes? It is not like I have several objects, only sheets and validation on columns.
The advantage of using classes instead of just subroutines is that classes create a level of abstraction that allow you to write cleaner code. Admittedly, if you've never used classes before in VBA, there is a learning curve, but I believe it's certainly worth the time to figure it out.
One key indication that you should switch to classes is if you're constantly adding parameters to your functions and subroutines. In this case, it's almost always best to use classes.
I've copied an explanation of classes from one of my previous Stack Overflow answers:
Here's a long example of how using a class might help you. Although this example is lengthy, it will show you how a few principles of object-oriented programming can really help you clean up your code.
In the VBA editor, go to Insert > Class Module. In the Properties window (bottom left of the screen by default), change the name of the module to WorkLogItem. Add the following code to the class:
Option Explicit
Private pTaskID As Long
Private pPersonName As String
Private pHoursWorked As Double
Public Property Get TaskID() As Long
TaskID = pTaskID
End Property
Public Property Let TaskID(lTaskID As Long)
pTaskID = lTaskID
End Property
Public Property Get PersonName() As String
PersonName = pPersonName
End Property
Public Property Let PersonName(lPersonName As String)
pPersonName = lPersonName
End Property
Public Property Get HoursWorked() As Double
HoursWorked = pHoursWorked
End Property
Public Property Let HoursWorked(lHoursWorked As Double)
pHoursWorked = lHoursWorked
End Property
The above code will give us a strongly-typed object that's specific to the data with which we're working. When you use multi-dimension arrays to store your data, your code resembles this: arr(1,1) is the ID, arr(1,2) is the PersonName, and arr(1,3) is the HoursWorked. Using that syntax, it's hard to know what is what. Let's assume you still load your objects into an array, but instead use the WorkLogItem that we created above. This name, you would be able to do arr(1).PersonName to get the person's name. That makes your code much easier to read.
Let's keep moving with this example. Instead of storing the objects in array, we'll try using a collection.
Next, add a new class module and call it ProcessWorkLog. Put the following code in there:
Option Explicit
Private pWorkLogItems As Collection
Public Property Get WorkLogItems() As Collection
Set WorkLogItems = pWorkLogItems
End Property
Public Property Set WorkLogItems(lWorkLogItem As Collection)
Set pWorkLogItems = lWorkLogItem
End Property
Function GetHoursWorked(strPersonName As String) As Double
On Error GoTo Handle_Errors
Dim wli As WorkLogItem
Dim doubleTotal As Double
doubleTotal = 0
For Each wli In WorkLogItems
If strPersonName = wli.PersonName Then
doubleTotal = doubleTotal + wli.HoursWorked
End If
Next wli
Exit_Here:
GetHoursWorked = doubleTotal
Exit Function
Handle_Errors:
'You will probably want to catch the error that will '
'occur if WorkLogItems has not been set '
Resume Exit_Here
End Function
The above class is going to be used to "do something" with a colleciton of WorkLogItem. Initially, we just set it up to count the total number of hours worked. Let's test the code we wrote. Create a new Module (not a class module this time; just a "regular" module). Paste the following code in the module:
Option Explicit
Function PopulateArray() As Collection
Dim clnWlis As Collection
Dim wli As WorkLogItem
'Put some data in the collection'
Set clnWlis = New Collection
Set wli = New WorkLogItem
wli.TaskID = 1
wli.PersonName = "Fred"
wli.HoursWorked = 4.5
clnWlis.Add wli
Set wli = New WorkLogItem
wli.TaskID = 2
wli.PersonName = "Sally"
wli.HoursWorked = 3
clnWlis.Add wli
Set wli = New WorkLogItem
wli.TaskID = 3
wli.PersonName = "Fred"
wli.HoursWorked = 2.5
clnWlis.Add wli
Set PopulateArray = clnWlis
End Function
Sub TestGetHoursWorked()
Dim pwl As ProcessWorkLog
Dim arrWli() As WorkLogItem
Set pwl = New ProcessWorkLog
Set pwl.WorkLogItems = PopulateArray()
Debug.Print pwl.GetHoursWorked("Fred")
End Sub
In the above code, PopulateArray() simply creates a collection of WorkLogItem. In your real code, you might create class to parse your Excel sheets or your data objects to fill a collection or an array.
The TestGetHoursWorked() code simply demonstrates how the classes were used. You notice that ProcessWorkLog is instantiated as an object. After it is instantiated, a collection of WorkLogItem becomes part of the pwl object. You notice this in the line Set pwl.WorkLogItems = PopulateArray(). Next, we simply call the function we wrote which acts upon the collection WorkLogItems.
Why is this helpful?
Let's suppose your data changes and you want to add a new method. Suppose your WorkLogItem now includes a field for HoursOnBreak and you want to add a new method to calculate that.
All you need to do is add a property to WorkLogItem like so:
Private pHoursOnBreak As Double
Public Property Get HoursOnBreak() As Double
HoursOnBreak = pHoursOnBreak
End Property
Public Property Let HoursOnBreak(lHoursOnBreak As Double)
pHoursOnBreak = lHoursOnBreak
End Property
Of course, you'll need to change your method for populating your collection (the sample method I used was PopulateArray(), but you probably should have a separate class just for this). Then you just add your new method to your ProcessWorkLog class:
Function GetHoursOnBreak(strPersonName As String) As Double
'Code to get hours on break
End Function
Now, if we wanted to update our TestGetHoursWorked() method to return result of GetHoursOnBreak, all we would have to do as add the following line:
Debug.Print pwl.GetHoursOnBreak("Fred")
If you passed in an array of values that represented your data, you would have to find every place in your code where you used the arrays and then update it accordingly. If you use classes (and their instantiated objects) instead, you can much more easily update your code to work with changes. Also, when you allow the class to be consumed in multiple ways (perhaps one function needs only 4 of the objects properties while another function will need 6), they can still reference the same object. This keeps you from having multiple arrays for different types of functions.
For further reading, I would highly recommend getting a copy of VBA Developer's Handbook, 2nd edition. The book is full of great examples and best practices and tons of sample code. If you're investing a lot of time into VBA for a serious project, it's well worth your time to look into this book.
If there are lots of subroutines or subroutines are very long then structuring the code in to classes may help. If there are only a couple of subroutines, say, each being only 10 lines of code each then this is over kill. The benefit of structuring the code in to classes is that it is easier to read and change when you come back to it down the line. So another reason to structuring the code into classes is if the code is likely to need changing down the line
There is one other thing you could add to the advantages other contributors have stated (sorry if it's somewhere in Ben McCormack's excellent answer and I missed it). Classes can have their uses if your VBA script is likely to be re-programmed at some point.
For instance, I am designing a sort of order management system. It is to be used by several colleagues for quite a while, but it may need re-progamming if ordering rules change. I have therefore designed a basic stock item class, which gathers all the information about a stock item. The rules about how this data is analyzed for any order are, however, written in easily accessible and well commented sub routines. By doing this, I hope that future VBA programmers can easily change the mathematical rules by which orders are generated, without having to deal with how all the data is gathered about a particular stock item (this is all done by subroutines and functions within the class, which are activated when the class is handed a stock number). A Class' public properties are also picked up by intellisense, allowing the next programmer, as well as yourself, to have an easier time of it.
I guess the point is that classes can make life easier for later users in this way if they encode some basic set of information, or some conceptual object, that is always likely to be relevant to the context of the program's use.