An ArrayList, at first glance might be kind of hard to get your head around, if you’re a beginner programmer, or coming from the Classic ASP/HTML world. But, at its base, it’s just a list of items in memory, and can be treated as an object, but it’s not a control, like a ListBox. It has properties and methods, and the count of the items is Zero-based. For instance, let’s say we have an arraylist consisting of 5 colors (Blue, Red, Green, Orange and Pink). Since ArrayLists are Zero-based, the numbering of the items starts with zero and ends with 4.
To create an ArrayList, first, we must dimension it:
Dim MyArrayList as ArrayList
Then, to create a space for it in memory:
MyArrayList=New Arraylist
Next, we must populate the ArrayList. This can be done any one of many ways. It can be done manually:
MyArrayList.add("Blue")
MyArrayList.add("Red")
MyArrayList.add("Green")
or, we can populate it from a database table, using a DataReader, like so:
While objDR.Read()
MyArrayList.add(objDR("FieldName"))
End While
Once it’s populated, then, we can ‘hook it up’ with a server control, like a ListBox or DropDownList:
DropDownList.datasource=MyArrayList DropDownList.databind
There are many properties and methods, and I won’t even begin to go into them all since this, like the title says, is the ‘Beginner’s ArrayList’, but if you go to the Quickstart Tutorials, you will find a very useful tool near the bottom of the TOC on the left, under Sample Applications, called ‘A Class Browser Application’. Look under the System.Collections Class to find it’s many properties and methods.
I will go into a few, just to show you a few common things that can be done with the ArrayList. As you probably have figured, from the tutorial so far, there is definitely a Count property. And, as logically as it sounds, it can be accessed like this:
Label1.text = MyArrayList.Count
We can iterate through the list, parsing or manipulating the items, but since it’s zero based, if we use a For/Next loop, we must do it like this:
for x=0 to MyArrayList.Count-1
'manipulate the items
next
We can use this to put the items of an ArrayList into a string:
Dim sNewString as String
for x=0 to MyArrayList.Count-1
'We step through the arraylist, one at a time
'Here we create the string, adding each item, followed by a comma
sNewString+=MyArrayList(x).ToString & ","
next
We can sort the ArrayList, in ascending order, easily like this:
MyArrayList.Sort
And, conversely, we can sort them descending like this:
MyArrayList.Reverse
As you’ve seen earlier, to add an item, we just call the ‘Add’ Method, and it also has a ‘Remove’ method to - you guessed it – Remove an item from the ArrayList. Also, it has an Item property, which uses the numbered position in the ArrayList as a parameter, and can be accessed like this:
Label1.text=MyArraList.Item(3)
That’s pretty much where I’m going to leave it for now, with the exception of a couple of links to Code Samples on this site, concerning ArrayLists:
