Section 15.2. Array Lists

15.2 Array Lists

Imagine that your program asks the user for input or gathers input from a web site. As it finds objects (strings, books, values, etc.), you would like to add them to an array, but you have no idea how many objects you'll collect in any given session.

It is difficult to use an array for such a purpose because you must declare the size of an Array object at compile time. If you try to add more objects than you've allocated memory for, the Array class will throw an exception. If you do not know in advance how many objects your array will be required to hold, you run the risk of declaring either too small an array (and running out of room) or too large an array (and wasting memory).

The .NET Framework provides a class designed for just this situation. The ArrayList class is an array whose size is dynamically increased as required. The ArrayList class provides many useful methods and properties. A few of the most important are shown in Table 15-3.

Table 15-3. ArrayList members

Method or property	Purpose
Capacity	Property containing the number of elements the array can currently hold.
Count	Property that returns the number of elements currently in the array.
Item( )	Method that gets or sets the element at the specified index; this is the indexer for the ArrayList class.
Add( )	Method that adds an object to the ArrayList.
Clear( )	Method that removes all elements from the ArrayList.
GetEnumerator( )	Method that returns an enumerator to iterate an ArrayList.
Insert( )	Method that inserts an element into ArrayList.
RemoveAt( )	Method that removes the element at the specified index.
Reverse( )	Method that reverses the order of elements in the ArrayList.
Sort ( )	Method that alphabetically sorts the ArrayList.
ToArray( )	Method that copies the elements of the ArrayList to a new array.

When you create an ArrayList, you do not define how many objects it will contain. You add to the ArrayList using the Add( ) method, and the list takes care of its own internal bookkeeping, as illustrated in Example 15-2.

Example 15-2. Using an ArrayList

 Option Strict On Imports System Namespace ArrayListDemo     ' a class to hold in the array list     Public Class Employee         Private myEmpID As Integer         Public Sub New(ByVal empID As Integer)             Me.myEmpID = empID         End Sub 'New         Public Overrides Function ToString( ) As String             Return myEmpID.ToString( )         End Function 'ToString         Public Property EmpID( ) As Integer             Get                 Return myEmpID             End Get             Set(ByVal Value As Integer)                 myEmpID = Value             End Set         End Property     End Class 'Employee     Class Tester         Public Sub Run( )             Dim empArray As New ArrayList( )             Dim intArray As New ArrayList( )             ' populate the arraylists             Dim i As Integer             For i = 0 To 4                 empArray.Add(New Employee(i + 100))                 intArray.Add((i * 5))             Next i             ' print each member of the array             For Each i In intArray                 Console.Write("{0} ", i.ToString( ))             Next i             Console.WriteLine(ControlChars.Lf)             ' print each employee             Dim e As Employee             For Each e In empArray                 Console.Write("{0} ", e.ToString( ))             Next e             Console.WriteLine(ControlChars.Lf)             Console.WriteLine("empArray.Capacity: {0}", empArray.Capacity)         End Sub 'Run         Shared Sub Main( )             Dim t As New Tester( )             t.Run( )         End Sub 'Main     End Class 'Tester End Namespace 'ArrayListDemo 

  Output:  0 5 10 15 20 100 101 102 103 104 empArray.Capacity: 16 

Suppose you're defining two ArrayList objects, empArray to hold Employee objects, and intArray to hold integers:

 Dim empArray As New ArrayList( ) Dim intArray As New ArrayList( ) 

Each ArrayList object has a property, Capacity, which is the number of elements the ArrayList is capable of storing.

The default capacity for the ArrayList class is 16. You are free to set a different starting capacity for your ArrayList, but typically there is no need for you ever to do so.

You add elements to the ArrayList with the Add( ) method:

 empArray.Add(New Employee(i + 100)) intArray.Add((i * 5)) 

When you add the 17th element, the capacity is automatically doubled to 32. If you change the For loop to:

 For i = 0 To 17 

the output looks like this:

 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 empArray.Capacity: 32 

Similarly, if you added a 33 ^rd element, the capacity would be doubled to 64. The 65 ^th element increases the capacity to 128, the 129th element increases it to 256, and so forth.