Section 15.3. The Collection Class

15.3 The Collection Class

Visual Basic .NET offers a generic collection class named, aptly, Collection. In many ways, the Collection object serves as an object-oriented alternative to Array, much as ArrayList does.

These two constructs (ArrayList and Collection) are very similar. Both offer and Add( ) and Remove( ) methods as well as an Item property. The Collection class, however, overloads the Item property to take a string as a key into the collection. This allows the Collection class to act as a dictionary, associating keys with values. You can also use the Item property to access members of the collection by index value; however the Collection uses a one-based index (i.e., the first element is index 1 rather than 0).

Example 15-3 illustrates the use of a VB.NET Collection object.

Example 15-3. Using a Collection object

 Option Strict On Imports System Namespace CollectionDemo     ' 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 intCollection As New Collection( )             Dim empCollection As New Collection( )             Dim empCollection2 As New Collection( )             ' populate the Collections             Dim i As Integer             For i = 0 To 4                 empCollection.Add(New Employee(i + 100))                 intCollection.Add((i * 5))             Next i             ' add key/value pairs             empCollection2.Add(New Employee(1789), "George Washington")             empCollection2.Add(New Employee(1797), "John Adams")             empCollection2.Add(New Employee(1801), "Thomas Jefferson")             ' print each member of the array             For Each i In intCollection                 Console.Write("{0} ", i.ToString( ))             Next i             Console.WriteLine( )             Console.WriteLine("Employee collection...")             Dim e As Employee             For Each e In empCollection                 Console.Write("{0} ", e.ToString( ))             Next e             Console.WriteLine( )             Console.WriteLine("Employee collection 2...")             For Each e In empCollection2                 Console.Write("{0} ", e.ToString( ))             Next e             Console.WriteLine( )             ' retrieve an Employee by key              Dim emp As Employee             emp = empCollection2.Item("John Adams")             Console.WriteLine( _               "Key John Adams retrieved empID {0}", emp.ToString( ))             ' note that indexing is 1-based (rather than zero based)             emp = empCollection2.Item(1)             Console.WriteLine( _               "Index(1) retrieved empID {0}", emp.ToString( ))         End Sub 'Run         Shared Sub Main( )             Dim t As New Tester( )             t.Run( )         End Sub 'Main     End Class 'Tester End Namespace 'CollectionDemo 

  Output:  0 5 10 15 20 Employee collection... 100 101 102 103 104 Employee collection 2... 1789 1797 1801 Key John Adams retrieved empID 1797 Index(1) retrieved empID 1789 

Example 15-3 creates three Collection objects (intCollection, empCollection, and empCollection2):

 Dim intCollection As New Collection( ) Dim empCollection As New Collection( ) Dim empCollection2 As New Collection( ) 

The first two objects are populated in For loops , just as the ArrayList was created in Example 15-2.

 Dim i As Integer For i = 0 To 4     empCollection.Add(New Employee(i + 100))     intCollection.Add((i * 5)) Next i 

The third Collection object, empCollection2, is populated using key values. Each new Employee is associated with a string, representing the name of the Employee:

 empCollection2.Add(New Employee(1789), "George Washington") empCollection2.Add(New Employee(1797), "John Adams") empCollection2.Add(New Employee(1801), "Thomas Jefferson") 

You retrieve objects from the collection much as you did from the ArrayLists:

 For Each i In intCollection     Console.Write("{0} ", i.ToString( )) Next i Dim e As Employee For Each e In empCollection     Console.Write("{0} ", e.ToString( )) Next e For Each e In empCollection2     Console.Write("{0} ", e.ToString( )) Next e 

You can, however, retrieve objects from the collection using either the key value or an index value (one-based):

 Dim emp As Employee emp = empCollection2.Item("John Adams") Console.WriteLine("Key John Adams retrieved empID {0}", emp.ToString( )) emp = empCollection2.Item(1) Console.WriteLine("Index(1) retrieved empID {0}", emp.ToString( ))