Visual Basic 2005 for Programmers (2nd Edition)

25.2. Motivation for Generic Methods

Overloaded methods are often used to perform similar operations on different types of data. To motivate the concepte of generic methods, let's begin with an example (Fig. 25.1) that contains three overloaded PrintArray methods (lines 1925, 2834 and 3743). These methods display the elements of an Integer array, a Double array and a Char array, respectively. In Section 25.3, we reimplement this program more concisely and elegantly using a single generic method.

Figure 25.1. Displaying arrays of different types using overloaded methods

1 ' Fig. 25.1: OverloadedMethods.vb 2 ' Using overloaded methods to print arrays of different types. 3 Module OverloadedMethods 4 Sub Main() 5 ' create arrays of Integer, Double and Char types 6 Dim integerArray As Integer() = {1, 2, 3, 4, 5, 6} 7 Dim doubleArray As Double() = {1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7} 8 Dim charArray As Char() = {"H"c, "E"c, "L"c, "L"c, "O"c} 9 10 Console.WriteLine("Array integerArray contains:") 11 PrintArray(integerArray) ' pass an Integer array argument 12 Console.WriteLine("Array doubleArray contains:") 13 PrintArray(doubleArray) ' pass a Double array argument 14 Console.WriteLine("Array charArray contains:") 15 PrintArray(charArray) ' pass a Char array 16 End Sub ' Main 17 18 ' output Integer array 19 Sub PrintArray(ByVal inputArray() As Integer) 20 For Each element As Integer In inputArray 21 Console.Write(element.ToString() & " ") 22 Next element 23 24 Console.WriteLine(vbCrLf) 25 End Sub ' PrintArray 26 27 ' output Double array 28 Sub PrintArray(ByVal inputArray() As Double) 29 For Each element As Double In inputArray 30 Console.Write(element.ToString() & " ") 31 Next element 32 33 Console.WriteLine(vbCrLf) 34 End Sub ' PrintArray 35 36 ' output Char array 37 Sub PrintArray( ByVal inputArray()As Char) 38 For Each element As Char In inputArray 39 Console.Write(element.ToString() & " ") 40 Next element 41 42 Console.WriteLine(vbCrLf) 43 End Sub ' PrintArray 44 End Module ' OverloadedMethods

Array integerArray contains: 1 2 3 4 5 6 Array doubleArray contains: 1.1 2.2 3.3 4.4 5.5 6.6 7.7 Array charArray contains: H E L L O

The program begins by declaring and initializing three arrayssix-element Integer array integerArray (line 6), seven-element Double array doubleArray (line 7) and fiveelement Char array charArray (line 8). Then lines 1015 output the arrays.

When the compiler encounters a method call, it attempts to locate a method declaration that has the same method name and parameters that match the argument types in the method call. In this example, each PrintArray call exactly matches one of the PrintArray method declarations. For example, line 11 calls PrintArray with integerArray as its argument. At compile time, the compiler determines argument integerArray's type (i.e., Integer()), attempts to locate a method named PrintArray that specifies a single Integer() parameter (which it finds in lines 1925) and sets up a call to that method. Similarly, when the compiler encounters the PrintArray call in line 13, it determines argument doubleArray's type (i.e., Double()), then attempts to locate a method named PrintArray that specifies a single Double() parameter (which it finds in lines 2834) and sets up a call to that method. Finally, when the compiler encounters the PrintArray call in line 15, it determines argumentcharArray's type (i.e., Char()), then attempts to locate a method named PrintArray that specifies a single Char() parameter (which it finds in lines 3743) and sets up a call to that method.

Study each PrintArray method. Note that the array element type (Integer, Double or Char) appears in two locations in each methodthe method header (lines 19, 28 and 37) and the For Each statement header (lines 20, 29 and 38). If we replace the element type in each method with a generic namewe chose E to represent the "element" typethen all three methods would look like the one in Fig. 25.2. It appears that if we can replace the array element type in each of the three methods with a single "generic type parameter," then we should be able to declare one PrintArray method that can display the elements of any array. The method in Fig. 25.2 will not compile because its syntax is not correctwe declare a generic PrintArray method with the proper syntax in Fig. 25.3.

Figure 25.2. PrintArray method in which actual type names are replaced by convention with the generic name E (for "element"). [Note: This code will not compile.]

1 Public Sub PrintArray(ByVal inputArray() As E) 2 For Each element As E In inputArray 3 Console.Write(element.ToString() & " ") 4 Next element 5 6 Console.WriteLine(vbCrLf) 7 End Sub ' PrintArray

Figure 25.3. Printing array elements using generic method PrintArray

1 ' Fig. 25.3: GenericMethod.vb 2 ' Using overloaded methods to print arrays of different types. 3 Module GenericMethod 4 Sub Main() 5 ' create arrays of integer, double and char 6 Dim integerArray As Integer() = {1, 2, 3, 4, 5, 6} 7 Dim doubleArray As Double() = {1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7} 8 Dim charArray As Char() = {"H"c, "E"c, "L"c, "L"c, "O"c} 9 10 Console.WriteLine("Array integerArray contains:") 11 PrintArray(integerArray) ' pass an integer array argument 12 Console.WriteLine("Array doubleArray contains:") 13 PrintArray(doubleArray) ' pass a char array argument 14 Console.WriteLine( "Array charArray contains:") 15 PrintArray(charArray) ' pass a double array argument 16 End Sub ' Main 17 18 ' output array of all types 19 Public Sub PrintArray(Of E)(ByVal inputArray() As E) 20 For Each element As E In inputArray 21 Console.Write(element.ToString() & " ") 22 Next element 23 24 Console.WriteLine(vbCrLf) 25 End Sub ' PrintArray 26 End Module ' GenericMethod

Array integerArray contains: 1 2 3 4 5 6 Array doubleArray contains: 1.1 2.2 3.3 4.4 5.5 6.6 7.7 Array charArray contains: H E L L O

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