Visual Basic.NET How to Program (PDF)

Chapter 23 Data Structures and Collections 1185
Count property. This reduces the storage capacity of the ArrayList to the exact number
of elements currently in the ArrayList.
When users click Statistics, cmdStatistics_Click (lines 125–130) uses the
Count and Capacity properties to display the current number of elements in the
ArrayList and the maximum number of elements that can be stored without the allocation
of more memory to the ArrayList.
When users click Display, cmdDisplay_Click (lines 133–144) outputs the contents
of the ArrayList. This event handler uses an IEnumerator (sometimes called
an enumerator, or an iterator) to traverse the elements of an ArrayList one element at
a time. Interface IEnumerator defines methods MoveNext and Reset and property
Current. MoveNext moves the enumerator to the next element in the ArrayList. The
first call to MoveNext positions the enumerator at the first element of the ArrayList.
MoveNext returns True if there is at least one more element in the ArrayList; otherwise,
the method returns False. Method Reset positions the enumerator before the first
element of the ArrayList. Methods MoveNext and Reset throw an InvalidOperationException
if the contents of the collection are modified after the enumerator’s
creation. Property Current returns the object at the current location in the ArrayList.
Line 136 creates an IEnumerator, called enumerator, and assigns it the result of
a call to ArrayList method GetEnumerator. Lines 139–141 use enumerator to
iterate the ArrayList (as long as MoveNext returns True), retrieve the current item
via property Count and append it to StringBuilder buffer. When the loop terminates,
line 143 displays the contents of buffer.
23.7.3 Class Stack
The Stack class (namespace System.Collections) implements a stack data structure.
This class provides much of the functionality that we defined in our implementation
in Section 23.4. The application in Fig. 23.29 provides a GUI that enables the user to test
many Stack methods. Line 31 of the FrmStackTest constructor creates a Stack with
the default initial capacity (10 elements).
Class Stack provides methods Push and Pop to perform the basic stack operations.
Method Push takes an Object as an argument and adds it to the top of the Stack. If the
number of items on the Stack (the Count property) is equal to the capacity at the time of
the Push operation, the Stack grows to accommodate more Objects. Event handler
cmdPush_Click (lines 37–42) uses method Push to add a user-specified string to the
stack (line 40).
1 ' Fig. 23.29: StackTest.vb
2 ' Demonstrates class Stack functionality.
3
4 Imports System.Collections
5 Imports System.Text
6 Imports System.Windows.Forms
7
8 Public Class FrmStackTest
9 Inherits Form
Fig. 23.29 Stack class demonstration (part 1 of 4).