Visual Basic.NET How to Program (PDF)

Chapter 23 Data Structures and Collections 1141
RemoveFromFront (lines 57–81) and RemoveFromBack (lines 84–117) are the primary
methods of class CList. Each method uses a SyncLock block to ensure that
CList objects are multithread safe when used in a multithreaded program. This means
that, if one thread is modifying the contents of a CList object, no other thread can modify
the same CList object at the same time. Method IsEmpty (lines 120–132) is a predicate
method that determines whether the list is empty (i.e., whether the reference to the first node
of the list is Nothing). Predicate methods typically test a condition and do not modify the
object on which they are called. If the list is empty, method IsEmpty returns True; otherwise,
it returns False. Method Print (lines 135–159) displays the list’s contents. Both
method IsEmpty and method Print use SyncLock blocks, ensuring that the state of
the list does not change while the methods are performing their tasks.
Class EmptyListException (Fig. 23.6) defines an exception class to handle
illegal operations on an empty CList. For example, an EmptyListException occurs
if the program attempts to remove a node from an empty CList.
Module modListTest (Fig. 23.7) uses the linked-list library to create and manipulate
a linked list. Line 10 creates an instance of type CList named list. Then, lines 13–
16 create data to add to the list. Lines 19–29 use CList insertion methods to insert these
objects and use CList method Print to output the contents of list after each insertion.
The code inside the Try block (lines 35–70) removes objects (using CList deletion
methods), outputs the removed object and outputs list after every deletion. If there is an
attempt to remove an object from an empty list, the Catch block (lines 66–68) catches the
EmptyListException. Note that module modListTest uses namespace
LinkedListLibrary (Fig. 23.4); thus, the project containing module modListTest
must contain a reference to the LinkedListLibrary class library.
firstNode
H D ... Q
Fig. 23.3 Linked-list graphical representation.
1 ' Fig. 23.04: ListNodes.vb
2 ' Class to represent one node in a CList.
3
4 Public Class CListNode
5 Private mData As Object
6 Private mNextNode As CListNode
7
Fig. 23.4 Self-referential class CListNode (part 1 of 2).
lastNode