Buying Generic in C#.NET

Painting Code-39 in C#.NET Buying Generic

Wrappers are classes (or methods) that encapsulate complexity A wrapper may have an interface quite different from the interfaces of what s inside it that s an adapter The wrapper encapsulates three actual queues here (they could be generic), and the wrapper must manage what goes into which underlying queue and how The standard interface to the Queue class, as implemented in C#, includes these two key methods: Enqueue() (pronounced N-Q ) inserts items into a queue at the back Dequeue() (pronounced D-Q ) removes items from the queue at the front For the shipping-priority queue, the wrapper provides the same interface as a normal queue, thus pretending to be a normal queue itself It implements an Enqueue() method that determines an incoming package s priority and decides which underlying queue it gets to join The wrapper s Dequeue() method finds the highest-priority Package in any of the underlying queues The formal name of this wrapper class is PriorityQueue Here s the code for the PriorityQueue example on this book s Web site:

// PriorityQueue -- Demonstrates using lower-level queue collection objects // (generic ones at that) to implement a higher-level generic // Queue that stores objects in priority order using System; using SystemCollectionsGeneric; namespace PriorityQueue { class Program { // Main -- Fill the priority queue with packages, then // remove a random number of them static void Main(string[] args) { ConsoleWriteLine( Create a priority queue: ); PriorityQueue<Package> pq = new PriorityQueue<Package>();

ConsoleWriteLine( Add a random number (0 - 20) of random packages to queue: ); Package pack; PackageFactory fact = new PackageFactory(); // You want a random number less than 20 Random rand = new Random(); int numToCreate = randNext(20); // Random int from 0 - 20 ConsoleWriteLine( \tCreating {0} packages: , numToCreate); for (int i = 0; i < numToCreate; i++) { ConsoleWrite( \t\tGenerating and adding random package {0} , i); pack = factCreatePackage(); ConsoleWriteLine( with priority {0} , packPriority); pqEnqueue(pack); } ConsoleWriteLine( See what we got: ); int total = pqCount; ConsoleWriteLine( Packages received: {0} , total); ConsoleWriteLine( Remove a random number of packages (0-20): ); int numToRemove = randNext(20); ConsoleWriteLine( \tRemoving up to {0} packages , numToRemove); for (int i = 0; i < numToRemove; i++) { pack = pqDequeue(); if (pack != null) { ConsoleWriteLine( \t\tShipped package with priority {0} , packPriority); } } // See how many were shipped ConsoleWriteLine( Shipped {0} packages , total - pqCount); // Wait for user to acknowledge the results ConsoleWriteLine( Press Enter to terminate ); ConsoleRead(); } } // Priority enumeration -- Defines a set of priorities // instead of priorities like 1, 2, 3, these have names // For information on enumerations, // see the article Enumerating the Charms of the Enum // on csharp102info enum Priority { Low, Medium, High } // IPrioritizable interface -- Defines ability to prioritize // Define a custom interface: Classes that can be added to // PriorityQueue must implement this interface interface IPrioritizable { Priority Priority { get; } // Example of a property in an interface } //PriorityQueue -- A generic priority queue class // Types to be added to the queue *must* implement IPrioritizable interface class PriorityQueue<T> where T : IPrioritizable {

//Queues -- the three underlying queues: all generic! private Queue<T> _queueHigh = new Queue<T>(); private Queue<T> _queueMedium = new Queue<T>(); private Queue<T> _queueLow = new Queue<T>(); //Enqueue -- Prioritize T and add it to correct queue; an item of type T // The item must know its own priority public void Enqueue(T item) { switch (itemPriority) // Require IPrioritizable to ensure this property { case PriorityHigh: _queueHighEnqueue(item); break; case PriorityMedium: _queueMediumEnqueue(item); break; case PriorityLow: _queueLowEnqueue(item); break; default: throw new ArgumentOutOfRangeException( itemPriorityToString(), bad priority in PriorityQueueEnqueue ); } } //Dequeue -- Get T from highest-priority queue available public T Dequeue() { // Find highest-priority queue with items Queue<T> queueTop = TopQueue(); // If a non-empty queue is found if (queueTop != null & queueTopCount > 0) { return queueTopDequeue(); // Return its front item } // If all queues empty, return null (you could throw exception) return default(T); // What s this See discussion } //TopQueue -- What s the highest-priority underlying queue with items private Queue<T> TopQueue() { if (_queueHighCount > 0) // Anything in high-priority queue return _queueHigh; if (_queueMediumCount > 0) // Anything in medium-priority queue return _queueMedium; if (_queueLowCount > 0) // Anything in low-priority queue return _queueLow; return _queueLow; // All empty, so return an empty queue } //IsEmpty -- Check whether there s anything to deqeue public bool IsEmpty() { // True if all queues are empty return (_queueHighCount == 0) & (_queueMediumCount == 0) & (_queueLowCount == 0); }