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• How to generate a Program template by generating an abstract class

  - by Byron-Lim Timothy Steffan

  i have the following problem. The 1st step is to implement a program, which follows a specific protocol on startup. Therefore, functions as onInit, onConfigRequest, etc. will be necessary. (These are triggered e.g. by incoming message on a TCP Port) My goal is to generate a class for example abstract one, which has abstract functions as onInit(), etc. A programmer should just inherit from this base class and should merely override these abstract functions of the base class. The rest as of the protocol e.g. should be simply handled in the background (using the code of the base class) and should not need to appear in the programmers code. What is the correct design strategy for such tasks? and how do I deal with, that the static main method is not inheritable? What are the key-tags for this problem? (I have problem searching for a solution since I lack clear statements on this problem) Goal is to create some sort of library/class, which - included in ones code - results in executables following the protocol. EDIT (new explanation): Okay let me try to explain more detailled: In this case programs should be clients within a client server architecture. We have a client server connection via TCP/IP. Each program needs to follow a specific protocol upon program start: As soon as my program starts and gets connected to the server it will receive an Init Message (TcpClient), when this happens it should trigger the function onInit(). (Should this be implemented by an event system?) After onInit() a acknowledgement message should be sent to the server. Afterwards there are some other steps as e.g. a config message from the server which triggers an onConfig and so on. Let's concentrate on the onInit function. The idea is, that onInit (and onConfig and so on) should be the only functions the programmer should edit while the overall protocol messaging is hidden for him. Therefore, I thought using an abstract class with the abstract methods onInit(), onConfig() in it should be the right thing. The static Main class I would like to hide, since within it e.g. there will be some part which connects to the tcp port, which reacts on the Init Message and which will call the onInit function. 2 problems here: 1. the static main class cant be inherited, isn it? 2. I cannot call abstract functions from the main class in the abstract master class. Let me give an Pseudo-example for my ideas: public abstract class MasterClass { static void Main(string[] args){ 1. open TCP connection 2. waiting for Init Message from server 3. onInit(); 4. Send Acknowledgement, that Init Routine has ended successfully 5. waiting for Config message from server 6..... } public abstract void onInit(); public abstract void onConfig(); } I hope you get the idea now! The programmer should afterwards inherit from this masterclass and merely need to edit the functions onInit and so on. Is this way possible? How? What else do you recommend for solving this? EDIT: The strategy ideo provided below is a good one! Check out my comment on that.

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• Why can't I create an abstract constructor on an abstract C# class?

  - by Anthony D

  I am creating an abstract class. I want each of my derived classes to be forced to implement a specific signature of constructor. As such, I did what I would have done has I wanted to force them to implement a method, I made an abstract one. public abstract class A { abstract A(int a, int b); } However I get a message saying the abstract modifier is invalid on this item. My goal was to force some code like this. public class B : A { public B(int a, int b) : base(a, b) { //Some other awesome code. } } This is all C# .NET code. Can anyone help me out? Update 1 I wanted to add some things. What I ended up with was this. private A() { } protected A(int a, int b) { //Code } That does what some folks are saying, default is private, and the class needs to implement a constructor. However that doesn't FORCE a constructor with the signature A(int a, int b). public abstract class A { protected abstract A(int a, int b) { } } Update 2 I should be clear, to work around this I made my default constructor private, and my other constructor protected. I am not really looking for a way to make my code work. I took care of that. I am looking to understand why C# does not let you do this.

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• Make an abstract class or use a processor?

  - by Tim Murphy

  I'm developing a class to compare two directories and run an action when a directory/file in the source directory does not exist in destination directory. Here is a prototype of the class: public abstract class IdenticalDirectories { private DirectoryInfo _sourceDirectory; private DirectoryInfo _destinationDirectory; protected abstract void DirectoryExists(DirectoryInfo sourceDirectory, DirectoryInfo destinationDirectory); protected abstract void DirectoryDoesNotExist(DirectoryInfo sourceDirectory, DirectoryInfo destinationDirectory); protected abstract void FileExists(DirectoryInfo sourceDirectory, DirectoryInfo destinationDirectory); protected abstract void FileDoesNotExist(DirectoryInfo sourceDirectory, DirectoryInfo destinationDirectory); public IdenticalDirectories(DirectoryInfo sourceDirectory, DirectoryInfo destinationDirectory) { ... } public void Run() { foreach (DirectoryInfo sourceSubDirectory in _sourceDirectory.GetDirectories()) { DirectoryInfo destinationSubDirectory = this.GetDestinationDirectoryInfo(subDirectory); if (destinationSubDirectory.Exists()) { this.DirectoryExists(sourceSubDirectory, destinationSubDirectory); } else { this.DirectoryDoesNotExist(sourceSubDirectory, destinationSubDirectory); } foreach (FileInfo sourceFile in sourceSubDirectory.GetFiles()) { FileInfo destinationFile = this.GetDestinationFileInfo(sourceFile); if (destinationFile.Exists()) { this.FileExists(sourceFile, destinationFile); } else { this.FileDoesNotExist(sourceFile, destinationFile); } } } } } The above prototype is an abstract class. I'm wondering if it would be better to make the class non-abstract and have the Run method receiver a processor? eg. public void Run(IIdenticalDirectoriesProcessor processor) { foreach (DirectoryInfo sourceSubDirectory in _sourceDirectory.GetDirectories()) { DirectoryInfo destinationSubDirectory = this.GetDestinationDirectoryInfo(subDirectory); if (destinationSubDirectory.Exists()) { processor.DirectoryExists(sourceSubDirectory, destinationSubDirectory); } else { processor.DirectoryDoesNotExist(sourceSubDirectory, destinationSubDirectory); } foreach (FileInfo sourceFile in sourceSubDirectory.GetFiles()) { FileInfo destinationFile = this.GetDestinationFileInfo(sourceFile); if (destinationFile.Exists()) { processor.FileExists(sourceFile, destinationFile); } else { processor.FileDoesNotExist(sourceFile, destinationFile); } } } } What do you see as the pros and cons of each implementation?

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• How to create a dynamic array of an Abstract class?

  - by outsyncof

  Lets say I have an abstract class Cat that has a few concrete subclasses Wildcat, Housecat, etc. I want my array to be able to store pointers to a type of cat without knowing which kind it really is. When I try to dynamically allocate an array of Cat, it doesn't seem to be working. Please help? Cat* catArray = new Cat[200];

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• UIHint can not resolve template in abstract models

  - by Reza Owliaei

  Assume an abstract model like this: public abstract class MyClass : BaseEntity { [UIHint("File")] public long? DocumentFileId { get; set; } } The problem is Cannot resolve template 'File', while there is File.cshtml in View editor templates. The point is, if I don't define MyClass as an abstract class, error will be solved. My question is, why editor template can not resolve in abstract classes, and how can I handle it?

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• Derive abstract class from non-abstract class

  - by Jehof

  Is it OK to derive an abstract class from a non-abstract class or is there something wrong with this approach? Here´s a little example: public class Task { // Some Members } public abstract class PeriodicalTask : Task { // Represents a base class for task that has to be done periodicaly. // Some additional Members } public class DailyTask : PeriodicalTask { // Represents a Task that has to be done daily. // Some additional Members } public class WeeklyTask : PeriodicalTask { // Represents a Task that has to be done weekly. // Some additional Members } In the example above i do not want to make the class Task abstract, because i want to instantiate it directly. PeriodicalTask should inherit the functionality from Task and add some additional members but i do not want to instantiate it directly. Only derived class of PeriodicalTask should be instantiated.

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• abstract class NumberFormat - very confuse about getInstance()

  - by Alex

  Hi, I'm new to Java and I have a beginner question: NumberFormat is an abstract class and so I assume I can't make an instance of it. But there is a public static (factory?) method getInstance() that allow me to do NumberFormat nf = NumberFormat.getInstance(); I'm quite confuse. I'll be glad if someone could give me hints on: 1) If there is a public method to get an instance of this abstract class, why don't we have also a constructor? 2) This is an abstract class ; how can we have this static method giving us an instance of the class? 3) Why choosing such a design? If I assume it's possible to have an instance of an abstract class (???), I don't get why this class should be abstract at all. Thank you.

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• abstract class NumberFormat - very confused about getInstance()

  - by Alex

  Hi, I'm new to Java and I have a beginner question: NumberFormat is an abstract class and so I assume I can't make an instance of it. But there is a public static (factory?) method getInstance() that allow me to do NumberFormat nf = NumberFormat.getInstance(); I'm quite confuse. I'll be glad if someone could give me hints on: 1) If there is a public method to get an instance of this abstract class, why don't we have also a constructor? 2) This is an abstract class ; how can we have this static method giving us an instance of the class? 3) Why choosing such a design? If I assume it's possible to have an instance of an abstract class (???), I don't get why this class should be abstract at all. Thank you.

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• Question About Abstract Classes?

  - by IbrarMumtaz

  URL: Link (1) According to this wesbite .. you cannot implement Abstract classes but derive from them. This makes sense and I have read this many times. Like an interface, you cannot implement an instance of an abstract class, however you can implement methods, fields, and properties in the abstract class that can be used by the child class. But on MSDN URL: TextWriter CLass on MSDN TextWriter is an abstract class but it has two constructors defined ... and according to the MS 70-536 book, the following statement is valid: TextWriter tw = new File.CreateText("myFile.Txt") The static file class and it's CreateText method is fine by me as I have studied it on MSDN but can somebody explain this little contradiction I have found? Surely I am not the first? Why is instantaion of base abstract classes possible????

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• Implementing a Version check between an Abstract class and it's implementation

  - by Michael Stum

  I have this abstract class and concrete implementation (they are in different assemblies): public abstract class MyAbstractClass { private static readonly int MyAbstractClassVersion = 1; public abstract int ImplementedVersion { get; } protected MyAbstractClass() { CheckVersion(); } private void CheckVersion() { var message = string.Format( "MyAbstractClass implements Version {0}, concrete is Version {1}", RepositoryVersion, ImplementedVersion); if (!MyAbstractClassVersion.Equals(ImplementedVersion)) throw new InvalidOperationException(message); } } public class ConcreteClass : MyAbstractClass { public ConcreteClass() : base() { // ConcreteClass is guaranteed to have // a constructor that calls the base constructor // I just simplified the example } public override int ImplementedVersion { get { return 2; } } } As you see, I call CheckVersion() from the abstract constructor, to get rid of the "virtual member call in base constructor" message, but I am not sure if that's really the way to do it. Sure, it works, but that doesn't mean it will always work, will it? Also, I wonder if I can get the name of the Concrete Type from the CheckVersion() function? I know that adding new abstract members will force an error anyway (System.TypeLoadException) and I'm not sure if I want this type of strict Versioning, but I'm just curious how it would be done properly given only the abstract class and an implementation (I know I could do it by using interfaces and/or a Factory pattern).

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• Class Mapping Error: 'T' must be a non-abstract type with a public parameterless constructor

  - by Amit Ranjan

  Hi, While mapping class i am getting error 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method. Below is my SqlReaderBase Class public abstract class SqlReaderBase<T> : ConnectionProvider { #region Abstract Methods protected abstract string commandText { get; } protected abstract CommandType commandType { get; } protected abstract Collection<IDataParameter> GetParameters(IDbCommand command); **protected abstract MapperBase<T> GetMapper();** #endregion #region Non Abstract Methods /// <summary> /// Method to Execute Select Queries for Retrieveing List of Result /// </summary> /// <returns></returns> public Collection<T> ExecuteReader() { //Collection of Type on which Template is applied Collection<T> collection = new Collection<T>(); // initializing connection using (IDbConnection connection = GetConnection()) { try { // creates command for sql operations IDbCommand command = connection.CreateCommand(); // assign connection to command command.Connection = connection; // assign query command.CommandText = commandText; //state what type of query is used, text, table or Sp command.CommandType = commandType; // retrieves parameter from IDataParameter Collection and assigns it to command object foreach (IDataParameter param in GetParameters(command)) command.Parameters.Add(param); // Establishes connection with database server connection.Open(); // Since it is designed for executing Select statements that will return a list of results // so we will call command's execute reader method that return a Forward Only reader with // list of results inside. using (IDataReader reader = command.ExecuteReader()) { try { // Call to Mapper Class of the template to map the data to its // respective fields MapperBase<T> mapper = GetMapper(); collection = mapper.MapAll(reader); } catch (Exception ex) // catch exception { throw ex; // log errr } finally { reader.Close(); reader.Dispose(); } } } catch (Exception ex) { throw ex; } finally { connection.Close(); connection.Dispose(); } } return collection; } #endregion } What I am trying to do is , I am executine some command and filling my class dynamically. The class is given below: namespace FooZo.Core { public class Restaurant { #region Private Member Variables private int _restaurantId = 0; private string _email = string.Empty; private string _website = string.Empty; private string _name = string.Empty; private string _address = string.Empty; private string _phone = string.Empty; private bool _hasMenu = false; private string _menuImagePath = string.Empty; private int _cuisine = 0; private bool _hasBar = false; private bool _hasHomeDelivery = false; private bool _hasDineIn = false; private int _type = 0; private string _restaurantImagePath = string.Empty; private string _serviceAvailableTill = string.Empty; private string _serviceAvailableFrom = string.Empty; public string Name { get { return _name; } set { _name = value; } } public string Address { get { return _address; } set { _address = value; } } public int RestaurantId { get { return _restaurantId; } set { _restaurantId = value; } } public string Website { get { return _website; } set { _website = value; } } public string Email { get { return _email; } set { _email = value; } } public string Phone { get { return _phone; } set { _phone = value; } } public bool HasMenu { get { return _hasMenu; } set { _hasMenu = value; } } public string MenuImagePath { get { return _menuImagePath; } set { _menuImagePath = value; } } public string RestaurantImagePath { get { return _restaurantImagePath; } set { _restaurantImagePath = value; } } public int Type { get { return _type; } set { _type = value; } } public int Cuisine { get { return _cuisine; } set { _cuisine = value; } } public bool HasBar { get { return _hasBar; } set { _hasBar = value; } } public bool HasHomeDelivery { get { return _hasHomeDelivery; } set { _hasHomeDelivery = value; } } public bool HasDineIn { get { return _hasDineIn; } set { _hasDineIn = value; } } public string ServiceAvailableFrom { get { return _serviceAvailableFrom; } set { _serviceAvailableFrom = value; } } public string ServiceAvailableTill { get { return _serviceAvailableTill; } set { _serviceAvailableTill = value; } } #endregion public Restaurant() { } } } For filling my class properties dynamically i have another class called MapperBase Class with following methods: public abstract class MapperBase<T> where T : new() { protected T Map(IDataRecord record) { T instance = new T(); string fieldName; PropertyInfo[] properties = typeof(T).GetProperties(); for (int i = 0; i < record.FieldCount; i++) { fieldName = record.GetName(i); foreach (PropertyInfo property in properties) { if (property.Name == fieldName) { property.SetValue(instance, record[i], null); } } } return instance; } public Collection<T> MapAll(IDataReader reader) { Collection<T> collection = new Collection<T>(); while (reader.Read()) { collection.Add(Map(reader)); } return collection; } } There is another class which inherits the SqlreaderBaseClass called DefaultSearch. Code is below public class DefaultSearch: SqlReaderBase<Restaurant> { protected override string commandText { get { return "Select Name from vw_Restaurants"; } } protected override CommandType commandType { get { return CommandType.Text; } } protected override Collection<IDataParameter> GetParameters(IDbCommand command) { Collection<IDataParameter> parameters = new Collection<IDataParameter>(); parameters.Clear(); return parameters; } protected override MapperBase<Restaurant> GetMapper() { MapperBase<Restaurant> mapper = new RMapper(); return mapper; } } But whenever I tried to build , I am getting error 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method. Even T here is Restaurant has a Parameterless Public constructor.

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• When to use abstract classes?

  - by bala3569

  Here is the msdn article on abstract class but i really dont get it... When should i really use abstract classes? What are the advantages of using abstract classes?

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• nHibernate, Automapping and Chained Abstract Classes

  - by Mr Snuffle

  I'm having some trouble using nHibernate, automapping and a class structure using multiple chains of abstract classes It's something akin to this public abstract class AbstractClassA {} public abstract class AbstractClassB : AbstractClassA {} public class ClassA : AbstractClassB {} When I attempt to build these mappings, I receive the following error "FluentNHibernate.Cfg.FluentConfigurationException was unhandled Message: An invalid or incomplete configuration was used while creating a SessionFactory. Check PotentialReasons collection, and InnerException for more detail. Database was not configured through Database method." However, if I remove the abstract keyword from AbstractClassB, everything works fine. The problem only occurs when I have more than one abstract class in the class hierarchy. I've manually configured the automapping to include both AbstractClassA and AbstractClassB using the following binding class public class BindItemBases : IManualBinding { public void Bind(FluentNHibernate.Automapping.AutoPersistenceModel model) { model.IncludeBase<AbstractClassA>(); model.IncludeBase<AbstractClassB>(); } } I've had to do a bit of hackery to get around this, but there must be a better way to get this working. Surely nHibernate supports something like this, I just haven't figured out how to configure it right. Cheers, James

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• Abstract class and constructor

  - by Amutha

  As abstract class can be instantiated ,still why constructor is allowed inside abstract class? public abstract class SomeClass { private string _label; public SomeClass(string label) { _label=label; } }

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• Abstract class in Java

  - by keyur

  What is an "abstract class" in Java?

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• Inheritance of Custom Attributes on Abstract Properties

  - by Marty Trenouth

  I've got a custom attribute that I want to apply to my base abstract class so that I can skip elements that don't need to be viewed by the user when displaying the item in HTML. It seems that the properties overriding the base class are not inheriting the attributes. Does overriding base properties (abstract or virtual) blow away attributes placed on the original property? From Attribute class Defination [AttributeUsage(AttributeTargets.Property, Inherited = true, AllowMultiple = false)] public class NoHtmlOutput : Attribute { } From Abstract Class Defination [NoHtmlOutput] public abstract Guid UniqueID { get; set; } From Concrete Class Defination public override Guid UniqueID{ get{ return MasterId;} set{MasterId = value;}} From class checking for attribute Type t = o.GetType(); foreach (PropertyInfo pi in t.GetProperties()) { if (pi.GetCustomAttributes(typeof(NoHtmlOutput), true).Length == 1) continue; // processing logic goes here }

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• Static classes in PHP via abstract keyword?

  - by Boldewyn

  According to the PHP manual, a class like this: abstract class Example {} cannot be instantiated. If I need a class without instance, e.g. for a registry pattern: class Registry {} // and later: echo Registry::$someValue; would it be considered good style to simply declare the class as abstract? If not, what are the advantages of hiding the constructor as protected method compared to an abstract class? Rationale for asking: As far as I see it, it could a bit of feature abuse, since the manual refers to abstract classes more as like blueprints for later classes with instantiation possibility.

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• abstract method signature, inheritance, and "Do" naming convention

  - by T. Webster

  I'm learning about design patterns and in examples of code I've seen a convention where the abstract class declares a method, for example: public abstract class ServiceBase { ... public virtual object GetSomething(); and then protected abstract object DoGetSomething(); My question is on why these two methods exist, since they appear to serve the same purpose. Is this so that the base class GetSomething() method logic cannot be overridden by inherited classes? But then again, the method is marked virtual, so it can be overridden anyway. What is the usefulness here in requiring derived class implementers to implement the abstract method when the virtual method can be called anyway?

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• Scala's sealed abstract.

  - by Lukasz Lew

  What is the difference between sealed abstract and abstract Scala class?

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• Abstract classes in shared library

  - by JTom

  Hi, I have an ordinary abstract class that has couple of pure virtual methods. The class itself is a part of the shared library. The compilation of the shared library itself is OK. But when the library is linked to another program that has another class deriving from the abstract one in the shared library and defining the pure virtual methods, I get the following linker error: I compile like this..: g++ -I../path/to/the/library main.cpp derived.cpp -L../path/to/the/library -lsomename -o shared ...and the linker error is: libsomename.so: undefined reference to `AbstractClass::method()' It's like the abstract class cannot access its pure virtual methods but I do not try to make any instance of the abstract class anywhere in the library. What could be the problem?

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• Python 2.6, 3 abstract base class misunderstanding

  - by Aaron

  I'm not seeing what I expect when I use ABCMeta and abstractmethod. This works fine in python3: from abc import ABCMeta, abstractmethod class Super(metaclass=ABCMeta): @abstractmethod def method(self): pass a = Super() TypeError: Can't instantiate abstract class Super ... And in 2.6: class Super(): __metaclass__ = ABCMeta @abstractmethod def method(self): pass a = Super() TypeError: Can't instantiate abstract class Super ... They both also work fine (I get the expected exception) if I derive Super from object, in addition to ABCMeta. They both "fail" (no exception raised) if I derive Super from list. I want an abstract base class to be a list but abstract, and concrete in sub classes. Am I doing it wrong, or should I not want this in python?

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• Size of abstract class

  - by webgenius

  How can I find the size of an abstract class? class A { virtual void PureVirtualFunction() = 0; }; Since this is an abstract class, I can't create objects of this class. How will I be able to find the size of the abstract class A using the 'sizeof' operator?

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• Create a new instance in a static function of an abstract class

  - by arno

  abstract class db_table { static function get_all_rows() { ... while(...) { $rows[] = new self(); ... } return $rows; } } class user extends db_table { } $rows = user::get_all_rows(); I want to create instances of a class from a static method defined in the abstract parent class but PHP tells me "Fatal error: Cannot instantiate abstract class ..." How should I implement it correctly?

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• Why does Java allow multiple inheritance from interfaces but not from abstract/concrete classes

  - by dan

  Why Java allows multiple inheritance from interfaces but not from abstract or concrete classes

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• Inventory is not abstract and does not override abstract method

  - by Dan

  OK so my applet is not compiling and I Googled some answers and none worked. (Such as taking public out of public class)... Here's my code: http://www.so.pastebin.com/MBjZGneg Heere is my error: C:\Users\Dan\Documents\DanJavaGen\Inventory.java:12: Inventory is not abstract and does not override abstract method keyReleased(java.awt.event.KeyEvent) in java.awt.event.KeyListener public class Inventory extends Applet implements KeyListener { ... help? :) please.

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