abstract

class Base {} abstract class A { abstract public List<Base> Items { get; set; } } class Derived : Base {} class B : A { private List<Derived> items; public override List<Derived> Items { get { return items; } set { items = value; } } } The compiler says that B.Items must be List of Base elements "to match overridden member" A.Items. How can i make that work? Eugene Podskal What you've tried to accomplish initially is impossible - .NET does not support co(contra)variance for method overload . The same goes for properties, because properties are just the pair of methods . But you can make your

Are there are differences between hashmap and hashtable in theory? I don't mean in the concrete definitions given in Java (or the implementation), but in theory. Isn't a hashtable a map that uses hashing ... hence a hashmap? woliveirajr According to Wikipedia , they are the same: In computing, a hash table (hash map) is a data structure used to implement an associative array (...) According to Wikibooks , it's the same: A hash table, or a hash map, is a data structure that associates keys with values. Some answer on StackOverflow also states: Hashtable is often useful (they are also called

I'm trying to understand how Java handles cases of ambiguity that come out when a concrete class inherits (abstract or concrete) methods having the same name from different classes/interfaces. I've not been able to find a general rule, this is why I decided, once for all, to spend some time on this by using a practical approach. I considered 8 different cases, combining abstract methods non-abstract methods abstract classes interfaces resulting in this scheme: +-------------------------+ | INTERFACE | +----------+--------------| | abstract | non-abstract | | method | method | +-----------+----

An abstract class need not include any abstract methods. Is there any other reason to make a class abstract other than the fact that abstract classes can't be instantiated? The principal role of an abstract class is to provide an appropriate root class from which concrete, (i.e. non-abstract) subclasses can be derived. This is a powerful and versatile feature which promotes code re-use. Abstract classes encapsulate general features that are common to a range of data types - features which are too general to be meaningful in the abstract class, but which can be overridden in a subclass Any

I am a java developer with good understanding of Object orientation concepts( or maybe, I think like that ). And right now I am learning design patterns (From Head first design patterns). I have been reading about OOPS concept abstraction to understand it briefly, and reading more about it has made me more confusing than I earlier was. As I understand, abstraction refers to hiding the internal details of the program while exposing the interface to other programmers without worries of internal details. But, I don't understand How abstract classes fit into this concept of abstraction, where

I'm wondering if there is any way to do the following. I have an abstract class, Shape , and all its different subclasses and I want to override the clone method. All I want to do in the method is create a new Shape from the toString() of the current one. Obviously I can't do the following because Shape is abstract. Is there another way to do this because overriding clone in every subclass just for a simple name change seems useless. public abstract class Shape { public Shape(String str) { // Create object from string representation } public Shape clone() { // Need new way to do this return

I have an abstract base class class IThingy { virtual void method1() = 0; virtual void method2() = 0; }; I want to say - "all classes providing a concrete instantiation must provide these static methods too" I am tempted to do class IThingy { virtual void method1() = 0; virtual void method2() = 0; static virtual IThingy Factory() = 0; }; I know that doesnt compile, and anyway its not clear how to use it even if it did compile. And anyway I can just do Concrete::Factory(); // concrete is implementation of ITHingy without mentioning Factory in the base class at all. But I feel there should be