Just to chime in late here, I think that there’s no reason to stop somebody from instantiating the abstract class, especially because they can add methods to it on the fly. Duck-typing languages, like Ruby, use the presence/absence or behavior of methods at runtime to determine whether they should be called or not. Therefore your … Read more
You can think of an interface as a “contract”. You are defining a set of methods that classes which implement this interface must implement. An abstract class, on the other hand, is used when you have some code that could be common to all the child classes you want to implement. So you might have … Read more
Abstract classes are useful when you need a class for the purpose of inheritance and polymorphism, but it makes no sense to instantiate the class itself, only its subclasses. They are commonly used when you want to define a template for a group of subclasses that share some common implementation code, but you also want … Read more
Interfaces cannot have state associated with them. Abstract classes can have state associated with them. Furthermore, default methods in interfaces need not be implemented. So in this way, it will not break already existing code, as while the interface does receive an update, the implementing class does not need to implement it. As a result … Read more
Because there might be a standard way you want to instantiate data in the abstract class. That way you can have classes that inherit from that class call the base constructor. public abstract class A{ private string data; protected A(string myString){ data = myString; } } public class B : A { B(string myString) : … Read more
Python 3.3+ from abc import ABCMeta, abstractmethod class A(metaclass=ABCMeta): def __init__(self): # … pass @property @abstractmethod def a(self): pass @abstractmethod def b(self): pass class B(A): a = 1 def b(self): pass Failure to declare a or b in the derived class B will raise a TypeError such as: TypeError: Can’t instantiate abstract class B with … Read more
@Oddthinking’s answer is not wrong, but I think it misses the real, practical reason Python has ABCs in a world of duck-typing. Abstract methods are neat, but in my opinion they don’t really fill any use-cases not already covered by duck typing. Abstract base classes’ real power lies in the way they allow you to … Read more
You can’t instantiate abstract classes, thus a vector of abstract classes can’t work. You can however use a vector of pointers to abstract classes: std::vector<IFunnyInterface*> ifVec; This also allows you to actually use polymorphic behaviour – even if the class wasn’t abstract, storing by value would lead to the problem of object slicing.
Use the abc module to create abstract classes. Use the abstractmethod decorator to declare a method abstract, and declare a class abstract using one of three ways, depending upon your Python version. In Python 3.4 and above, you can inherit from ABC. In earlier versions of Python, you need to specify your class’s metaclass as … Read more
I will give you an example first: public interface LoginAuth{ public String encryptPassword(String pass); public void checkDBforUser(); } Suppose you have 3 databases in your application. Then each and every implementation for that database needs to define the above 2 methods: public class DBMySQL implements LoginAuth{ // Needs to implement both methods } public class … Read more