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A class diagram exemplifying the singleton pattern.. In object-oriented programming, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance.
Some languages take consideration of some special types of constructors. Constructors, which concretely use a single class to create objects and return a new instance of the class, are abstracted by factories, which also create objects but can do so in various ways, using multiple classes or different allocation schemes such as an object pool.
By default, all methods in all classes are concrete, unless the abstract keyword is used. An abstract class may include abstract methods, which have no implementation. By default, all methods in all interfaces are abstract, unless the default keyword is used. The default keyword can be used to specify a concrete method in an interface.
Other languages, notably Java and C#, support a variant of abstract classes called an interface via a keyword in the language. In these languages, multiple inheritance is not allowed, but a class can implement multiple interfaces. Such a class can only contain abstract publicly accessible methods. [22] [28] [29]
C# supports classes with properties. The properties can be simple accesor functions with a backing field, or implement arbitrary getter and setter functions. A property is read-only if there's no setter. Like with fields, there can be class and instance properties. The underlying methods can be virtual or abstract like any other method. [82]
In class-based programming, a factory is an abstraction of a constructor of a class, while in prototype-based programming a factory is an abstraction of a prototype object. A constructor is concrete in that it creates objects as instances of one class, and by a specified process (class instantiation), while a factory can create objects by instantiating various classes, or by using other ...
Interfaces and abstract classes are similar. The following describes some important differences: An abstract class may have member variables as well as non-abstract methods or properties. An interface cannot. A class or abstract class can only inherit from one class or abstract class. A class or abstract class may implement one or more interfaces.
The factory method design pattern solves problems such as: How can an object's subclasses redefine its subsequent and distinct implementation? The pattern involves creation of a factory method within the superclass that defers the object's creation to a subclass's factory method.