Abstract type should not be confused with Abstract data type.
In programming languages, an abstract type (also known as existential types)[1] is a type in a nominative type system that cannot be instantiated directly; by contrast, a concrete type be instantiated directly. Instantiation of an abstract type can occur only indirectly, via a concrete subtype.
An abstract type may provide no implementation, or an incomplete implementation. In some languages, abstract types with no implementation (rather than an incomplete implementation) are known as protocols, interfaces, signatures, or class types. In class-based object-oriented programming, abstract types are implemented as abstract classes (also known as abstract base classes), and concrete types as concrete classes. In generic programming, the analogous notion is a concept, which similarly specifies syntax and semantics, but does not require a subtype relationship: two unrelated types may satisfy the same concept.
Often, abstract types will have one or more implementations provided separately, for example, in the form of concrete subtypes that be instantiated. In object-oriented programming, an abstract class may include abstract methods or abstract properties[2] that are shared by its subclasses. Other names for language features that are (or may be) used to implement abstract types include traits, mixins, flavors, roles, or type classes.
Abstract types may also include any number of non-abstract methods and properties, such as when implementing the Template Method Pattern which uses a mixture of invariant methods with fixed implementations and hook methods which can be overridden in concrete subclasses to provide custonised logic.
Abstract classes can be created, signified, or simulated in several ways:
- [NSObject doesNotRecognizeSelector:(SEL)selector] is invoked upon detection of an unimplemented method).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.
//By default, all methods in all classes are concrete, unless the abstract keyword is used.public abstract class Demo
//By default, all methods in all interfaces are abstract, unless the default keyword is used.interface DemoInterface
Abstract types are an important feature in statically typed OOP languages. Many dynamically typed languages have no equivalent feature (although the use of duck typing makes abstract types unnecessary); however traits are found in some modern dynamically-typed languages.
Some authors argue that classes should be leaf classes (have no subtypes), or else be abstract.[4] [5]
Abstract types are useful in that they can be used to define and enforce a protocol; a set of operations that all objects implementing the protocol must support.
Abstract types are also an essential part of the Template Method Pattern.