Abstract type

In programming languages, an abstract type is a type in a nominative type system that cannot be instantiated directly; a type that is not abstract – which can be instantiated – is called a concrete type. Every instance of an abstract type is an instance of some concrete subtype. Abstract types are also known as existential types.
An abstract type may provide no implementation, or an incomplete implementation. In some languages, abstract types with no implementation are known as protocols, interfaces, signatures, or class types. In class-based object-oriented programming, abstract types are implemented as abstract 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 can be instantiated. In object-oriented programming, an abstract class may include abstract methods or abstract properties that are shared by its subclasses. Other names for language features that are used to implement abstract types include traits, mixins, flavors, roles, or type classes.

Signifying abstract types

Abstract classes can be created, signified, or simulated in several ways:

//By default, all methods in all classes are concrete, unless the abstract keyword is used.
abstract class Demo
//By default, all methods in all interfaces are abstract, unless the default keyword is used.
interface DemoInterface

Use of abstract types

Abstract types are an important feature in statically typed OOP languages. Many dynamically typed languages have no equivalent feature ; however traits are found in some modern dynamically-typed languages.
Some authors argue that classes should be leaf classes, or else be abstract.
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.