Abstract type
（重定向自Abstract base class）
In programming languages, an abstract type is a type in a nominative type system that cannot be instantiated directly. Abstract types are also known as existential types. An abstract type may provide no implementation, or an incomplete implementation. Often, abstract types will have one or more implementations provided separately, for example, in the form of concrete subclasses that can be instantiated. It may include abstract methods or abstract properties that are shared by its subtypes.