Julia 1.0 Programming Complete Reference Guide

Type hierarchy and inheritance

Like all programming languages that implement OOP features, Julia allows developers to define rich and expressive type hierarchies. However, unlike most OOP languages, there is a very important difference—only the final (upper) type in the hierarchy can be instantiated in Julia. All its parents are just nodes in the type graph, and we can't create instances of them. They are abstract types and are defined using the abstract type keywords:

julia> abstract type Person end 

We can use the <: operator to indicate that a type is a subtype of an existing parent:

julia> abstract type Mammal end 
julia> abstract type Person <: Mammal end 
julia> mutable struct Player <: Person 
           username::String 
           score::Int 
       end 

Or, in another ...