In Julia, as a developer, you can define your own types to structure data used in applications. For example, if you need to represent points in a three-dimensional space, you can define a type Point as follows:

# see the code in Chapter 6\user_defined.jl:
type Point
    x::Float64
    y::Float64
    z::Float64
end

The type Point is a concrete type, objects of this type can be created as p1 = Point(2, 4, 1.3), and it has no subtypes: typeof(p1) returns Point (constructor with 2 methods), subtypes(Point) returns 0-element Array{Any,1}.

Such a user-defined type is composed of a set of named fields with an optional type annotation; that's why it is a composite type, and its type is also DataType. If the type of a named field is not given, then it is Any. A composite type is similar to a struct in C or a class without methods in Java.

Unlike in other object-oriented languages such as Python or Java, where you call a function on an object such as object.func(args)...