oop - 如何在 julia 中创建一个 "single dispatch, object-oriented Class"，其行为类似于具有公共(public)/私有(private)字段和方法的标准 Java 类

Question

我在一本书中读到“你不能在 julia 中使用像 obj.myfunc() 这样的单调度方法创建传统的‘类’”……我认为这听起来更像是一个挑战而不是事实。

所以这是我的JavaClass使用公共(public)/私有(private)字段和方法键入只是为了在 Julia 中出现像这样丑陋的东西的纯粹震惊和恐怖因素，毕竟开发人员已经竭尽全力避免它:

type JavaClass

    # Public fields
    name::String

    # Public methods
    getName::Function
    setName::Function
    getX::Function
    getY::Function
    setX::Function
    setY::Function

    # Primary Constructor - "through Whom all things were made."
    function JavaClass(namearg::String, xarg::Int64, yarg::Int64)

        # Private fields - implemented as "closed" variables
        x = xarg
        y = yarg

        # Private methods used for "overloading"
        setY(yarg::Int64) = (y = yarg; return nothing)
        setY(yarg::Float64) = (y = Int64(yarg * 1000); return nothing)

        # Construct object
        this = new()
        this.name = namearg
        this.getName = () -> this.name
        this.setName = (name::String) -> (this.name = name; return nothing)
        this.getX = () -> x
        this.getY = () -> y
        this.setX = (xarg::Int64) -> (x = xarg; return nothing)
        this.setY = (yarg) -> setY(yarg) #Select appropriate overloaded method

        # Return constructed object
        return this
    end

    # a secondary (inner) constructor
    JavaClass(namearg::String) = JavaClass(namearg, 0,0)
end

示例使用:

julia> a = JavaClass("John", 10, 20);

julia> a.name # public
"John"

julia> a.name = "Jim";

julia> a.getName()
"Jim"

julia> a.setName("Jack")

julia> a.getName()
"Jack"

julia> a.x # private, cannot access
ERROR: type JavaClass has no field x

julia> a.getX()
10

julia> a.setX(11)

julia> a.getX()
11

julia> a.setY(2) # "single-dispatch" call to Int overloaded method

julia> a.getY()
2

julia> a.setY(2.0)

julia> a.getY()  # "single-dispatch" call to Float overloaded method
2000

julia> b = JavaClass("Jill"); # secondary constructor

julia> b.getX()
0

本质上，构造函数变成了一个闭包，这就是创建“私有(private)”字段和方法/重载的方式。
有什么想法吗？ (除了“天哪，为什么？？？你为什么要这样做？？”)
还有其他方法吗？
你能想象到什么情况下这可能会严重失败？

Answer 1

Jeff Bezanson's answer很好，但是正如评论中提到的那样，字段可能会被装箱，这很烦人。
这个问题有一个更好的解决方案。
备选方案 1(与问题中提出的方法基本相同):

# Julia
mutable struct ExampleClass
    field_0
    field_1
    method_0
    method_1
    method_2

    function ExampleClass(field_0, field_1)
        this = new()

        this.field_0 = field_0
        this.field_1 = field_1

        this.method_0 = function()
            return this.field_0 * this.field_1
        end

        this.method_1 = function(n)
            return (this.field_0 + this.field_1) * n
        end

        this.method_2 = function(val_0, val_1)
            this.field_0 = val_0
            this.field_1 = val_1
        end

        return this
    end
end

ex = ExampleClass(10, 11)
ex.method_0()
ex.method_1(1)
ex.method_2(20, 22)

备选方案 2:

mutable struct ExampleClass
    field_0
    field_1

    function ExampleClass(field_0, field_1)
        this = new()

        this.field_0 = field_0
        this.field_1 = field_1

        return this
    end
end

function Base.getproperty(this::ExampleClass, s::Symbol)
    if s == :method_0
        function()
            return this.field_0 * this.field_1
        end
    elseif s == :method_1
        function(n)
            return (this.field_0 + this.field_1) * n
        end
    elseif s == :method_2
        function(val_0, val_1)
            this.field_0 = val_0
            this.field_1 = val_1
        end
    else
        getfield(this, s)
    end
end

ex = ExampleClass(10, 11)
ex.method_0()
ex.method_1(1)
ex.method_2(20, 22)

备选方案 1 看起来更好，但备选方案 2 表现更好。
我对此事进行了更深入的分析，您可以在这里查看: https://acmion.com/blog/programming/2021-05-29-julia-oop/