Android Question Create Object Instance of B4A Object Using The Typename

Andrew King · Monday at 8:01 PM

I am trying to find a way to create an instance of any type of object using the typename.

I need to create objects at runtime. I know this is kind of frowned upon but in some cases when writing classes to be used in other projects, this function is needed.

To be used like this:

B4X:

Dim obj as object
obj = CreateInstance("b4a.example.fruit")

This is a very simple example that doesn't make much sense to any application but it demonstrates how it could be used.

Is there a function or existing library that can do this and if not what would be the best way to accomplish this?

Daestrum · Monday at 11:58 PM

Couldn't you just use the JavaObject library?

Andrew King · Tuesday at 2:33 AM

Daestrum said:
Couldn't you just use the JavaObject library?

JavaObject.InitializeNewInstance() returns a JavaObject. I need the object to be a B4A object that is typed by using the specified typename string for my purposes.

Andrew King · Tuesday at 3:08 AM

What I'm trying to do is use reflector to read each objects' fields and values that are stored in a list and then transform the data with a function to say a format like XML, CSV then write it to a file.

Then I want to use another function to read the data from the XML or CSV or however I stored it and it create an instance of each object by passing it the typename in a parameter.

I don't want to use stringutils or any other library. I have a specific need for doing it this way. I've coded this in VB.net with ease but b4x doesnt have a GetType(Type) function or Type as Type. It does however have a reflector and javaObject class. Then the only way is to have a CreateInstance function that you pass the typename in string format to, but it must create the instance of the object in a type not derived from javaobject but as the type specified in the typename string parameter. This is needed because the object will later be used in its typed form.

aeric · Tuesday at 3:52 AM

I don't get it.
Isn't you can create instance of a Class or Custom Type?

teddybear · Tuesday at 4:56 AM

Andrew King said:
I don't want to use stringutils or any other library. I have a specific need for doing it this way. I've coded this in VB.net with ease but b4x doesnt have a GetType(Type) function or Type as Type.

Why do you say that? B4X also has the method GetType(object) to get the type of the Object.

drgottjr · Tuesday at 5:58 AM

the problem with b4x's gettype(object) is that it returns a string. i think op wants to capture the object's type (not its stringified representation) and then use it later in some method which uses the type (not a string) as the passed parameter. if i understand what he wants, and i'm not sure i do. i don't understand whether he's looking for an instance of b4a.example.fruit or an instance of "b4a.example.fruit"

aeric · Tuesday at 6:27 AM

B4X:

Dim Mango As Fruit
Mango.Initialize
Mango.Name = "Mango"
Mango.Origin = "Thailand"
Log(GetType(Mango))            ' b4a.example.main$_fruit

Dim Durian As TropicalFruit
Durian.Initialize
Durian.Name = "Musang King"
Durian.Origin = "Malaysia"
Log(GetType(Durian))           ' b4a.example.tropicalfruit

Dim afruit As Object
afruit = Durian
If GetType(afruit) = "b4a.example.tropicalfruit" Then
    MsgboxAsync("I love Tropical Fruit", afruit.As(TropicalFruit).Name)
End If

Andrew King · Tuesday at 6:20 PM

drgottjr said:
the problem with b4x's gettype(object) is that it returns a string. i think op wants to capture the object's type (not its stringified representation) and then use it later in some method which uses the type (not a string) as the passed parameter. if i understand what he wants, and i'm not sure i do. i don't understand whether he's looking for an instance of b4a.example.fruit or an instance of "b4a.example.fruit"

Right. I'm wanting to get the type by whatever means and then create a new instance of it at runtime not at compile time. Instance of b4a.example.fruit or whatever type that is passed to the function at runtime not compile time.

Andrew King · Tuesday at 6:21 PM

aeric said:

B4X:

Dim Mango As Fruit
Mango.Initialize
Mango.Name = "Mango"
Mango.Origin = "Thailand"
Log(GetType(Mango))            ' b4a.example.main$_fruit

Dim Durian As TropicalFruit
Durian.Initialize
Durian.Name = "Musang King"
Durian.Origin = "Malaysia"
Log(GetType(Durian))           ' b4a.example.tropicalfruit

Dim afruit As Object
afruit = Durian
If GetType(afruit) = "b4a.example.tropicalfruit" Then
    MsgboxAsync("I love Tropical Fruit", afruit.As(TropicalFruit).Name)
End If

View attachment 158536

So now I want to create an instance of this tropicalfruit or fruit not at compile time but at runtime.

aeric · Tuesday at 6:26 PM

Andrew King said:
So now I want to create an instance of this tropicalfruit or fruit not at compile time but at runtime.

so now you are asking a new question?

Andrew King · Tuesday at 6:34 PM

In vb.net I can:

VB.NET:

Public Class Fruit
    Public Property Name as string
    Public Property Color as string
End Class

Public Class Person
    Public Property FirstName as string
    Public Property LastName as string
End Class

Public Module Main
    Public Function RandomObject() as object
        Dim obj as object

        Dim rand = random.next(0,1)

        Select case rand
            case 0
                CreateInstance(gettype(Fruit))
            case 1
                CreateInstance(gettype(Person))
        End Case
    
        Return obj
    End Function

    Public Function CreateInstance(type as type) as Object
        Dim obj = Activator.CreateInstance(type)
        Return obj
    End Function

End Module

I'm trying to do something exactly like this in b4A. Something like vb.net's activator.createinstance. There is JavaObject.InitializeNewInstance but it returns a JavaObject. How to convert this javaobject into a b4a object? Or what other function can create an instance of a b4a object by providing the type or typeName?

Andrew King · Tuesday at 6:34 PM

aeric said:
so now you are asking a new question?

No... I think I'm not explaining myself sufficiently. See the next example I posted.

Andrew King · Tuesday at 6:51 PM

I need to be able to pass the type or typename as a parameter in a function and use that to create an instance of that specified type.

Andrew King · Tuesday at 7:01 PM

aeric said:
so now you are asking a new question?

The .As function will not let me pass a type as a parameter. I have to hard code it. But I want to be able to use any type when I call that function.
I want to use the type as a variable.

Andrew King · Tuesday at 7:11 PM

I want to create a class

B4X:

Public Class Fruit
    Public Name as string
    Public Color as string
End Class

Then I want to have one function that gets the type

B4X:

Dim typeName as string = GetType(fruit)

Then I want to have another function I can pass the type to and create an instance

B4X:

Sub CreateInstance(PassedType as string) as object
    Dim obj as object
    'code goes here to cast the object into the passedtype
    return obj
End Sub

aeric · Tuesday at 7:34 PM

Andrew King said:
I want to create a class

B4X:

Public Class Fruit Public Name as string Public Color as string End Class

Then I want to have one function that gets the type

B4X:

Dim typeName as string = GetType(fruit)

Then I want to have another function I can pass the type to and create an instance

B4X:

Sub CreateInstance(PassedType as string) as object Dim obj as object 'code goes here to cast the object into the passedtype return obj End Sub

I still not get you.
You have everything already.
When you want to use it, you just need to check the type.

aeric · Tuesday at 7:39 PM

Andrew King said:
In vb.net I can:

VB.NET:

Public Class Fruit Public Property Name as string Public Property Color as string End Class Public Class Person Public Property FirstName as string Public Property LastName as string End Class Public Module Main Public Function RandomObject() as object Dim obj as object Dim rand = random.next(0,1) Select case rand case 0 CreateInstance(gettype(Fruit)) case 1 CreateInstance(gettype(Person)) End Case Return obj End Function Public Function CreateInstance(type as type) as Object Dim obj = Activator.CreateInstance(type) Return obj End Function End Module

I'm trying to do something exactly like this in b4A. Something like vb.net's activator.createinstance. There is JavaObject.InitializeNewInstance but it returns a JavaObject. How to convert this javaobject into a b4a object? Or what other function can create an instance of a b4a object by providing the type or typeName?

B4X:

TxtName.Text = "B4X"
ShowTheObject(RandomObject)

B4X:

Sub RandomObject As Object
    Select Rnd(0, 2)
        Case 0
            Return CreateFruit(TxtName.Text, "", "")
        Case Else
            Dim someone As Person
            someone.Initialize
            someone.FirstName = TxtName.Text
            Return someone
    End Select
End Sub

Sub ShowTheObject (obj As Object)
    Select GetType(obj)
        Case "b4a.example.person"
            MsgboxAsync("The person first name is " & obj.As(Person).FirstName, "Person")
        Case "b4a.example.main$_fruit"
            MsgboxAsync("The fruit name is " & obj.As(Fruit).Name, "Fruit")
    End Select
End Sub

Andrew King · Tuesday at 7:42 PM

I believe the solution is:

B4X:

Public Sub CreateInstance(TypeName As String, Params() As Object) As Object
    Dim jo As JavaObject
    Return jo.InitializeNewInstance(TypeName,Params).As(Object)
End Sub

Thanks to aeric for his examples that led me to the solution. Thank you all for your help and time

aeric · Tuesday at 7:54 PM

Andrew King said:
I believe the solution is:

B4X:

Public Sub CreateInstance(TypeName As String, Params() As Object) As Object Dim jo As JavaObject Return jo.InitializeNewInstance(TypeName,Params).As(Object) End Sub

Thanks to aeric for his examples that led me to the solution. Thank you all for your help and time

edit: ~~How do you know the parameters for the constructor?~~ ok, it is possible.
I don't think you need to use JavaObject in this case.

Android Question Create Object Instance of B4A Object Using The Typename

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