Android Question Is this a bug or...?

[kostefar]

I´ve made a simplified version of a problem I struggled with for hours. I´m pretty sure that in an earlier version of b4a, this worked fine.

Dim l As List
Dim m As Map
l.Initialize
m.Initialize
m.Put ("1",2)
l.Add (m)
Log (l)
m.Put ("1",1)
Log (l)

The expected result is that the only item in the list remains the same, since only once in the code is a value being added to it. However, when the content of the map that is previously added to the list, the item in the list is also changing.

(ArrayList) [{1=2}]
(ArrayList) [{1=1}]

I tried with an Int instead of a map, and here the value remains the same = succesful.
I could just do m.initialize before the new value is replacing the old one in the map, but in the app I´m working on, there are multiple values in the map which I don´t want to be reset.
I then tried creating another map to which the change is made, that copies the first map (m2=m) but still the list seems to have a need to stay in sync with the map change.
My solution was to create a new map, and then manually copy each element of the first map to it, m2.put("1",m.get("1") etc. which seems pretty clumsy to me.
But perhaps there´s a good explanation to this, and something I can learn from.

Thanks in advance!

 

[fixit30]

Map keys must be unique, therefore the second m.Put will overwrite the first entry.

 

[Emme Developer]

As write by fixit30:

 

[kostefar]

Map keys must be unique, therefore the second m.Put will overwrite the first entry.

Certainly, but here the value of of the list item is also overwritten. That´s where the problem is.

 

[JordiCP]

The explanation is that non-primitive objects are passed by reference, not copied. So any changes made to m after adding it to l will also be visible from there.

The workaround you describe is the way to go if you want a "snapshot" of the map at a given moment

 

[kostefar]

As write by fixit30:
View attachment 55398

Yes, that explains what maps are supposed to do, but if you look at the code I provided, the value of the list item is updated as well when only the value of the map key is updated.

 

[kostefar]

The explanation is that non-primitive objects are passed by reference, not copied. So any changes made to m after adding it to l will also be visible from there.

The workaround you describe is the way to go if you want a "snapshot" of the map at a given moment

Thanks JordiCP, that made it all more clear. I wonder why I have not struggled more with that then, but now that I know perhaps I can use it in a creative way at some point

 

[Emme Developer]

Certainly, but here the value of of the list item is also overwritten. That´s where the problem is.

Map inside the list is a reference object, so map in list and map external are the same object. When you put a value in it, you are putting a value in list. Right answer is from Jordi

 

[kostefar]

Map inside the list is a reference object, so map in list and map external are the same object. When you put a value in it, you are putting a value in list. Right answer is from Jordi

Yeah, things make sense now!