I have a two-level hierarchy with a root and one or more children. Each of these children have a lis...

# getting-started

[elect]

I have a two-level hierarchy with a root and one or more children. Each of these children have a list of elements they need to track, and I'd like also to have somehow a "root" list in the root that incorporates all the children lists.
Is there a (easy/stdlib) way to have just the root list and then each child simply manages a subset/view of this list (removing/adding)?
Ps: no synch problems, everything is serial

[elect]

something in the direction of `subList`(here) but I guess if the root or the first child modifies the list, then I have to update indices for all the other children..

[ephemient]

if the children are immutable, sure. if they are mutable you will have to define your requirements more specifically

[ephemient]

val child = object : AbstractCollection() {
    override val size get() = root.count { isChild }
    override fun iterator() = root.filter { isChild }.iterator()
}

[elect]

each child has a consequential view on the (root) list, ie the first [0-10], the secon [11-40], etc

[Michael de Kaste]

I had the same issue when I had some sealed class which was viewed by the root and the implementations managed by its children.

depending on your usage combination:
• root view, child view
• root view, child mutate
• root mutate, child mutate
• root mutate, child view
there are different solutions.

if you have root view and child mutate, you can simply have root be a value that on every request returns a new list based on the children. You can make it better by defining your own concatenation iterator

[elect]

i have the worst I guess, root mutate, child mutate

[Michael de Kaste]

what needs to happen when root adds? Just add it to the final sublist?

[elect]

yep

[elect]

atm I'm leaning toward a map<name, ArrayList> on the root containing the corresponding children lists and then forwarding the methods I need

[ephemient]

if the root exposes a MutableList, you'll have to figure out what to do with insertions and removals at any index and before or after any listiterator

[ephemient]

if it's a MutableCollection, then you "only" have to figure out what to with removals before any mutableiterator and additions to the whole collection

[nkiesel]

do I understand correctly that each child is defined as an index range of the root list? Thus, a list of items and a list (or map if children are not identified by index) of IntRange would do the trick?