Kotlin merge two nullable mutable list

Question

val mutableList1: MutableList<TeamInvitationData?>?
val mutableList2: MutableList<TeamInvitationData?>?

addAll method can be use to merge nullable mutable list but, here it throws me compile time error.

Example:

val map1 = listOne?.map { TeamInvitationData(it) }
val map2 = listTwo?.map { TeamInvitationData(it) }
map1.addAll(map2)

Type interface failed ,Please try to specify type argument explicitly.

Here Any way can I merge this two array , thanks in advance.

Answer 1

Here are couple of solutions.

1. In case if you need to add all elements to mutableList1:

   val mutableList1: MutableList<Any?>? = ...
   val mutableList2: MutableList<Any?>? = ...
   
   mutableList1?.let { list1 -> mutableList2?.let(list1::addAll) }
   

2. In case if you need new nullable list as result:

   val mutableList1: MutableList<Any?>? = ...
   val mutableList2: MutableList<Any?>? = ...
   
   val list3: List<Any?>? = mutableList1?.let { list1 ->
       mutableList2?.let { list2 -> list1 + list2 }
   }
   

3. In case if you need new nullable mutable list as result:

   val mutableList1: MutableList<Any?>? = ...
   val mutableList2: MutableList<Any?>? = ...
   
   val list3: MutableList<Any?>? = mutableList1
           ?.let { list1 -> mutableList2?.let { list2 -> list1 + list2 } }
           ?.toMutableList()
   

4. In case if you need new non-null list as result:

   val mutableList1: MutableList<Any?>? = ...
   val mutableList2: MutableList<Any?>? = ...
   
   val list3: List<Any?> = mutableList1.orEmpty() + mutableList2.orEmpty()