The outer type (MutableList in MutableList<String>) can also be marked with ? to indicate that it could be null. So, we have a few combinations:

  • MutableList<String?> is a list of possibly-null strings
  • MutableList<String>? is a possibly-null list of strings
  • MutableList<String?>? is a possibly-null list of possibly-null strings
You can learn more about this in:
Tags:
Nullable Generic Types Nullable Generic Types, Continued
Elvis Returns! Functions on Nullable Types Nullable Casts Nullable Casts, Continued Nullable Generic Types Nullable Generic Types, Continued Nullable Types Safe Call Chains Safe Calls Smart Contracts The Elvis Operator
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