UselessCallOnCollection

Reports redundant calls on collections, iterables, and sequences from the standard library.

This inspection detects two kinds of redundancy:

• Redundant filter… calls on already filtered collections. Functions such as filterNotNull() or filterIsInstance have sense only when they are called on receivers that have types distinct from the resulting one. Otherwise, such calls can be omitted as the result will be the same.
• Redundant mapNotNull calls (and its variants). mapNotNull is redundant when the transformation result is already guaranteed to be non-nullable. In that case, the NotNull suffix can be dropped (e.g., mapNotNull → map).

A quick-fix is available to omit the redundant call or replace it with a simpler alternative.

Examples:

fun test(list: List<String>) {
      val x = list.filterNotNull() // can be simplified to 'list'
      val y = list.filterIsInstance<String>() // can be simplified to 'list'
  }

fun test(list: List<Int>) {
      val x = list.mapNotNull { it + 1 } // can be replaced with 'map'
  }