Kotlin List - iterator() Function

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The Kotlin List iterator() function is used to return an iterator for traversing elements in a list. Iterators are useful when we need to process each element of the list one by one.

There are the following use cases of the iterator() function:

• Custom Traversal Logic: It provides fine-grained control over traversal, including conditional checks or early exit.
• Compatibility: This function works with all Kotlin collection.
• Mutable Iterators: For mutable collections, the MutableIterator also includes the remove() function to remove elements during iteration safely.

Syntax

Following is the syntax of Kotlin list iterator() function −

val iterator = List.iterator()

Parameters

This function does not accepts any parameters

Return value

This function returns an iterator object of type Iterator<T>. For mutable collection, it returns a MutableIterator<T>.

Example 1

Let's see a basic example of the iterator() function, which iterates over elements of list.

fun main(args: Array<String>) {
   val list = listOf(10, 20, 30, 40, 50)

   // Get the iterator
   val iterator = list.iterator()

   // Traverse using iterator functions
   while (iterator.hasNext()) {
      val element = iterator.next()
      println("Element: $element")
   }
}

Output

Following is the output −

Element: 10
Element: 20
Element: 30
Element: 40
Element: 50

Example 2: Removing Element During Iteration

The following example demonstrate the flexibility of iterator() function, when working with mutable collection −

fun main(args: Array<String>) {
   val numbers = mutableListOf(1, 2, 3, 4, 5)
   val iterator = numbers.iterator()

   while (iterator.hasNext()) {
      val number = iterator.next()
      if (number % 2 == 0) {
         // Remove even numbers
         iterator.remove()
      }
   }
   println(numbers)
}

Output

Following is the output −

[1, 3, 5]

Example 3: Removing String

This is another example of iterator() function to remove strings having less than 5 character in the mutableList −

fun main(args: Array<String>) {
   val words = mutableListOf("apple", "bat", "cat", "elephant", "dog", "frog")
   
   // Get the iterator
   val iterator = words.iterator()
   
   // Remove words less than 5 characters
   while (iterator.hasNext()) {
      val word = iterator.next()
      if (word.length < 5) {
         iterator.remove()
      }
   }
   println("Filtered List: $words")
}

Output

Following is the output −

Filtered List: [apple, elephant]