How to use the firstOrNull() method of ArrayList in Kotlin

Overview

The firstOrNull() method can be used in two ways.

  1. To get the first element of the ArrayList. If the ArrayList is empty, then null is returned.
  2. To get the first element of the ArrayList that satisfies the provided predicate functionPredicate is a Functional interface, which takes one argument and returns either true or false based on the condition defined. If no element matches the provided predicate function, then null is returned.

Syntax to get the first element

// to get first element 
fun <T> List<T>.firstOrNull(): T?

Parameter

This method doesn’t take any argument.

Return value

Returns the first element if the ArrayList is not empty. Otherwise, null is returned.

Syntax to get the first element that matches the predicate

fun <T> List<T>.firstOrNull(
   predicate: (T) -> Boolean
): T?

Parameter

This method takes a predicate function as an argument.

Return value

This method returns the first element from the list that satisfies the provided the predicate.

If no element matches the given predicate function, then null is returned.

Code

The method below demonstrates how to use the firstOrNull() method.

fun main() {
  var list = ArrayList<Int>()
  println("The ArrayList is $list")
  var firstEle = list.firstOrNull();
  println("The First element is $firstEle");
  list.add(1)
  list.add(2)
  list.add(3)
  list.add(4)
  println("\nThe ArrayList is $list")
  firstEle = list.firstOrNull();
  println("The First element is $firstEle");
  firstEle = list.firstOrNull({it % 2 == 0 })
  println("\nThe first even element is $firstEle")
  firstEle = list.firstOrNull({it > 4})
  println("\nThe firstEle element > 4 is $firstEle")
}

Explanation

In the above code, we see the following:

  • Line 3: We create a new ArrayList with the name list. The list contains no element [].

  • Line 5: We use the firstOrNull method to get the first element of the list. In our case, the list is empty, so null is returned.

  • Lines 8-11: We add four elements to the list using the add method. Now the list is [1,2,3,4].

  • Line 13: We use the firstOrNull method to get the list’s first element(1). The list is not empty, so the first element of the list is returned.

  • Line 16: We use the firstOrNull method with a predicate function as an argument. This function will return true if the element is even. The firstOrNull method will return the first even element of the list. In our case, 2 will be returned.

  • Line 19: We use the firstOrNull method with a predicate function as an argument. This function will return true if the element is greater than 4. In our ArrayList, there is no element that is greater than 4, so null will be returned.

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