Kotlin Tutorials

Kotlin Map plus()
Syntax & Examples

Syntax of Map.plus()

There are 5 variations for the syntax of Map.plus() extension function. They are:

1.
operator fun <K, V> Map<out K, V>.plus( pair: Pair<K, V> ): Map<K, V>

This extension function creates a new read-only map by replacing or adding an entry to this map from a given key-value pair.

2.
operator fun <K, V> Map<out K, V>.plus( pairs: Iterable<Pair<K, V>> ): Map<K, V>

This extension function creates a new read-only map by replacing or adding entries to this map from a given collection of key-value pairs.

3.
operator fun <K, V> Map<out K, V>.plus( pairs: Array<out Pair<K, V>> ): Map<K, V>

This extension function creates a new read-only map by replacing or adding entries to this map from a given array of key-value pairs.

4.
operator fun <K, V> Map<out K, V>.plus( pairs: Sequence<Pair<K, V>> ): Map<K, V>

This extension function creates a new read-only map by replacing or adding entries to this map from a given sequence of key-value pairs.

5.
operator fun <K, V> Map<out K, V>.plus( map: Map<out K, V> ): Map<K, V>

This extension function creates a new read-only map by replacing or adding entries to this map from another map.


✐ Examples

1 Add a single entry to the map

In this example,

  • We create a map named map1 containing key-value pairs 1 to 'a' and 2 to 'b'.
  • We create a new key-value pair 3 to 'c'.
  • We use the plus() operator to add the new pair to map1.
  • The resulting map contains the added entry.
  • We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf(1 to 'a', 2 to 'b');
    val pair = 3 to 'c';
    val result = map1.plus(pair);
    print(result);
}

Output

{1=a, 2=b, 3=c}

2 Add multiple entries to the map

In this example,

  • We create a map named map2 containing key-value pairs 'a' to 1 and 'b' to 2.
  • We create a list of key-value pairs ('c' to 3) and ('d' to 4).
  • We use the plus() operator to add the list of pairs to map2.
  • The resulting map contains the added entries.
  • We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf('a' to 1, 'b' to 2);
    val pairs = listOf('c' to 3, 'd' to 4);
    val result = map2.plus(pairs);
    print(result);
}

Output

{a=1, b=2, c=3, d=4}

3 Add entries from an array to the map

In this example,

  • We create a map named map3 containing key-value pairs 1 to "apple" and 2 to "banana".
  • We create an array of key-value pairs (3 to "cherry") and (4 to "date").
  • We use the plus() operator to add the array of pairs to map3.
  • The resulting map contains the added entries.
  • We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf(1 to "apple", 2 to "banana");
    val array = arrayOf(3 to "cherry", 4 to "date");
    val result = map3.plus(array);
    print(result);
}

Output

{1=apple, 2=banana, 3=cherry, 4=date}

Summary

In this Kotlin tutorial, we learned about plus() extension function of Map: the syntax and few working examples with output and detailed explanation for each example.