Kotlin Map withDefault()
Syntax & Examples

Syntax of Map.withDefault()

The syntax of Map.withDefault() extension function is:

fun <K, V> Map<K, V>.withDefault( defaultValue: (key: K) -> V ): Map<K, V>

This withDefault() extension function of Map returns a wrapper of this read-only map, having the implicit default value provided with the specified function defaultValue.

✐ Examples

1 Map with default value

In this example,

We create a map named map1 containing key-value pairs "key1" to "value1" and "key2" to "value2".
We use the withDefault extension function with a default value function that returns "defaultValue".
The resulting map with default value is accessed with a key "key3" that is not present in the original map, returning the default value.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map1 = mapOf("key1" to "value1", "key2" to "value2");
    val mapWithDefault = map1.withDefault { "defaultValue" };
    println(mapWithDefault["key3"]);
}

Output

defaultValue

2 Map with custom default value

In this example,

We create a map named map2 containing key-value pairs "name" to "John Doe" and "age" to "30".
We use the withDefault extension function with a custom default value function that returns a string with the key name for unknown keys.
The resulting map with custom default value is accessed with a key "gender" that is not present in the original map, returning the custom default value.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map2 = mapOf("name" to "John Doe", "age" to "30");
    val mapWithDefault = map2.withDefault { key -> "Unknown key: $key" };
    println(mapWithDefault["gender"]);
}

Output

Unknown key: gender

3 Map with location not found default value

In this example,

We create a map named map3 containing key-value pairs "city" to "New York" and "country" to "USA".
We use the withDefault extension function with a default value function that returns a string indicating the location was not found for the key.
The resulting map with default value is accessed with a key "state" that is not present in the original map, returning the default value indicating the location was not found.
We print the result to standard output.

Kotlin Program

fun main(args: Array<String>) {
    val map3 = mapOf("city" to "New York", "country" to "USA");
    val mapWithDefault = map3.withDefault { key -> "Location not found for: $key" };
    println(mapWithDefault["state"]);
}

Output

Location not found for: state

Summary

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