How to work with Maps in Kotlin

Solution 1:

The reason for your confusion is that plus is not a mutating operator, meaning that it works on (read-only) Map, but does not change the instance itself. This is the signature:

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

What you want is a mutating operator set, defined on MutableMap:

operator fun <K, V> MutableMap<K, V>.set(key: K, value: V)

So your code may be rewritten (with some additional enhancements):

class Person(var name: String, var lastName: String, var age: Int)

val nameTable = mutableMapOf<String, Person>()
val example = Person("Josh", "Cohen", 24)

fun main (args: Array<String>) {
    nameTable["person1"] = example

    for((key, value) in nameTable){
        println(value.age)
    }
}

Solution 2:

The plus-method on Map creates a new map that contains the new entry. It does not mutate the original map. If you want to use this method, you would need to do this:

fun main() {
    val table = nameTable.plus(Pair("person1", example))
    for (entry in table) {
        println(entry.value.age)
    }
}

If you want to add the entry to the original map, you need to use the put method like in Java.

This would work:

fun main() {
    nameTable.put("person1", example)
    for (entry in nameTable) {
        println(entry.value.age)
   }
} 

To get and remove entries from the MutableMap, you can use this:

nameTable["person1"] // Syntactic sugar for nameTable.get("person1")
nameTable.remove("person1")