How to work with Maps in Kotlin - maps

Question

The code below is creating a new map called nameTable, then adding an entry named example to it, then trying to print the name property of the Value.
When I run it, it seems that the plus operation didn't add a new entry to the map like I thought it would.
So what am I doing wrong?
class Person(name1: String, lastName1: String, age1: Int){
var name: String = name1
var lastName: String = lastName1
var age: Int = age1
}
var nameTable: MutableMap<String, Person> = mutableMapOf()
var example = Person("Josh", "Cohen", 24)
fun main (args: Array<String>){
nameTable.plus(Pair("person1", example))
for(entry in nameTable){
println(entry.value.age)
}
}
While we're at it, I would love some examples of how to add, remove, and get an entry from a map.

Answer 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)
}
}

Answer 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")

Answer 3

It's too much trouble，You can assign values directly，like this:
#Test
#Throws(Exception::class)
fun main(){
val map:MutableMap<String,Person> = mutableMapOf()
map["Josh"]= Person("Josh", "Cohen", 24)
map.forEach { t, u ->
println("map key:$t,map value:${u.toString()}")
}
}
class Person(name1:String, lastName1:String, age1:Int){
var name:String = name1
var lastName:String = lastName1
var age:Int = age1
override fun toString(): String {
return "name:$name,lastNam:$lastName,age:$age \n"
}
}

Answer 4

You have to use
put
method.
class Person(name1:String, lastName1:String, age1:Int){
var name:String = name1
var lastName:String = lastName1
var age:Int = age1
}
var nameTable:MutableMap<String, Person> = mutableMapOf()
var example = Person("Josh", "Cohen", 24)
fun main (args: Array<String>){
nameTable.put("person1", example)
for(entry in nameTable){
println(entry.value.age)
}
}

Answer 5

val params: MutableMap<String, String> = HashMap<String, String>()
params.put("1", "A")
params.put("2", "B")