Your Question

When I run your code, this is the output I see:

ArrayList of objects  :
[[Ljava.lang.String;@5b480cf9, [Ljava.lang.String;@6f496d9f, [Ljava.lang.String;@723279cf]
my list of String result
[fruit-Africa, Milk-Europ, Meat-Asia]

And in your question, you have:

I want to have this list of String in my result:

[fruit-africa,milk-Europ,Meat-Asia]

So, it looks like you already have the output you want for result. The only difference from the actual output is the lack of a space after each comma. If you don't want that space, then use joinToString to customize the output:

println(result.joinToString(",", "[", "]"))

As for:

[[Ljava.lang.String;@5b480cf9, [Ljava.lang.String;@6f496d9f, [Ljava.lang.String;@723279cf]

You see that output because arrays don't override the toString() function, and therefore use the default implementation. In Kotlin, you can use contentToString() to get similar output as you see when printing a List.

println(myArrayList.joinToString(", ", "[", "]") { it.contentToString() })

So, here is the updated code with the above changes:

fun main(args: Array<String>) {
    val result: List<String>
    val obj1 = arrayOf("fruit", "Mangue", "Africa")
    val obj2 = arrayOf("Milk", "Soja", "Europ")
    val obj3 = arrayOf("Meat", "cochon","Asia")
    val myArrayList: ArrayList<Array<String>> = ArrayList<Array<String>>(3)
    val myList: MutableList<Array<String>> = mutableListOf<Array<String>>()
    myList.add(obj1)
    myList.add(obj2)
    myList.add(obj3)
    myArrayList.addAll(myList)
    result = myArrayList.stream().map{it -> ("${it[0]}-${it[2]}")}.toList()
    println("ArrayList of objects  :")
    println(myArrayList.joinToString(", ", "[", "]") { it.contentToString() })
    println("my list of String result")
    println(result.joinToString(",", "[", "]"))
}

Which gives this output:

ArrayList of objects  :
[[fruit, Mangue, Africa], [Milk, Soja, Europ], [Meat, cochon, Asia]]
my list of String result
[fruit-Africa,Milk-Europ,Meat-Asia]

Potential Improvements

With all that said, there are a few things you can simplify in your code:

• This is a minor point, but since you don't use the args parameter you can actually omit it.

• Your myList is not necessary; you can add your arrays directly to myArrayList.

• Given the small number of elements in each array, and the small number of arrays, you can actually create the List<Array<String>> and populate it with a single listOf.

  • For variable types, you should prefer using List, the interface, rather than ArrayList, the implementation. This is known as "programming to an interface". Preferring List also means better use of listOf and mutableListOf, which are the idiomatic ways of creating lists in Kotlin.

• You should prefer using List over arrays. In other words, create a List<List<String>> instead of a List<Array<String>>.

  • Lists do override the toString() method, providing readable output. Also, lists have better API support and work better with generics.

• You don't need to use stream(). Kotlin provides many extension functions for arrays and Iterables, one of those being map which returns a List. Yes, these transformation functions are eagerly evaluated, unlike with streams, but given you're only performing one transfomration this doesn't really matter (in fact, the stream is likely less performant).

  • See kotlin.collections for the available built-in extension functions.

• Given you want the first and last elements of each array, I would use first() and last().

Here is the simplified code (I added explicit types to make it clearer what the variables reference):

fun main() {
    val lists: List<List<String>> = listOf(
        listOf("Fruit", "Mangue", "Africa"),
        listOf("Milk",  "Soja",   "Europe"),
        listOf("Meat",  "Cochon", "Asia")
    )
    println("List of lists of strings:")
    println(lists)

    val result: List<String> = lists.map { "${it.first()}-${it.last()}" }
    println("Result:")
    println(result)
}

Output:

List of Arrays:
[[Fruit, Mangue, Africa], [Milk, Soja, Europe], [Meat, Cochon, Asia]]
Result:
[Fruit-Africa, Milk-Europe, Meat-Asia]