Effective way to append strings separated with comma [Kotlin]

Question

I was wondering if there is an effective way of separating different strings with comma as the separator?

In Java8 there was StringUtils.join(java.lang.Iterable,char).

For Kotlin I only found joinToString, which converts from array/list to string. As I understand, joinToString converts whole list/array. What if I want to convert a few items from array to comma separated string not all of them? How would one do that? Is there something short and elegant from Kotlin (obviously, I can make my own function for this, but I was just wondering).

list.forEachIndexed { index, item ->
   if (item.isGreen) {
       ...        
   }
}

Answer 1

val greenString = list.filter(ItemClass::isGreen).joinToString()

Here, ItemClass is the type of your item which defines the isGreen function or property. ItemClass::isGreen is a reference to this method/property.

You could also use a lambda for the same effect (see other answer).

---

Edit: You can specify how the object should be represented as a String in the joinToString function with the transform argument.
Because this is the last parameter it can be given outside of the regular parentheses:

list.filter(ItemClass::isGreen).joinToString() { it.content.text }

You could even leave off the parentheses all together now but they may be used for other arguments.
You can not use the reference style (::) here because it is a complex expression and not a direct reference to a specific method or property.

Answer 2

For this example you can do this:

list
    .filter { it.isGreen }
    .joinToString()