Effective way to append strings separated with comma [Kotlin]

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).

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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.

For this example you can do this:

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