Mastering High Performance with Kotlin

StringBuilder

Strings are immutable, and whenever you try to modify an existing object, a new instance of the String class is created. The following example demonstrates this:

fun main(vars: Array<String>) {    val cat1 = "Cat"    val cat2 = cat1.plus("Dog")    println(cat1)    println(cat2)    println(cat1 === cat2)    println(cat1 === cat1)}

Here's the output:

CatCatDogfalsetrue

So it's bad practice to use concatenation:

val query = "SELECT id, firstName, lastName FROM Building " + building + " WHERE firstName = " + user.firstName

Instead, you should use the string templates feature:

val query = "SELECT id, firstName, lastName FROM Building $building WHERE firstName = ${user.firstName}"

Under the hood, the string templates feature uses the StringBuilder ...