String a = "Hello ";
String b = a.concat("World"); // b = Hello World

val a = "Hello"
val b = "World"
val c = "$a $b"

val a = "Hello"
val b = "World"
val c = a + b   // same as calling operator function a.plus(b)

print(c)

val a = "Hello"
val b = "World"

val sb = StringBuilder()
sb.append(a).append(b)
val c = sb.toString()

print(c)

var fn = "Hello"
var ln = "World"

a.plus(b)