String interpolation

Kotlin allows access to variables (and other expressions) directly from within string literals, usually eliminating the need for string concatenation.

Consider the following Java code:

String name = "Alex";
String greeting = "Hello " + name + "!";

which becomes this in Kotlin:

val name = "Alex"
val greeting = "Hello $name!"

$name is replaced by the contents of the variable name, more precisely by a call to its toString() method.

The general syntax is $variable.

Please note in the example that the final ! is automatically interpreted as not belonging to the variable name. This is because the Kotlin compiler interprets everything after the introductory $ as a reference name until it encounters a character that is not part of a common identifier.

To resolve all the ambiguities, or to use more complex expressions, there is the alternative syntax ${expression}, which is to enclose the expression in curly braces.

Thus, the following is also possible:

val person = Person(firstName = "Alex", lastName = "Example")
val greeting = "Hello ${person.firstName}!"