Strings

Unicode correctness can be onerous in Python 2, since the "default" string type str is really just a byte array, while unicode is actually a sequence of code units (see below) - and whether the code units are 16 or 32 bits wide depends on how your Python distribution was built. In Kotlin, there's no such confusion: String, which is what you get when you make a string literal (which you can only do with double quotes), is an immutable sequence of UTF-16 code units. ByteArray is a fixed-size (but otherwise mutable) byte array (and String can specifically not be used as a byte array).

A UTF-16 code unit is a 16-bit unsigned integral value that represents either one Unicode code point (character code) or must be combined with another code unit to form a code unit. If this makes no sense, I strongly recommend Joel Spolsky's excellent essay on Unicode and its encodings. For most Western scripts, including English, all code points fit inside one code unit, so it's tempting to think of a code unit as a character - but that will lead astray once your code encounters non-Western scripts. A single UTF-16 code unit can be represented with single quotes, and has the type Char:

val c = 'x' // Char
val message = "Hello" // String
val m = message[0] // Char

Thus, single quotes can not be used to form string literals.

Given a string s, you can get a ByteArray with the UTF-8 encoding of the string by calling s.toByteArray(), or you can specify another encoding, e.g. s.toByteArray(Charsets.US_ASCII) - just like encode() in Python. Given a byte array b that contains a UTF-8-encoded string, you can get a String by calling String(b); if you've got a different encoding, use e.g. String(b, Charsets.US_ASCII), just like decode() in Python. You can also call e.g. b.toString(Charsets.US_ASCII), but do not call b.toString() without parameters (this will just print an internal reference to the byte array).

You can do string interpolation with $, and use curly braces for expressions:

val name = "Anne"
val yearOfBirth = 1985
val yearNow = 2018
val message = "$name is ${yearNow - yearOfBirth} years old"

If you want a literal $, you need to escape it: \$. Escaping generally works the same way as in Python, with a similar set of standard escape sequences.

This material was written by Aasmund Eldhuset; it is owned by Khan Academy and is licensed for use under CC BY-NC-SA 3.0 US. Please note that this is not a part of Khan Academy's official product offering.