Kotlin Primer

Strings

Strings in Kotlin have some really nice enhancements over Java

In-String Expressions

You can evaluate expressions inside Strings, which makes it much simpler than the concatenation needed in Java.

class Person(
    var name: String
)

fun main() {
    val person = Person("Scott")
    val numberOfChaptersRead = 42

    val message = "${person.name} has read $numberOfChaptersRead chapters!"

    println(message)
}

Expressions can be included inside ${...}. If the expression is just a simple reference, such as numberOfChaptersRead, you can usually just prepend $ in front of it, and it will be evaluated.

Sometimes, even for simple references, you need the full ${...}. For example, if the reference is followed by a character that can be part of a property name, you need the ${...} to mark where your actual property name ends.

fun main() {
    val term = "fall"
    val termMessage = 
        if (term == "summer")
            "Note: Summer terms are held online. "
        else
            ""

    val uglyMessageWithExtraSpace = 
        "Welcome to the class! $termMessage We'll be exploring ..."
    val betterLookingMessage =
        "Welcome to the class! ${termMessage}We'll be exploring ..."

    println(uglyMessageWithExtraSpace)
    println(betterLookingMessage)
}

In this example, because we didn't want two spaces when the termMessage was blank, we either need to append the "$" directly after the "!" or use the full syntax when adjacent to the "W". Try changing the term to fall to see the difference

Multi-line Strings

Sometimes we need longer Strings that may span multiple lines. While we can use "+" like in Java to concatenate

fun main() {
    val message = 
        "Hello there.\n" +
        "This is a message that spans " +
        "multiple lines in the source code."
    println(message)
}

The \n is a literal new-line character and splits the output across lines.

We can use a simpler format called a "Multi-line String" (sometimes called "Raw String", surrounded by three quotes on each end

fun main() {
    val message = """
    Hello there.
    This is a message that spans multiple lines in the source code.
    """
    println(message)
}

This preserves the line breaks and indentation inside the string. Note that when we run we see a blank line, followed by the text, indented.

You can reduce the indents by the least indented line using

fun main() {
    val message = """
    Hello there.
    This is a message that spans multiple lines in the source code.
       This line is indented.
    This one is not.
    """.trimIndent()
    println(message)
}

Or explicitly control the indentation using a margin indicator (| is the default)

fun main() {
    val message = """
    |Hello there.
    |This is a message that spans multiple lines in the source code.
    |   This line is indented.
    |This one is not.
    """.trimMargin()
    println(message)
}

Note that escape sequences like \n won't work inside multiline strings.