Kotlin Strings

Introduction

Strings in Kotlin are objects that represent sequences of characters. They are immutable, meaning that once a string is created, its value cannot be changed. Kotlin provides a rich set of methods and properties to manipulate strings effectively. This chapter will cover the basics of creating strings, common string operations, string templates, and string interpolation in Kotlin.

Creating Strings

In Kotlin, strings can be created using string literals, which are enclosed in double quotes.

Example

fun main() {
    val greeting = "Hello, Kotlin!"
    println(greeting)
}

Explanation:

• val greeting = "Hello, Kotlin!": Declares a string variable named greeting and initializes it with the value "Hello, Kotlin!".
• println(greeting): Prints the value of the greeting variable.

Output:

Hello, Kotlin!

Common String Operations

1. String Length

You can get the length of a string using the length property.

Example

fun main() {
    val text = "Kotlin"
    println("Length: ${text.length}")
}

Explanation:

• text.length: Returns the number of characters in the text string.

Output:

Length: 6

2. Accessing Characters

You can access individual characters in a string using the index operator [].

Example

fun main() {
    val text = "Kotlin"
    println("First character: ${text[0]}")
    println("Last character: ${text[text.length - 1]}")
}

Explanation:

• text[0]: Returns the first character of the text string.
• text[text.length - 1]: Returns the last character of the text string.

Output:

First character: K
Last character: n

3. Substring

You can extract a substring from a string using the substring function.

Example

fun main() {
    val text = "Hello, Kotlin!"
    val substring = text.substring(7, 13)
    println("Substring: $substring")
}

Explanation:

• text.substring(7, 13): Extracts a substring from the text string starting at index 7 and ending at index 12 (excluding 13).

Output:

Substring: Kotlin

4. String Concatenation

You can concatenate strings using the + operator or string templates.

Example

fun main() {
    val firstName = "John"
    val lastName = "Doe"
    val fullName = firstName + " " + lastName
    println("Full name: $fullName")
}

Explanation:

• firstName + " " + lastName: Concatenates the firstName and lastName strings with a space in between.

Output:

Full name: John Doe

String Templates

String templates allow you to embed expressions inside string literals. The expressions are evaluated and their results are concatenated into the string.

Example

fun main() {
    val name = "Alice"
    val age = 25
    val message = "My name is $name and I am $age years old."
    println(message)
}

Explanation:

• $name: Embeds the value of the name variable into the string.
• $age: Embeds the value of the age variable into the string.

Output:

My name is Alice and I am 25 years old.

Complex Expressions

For complex expressions, you can use curly braces {} inside string templates.

Example

fun main() {
    val a = 5
    val b = 10
    val sum = "The sum of $a and $b is ${a + b}."
    println(sum)
}

Explanation:

• ${a + b}: Evaluates the expression a + b and embeds the result into the string.

Output:

The sum of 5 and 10 is 15.

String Functions

Kotlin provides a variety of built-in functions to manipulate strings.

1. toUpperCase and toLowerCase

Converts all characters in a string to uppercase or lowercase.

Example

fun main() {
    val text = "Kotlin"
    println("Uppercase: ${text.toUpperCase()}")
    println("Lowercase: ${text.toLowerCase()}")
}

Explanation:

• text.toUpperCase(): Converts all characters in text to uppercase.
• text.toLowerCase(): Converts all characters in text to lowercase.

Output:

Uppercase: KOTLIN
Lowercase: kotlin

2. trim

Removes leading and trailing whitespace from a string.

Example

fun main() {
    val text = "   Kotlin   "
    println("Trimmed: '${text.trim()}'")
}

Explanation:

• text.trim(): Removes leading and trailing whitespace from text.

Output:

Trimmed: 'Kotlin'

3. replace

Replaces occurrences of a substring within a string with another substring.

Example

fun main() {
    val text = "Hello, Kotlin!"
    val newText = text.replace("Kotlin", "World")
    println(newText)
}

Explanation:

• text.replace("Kotlin", "World"): Replaces "Kotlin" with "World" in the text string.

Output:

Hello, World!

4. split

Splits a string into a list of substrings based on a delimiter.

Example

fun main() {
    val text = "one,two,three"
    val parts = text.split(",")
    println(parts)
}

Explanation:

• text.split(","): Splits the text string into a list of substrings using the comma , as the delimiter.

Output:

[one, two, three]

5. joinToString

Joins elements of a collection into a single string with a specified delimiter.

Example

fun main() {
    val parts = listOf("one", "two", "three")
    val text = parts.joinToString(", ")
    println(text)
}

Explanation:

• parts.joinToString(", "): Joins the elements of the parts list into a single string, separated by ,.

Output:

one, two, three

Example Program with Strings

Here is an example program that demonstrates the use of various string operations and functions in Kotlin:

fun main() {
    // Creating strings
    val greeting = "Hello, Kotlin!"
    println(greeting)

    // String length
    val text = "Kotlin"
    println("Length: ${text.length}")

    // Accessing characters
    println("First character: ${text[0]}")
    println("Last character: ${text[text.length - 1]}")

    // Substring
    val substring = greeting.substring(7, 13)
    println("Substring: $substring")

    // String concatenation
    val firstName = "John"
    val lastName = "Doe"
    val fullName = firstName + " " + lastName
    println("Full name: $fullName")

    // String templates
    val name = "Alice"
    val age = 25
    val message = "My name is $name and I am $age years old."
    println(message)

    // Complex expressions in string templates
    val a = 5
    val b = 10
    val sum = "The sum of $a and $b is ${a + b}."
    println(sum)

    // String functions
    println("Uppercase: ${text.toUpperCase()}")
    println("Lowercase: ${text.toLowerCase()}")
    println("Trimmed: '${text.trim()}'")
    val newText = greeting.replace("Kotlin", "World")
    println(newText)

    // Splitting and joining strings
    val splitText = "one,two,three"
    val parts = splitText.split(",")
    println(parts)
    val joinedText = parts.joinToString(", ")
    println(joinedText)
}

Output:

Hello, Kotlin!
Length: 6
First character: K
Last character: n
Substring: Kotlin
Full name: John Doe
My name is Alice and I am 25 years old.
The sum of 5 and 10 is 15.
Uppercase: KOTLIN
Lowercase: kotlin
Trimmed: 'Kotlin'
Hello, World!
[one, two, three]
one, two, three

Conclusion

In this chapter, you learned about strings in Kotlin, including their syntax and usage for creating strings, performing common string operations, using string templates, and utilizing built-in string functions. Understanding how to work with strings is essential for manipulating and formatting text in your Kotlin programs.