Using Kotlin's 'when' Expression

Kotlin is a modern, concise, and expressive programming language that has gained popularity among programmers for its simplicity and ease of use. One of the core features that makes Kotlin stand out from other programming languages is the 'when' expression, which allows you to perform conditional operations in a concise and readable way. In this article, we will explore the power of Kotlin's 'when' expression and how you can use it to simplify your code.

The Basics of Kotlin's 'when' Expression

The 'when' expression is used to perform conditional operations based on the value of a variable or an expression. It works similar to the switch statement in Java or C++, but with some added features that make it more flexible and powerful. Here's the basic syntax of the 'when' expression:

Copycopy code to clipboard
1when (variable) {
2    value1 -> operation1()
3    value2 -> operation2()
4    ...
5    else -> defaultOperation()
6}

In this syntax, variable is the variable or expression whose value you want to match against different cases, value1, value2, etc. are the values that you want to compare against the variable, and operation1(), operation2(), etc. are the operations that you want to perform if the corresponding value matches the variable. You can have any number of cases, and you can also include an optional else branch that will be executed if none of the values match.

The 'when' expression can also be used as an expression, which means it can return a value. Here's an example:

Copycopy code to clipboard
1val result = when (variable) {
2    value1 -> operation1()
3    value2 -> operation2()
4    ...
5    else -> defaultOperation()
6}

In this code snippet, the value of result will be the result of the operation that matches the value of variable. If none of the values match, it will be the result of the default operation.

Examples of Using Kotlin's 'when' Expression

Let's see some examples of how you can use Kotlin's 'when' expression to simplify your code and make it more readable.

Example 1: Checking for Multiple Conditions

Suppose you have a variable score that represents the score of a player in a game, and you want to perform different operations based on the score. Here's how you can do it using Kotlin's 'when' expression:

Copycopy code to clipboard
1when (score) {
2    in 0..50 -> println("You need to practice more.")
3    in 51..70 -> println("Not bad, but you can do better.")
4    in 71..90 -> println("Good job!")
5    in 91..100 -> println("Excellent!")
6    else -> throw IllegalArgumentException("Score must be between 0 and 100.")
7}

In this code snippet, we are checking the value of score against multiple ranges using the in operator. For example, if score is between 0 and 50, we print the message "You need to practice more." Similarly, we have different messages for different score ranges. If the score is outside the range of 0 to 100, we throw an exception.

Example 2: Matching Against Types

Kotlin's 'when' expression can also be used to match against different types. Suppose you have a variable value that can be of different types, and you want to perform different operations based on the type of value. Here's how you can do it using Kotlin's 'when' expression:

Copycopy code to clipboard
1when (value) {
2    is String -> println("The value is a string.")
3    is Int -> println("The value is an integer.")
4    is Double -> println("The value is a double.")
5    else -> throw IllegalArgumentException("Unsupported type.")
6}

In this code snippet, we are checking the type of value using the is operator. If value is a string, we print the message "The value is a string." Similarly, we have different messages for different types. If the type of value is unsupported, we throw an exception.

Example 3: Using When as an Expression

As mentioned earlier, Kotlin's 'when' expression can also be used as an expression, which means it can return a value. Let's see an example of how you can use'when' as an expression:

Copycopy code to clipboard
1val message = when (dayOfWeek) {
2    Calendar.MONDAY -> "It's Monday!"
3    Calendar.TUESDAY -> "It's Tuesday!"
4    Calendar.WEDNESDAY -> "It's Wednesday!"
5    Calendar.THURSDAY -> "It's Thursday!"
6    Calendar.FRIDAY -> "It's Friday!"
7    else -> "It's a weekend day."
8}

In this code snippet, we are using Kotlin's 'when' expression to return a different message based on the day of the week. If dayOfWeek is Monday, we return the message "It's Monday!", and so on. If dayOfWeek is not any of the weekdays, we return the message "It's a weekend day." The value of message will be the result of the operation that matches the value of dayOfWeek.

In Summary

Kotlin's 'when' expression is a powerful feature that can simplify your code and make it more readable. It allows you to perform conditional operations in a concise and flexible way, and it can also be used as an expression that returns a value. By mastering the 'when' expression, you can write cleaner and more maintainable code.