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Kotlin Crash Course for Programmers

Conditions with `if`

Explore Kotlin's conditional control flow by learning how to use if statements effectively. Understand how to apply comparison and logical operators, how Kotlin handles equality differently from Java, and how to write clear, concise conditional code. This lesson prepares you to write complex conditions and sets the foundation for using when statements.

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For conditional control flow, Kotlin uses the if and when keywords. The syntax of if-blocks is the same as in C and Java. Similarly, the when keyword is similar to switch in these languages but more powerful, as you will learn.

Conditions using if

Conditions with if can be written as follows:

Kotlin
if (planet == "Jupiter") {
  println("Radius of Jupiter is 69,911km")
} else if (planet == "Saturn") {
  println("Radius of Saturn is 58,232km")
} else {
  println("No data for planet $planet")
}

Each condition is followed by a block of code in curly braces. There can be any number of else-if blocks (including zero) and up to one else-block.

Note: In the else-block, the planet variable is included in the output string using string interpolation. To do so, you just prepend the variable with a ＄. More complex expressions must be wrapped with curly braces: "＄{user.name} logged in".

Equality and Comparison Operators

Kotlin’s operators to formulate conditions are the following:

Operator Meaning
== Structural equality ("equals")
!= Structural inequality ("!equals")
=== Referential equality (same memory address)
!== Referential inequality
> Greater than
< Less than
>= Greater or equal than
<= Less than or equal

Note that the comparison operators work on any Comparable by using its compareTo method.

Note for Java developers: In Kotlin, you use == where you would use equals in Java and === where you would use Java’s ==.

Logical Operators

To build up more complex conditions from primitive conditions (using the operators above), Kotlin provides the standard logical operators:

Operator Meaning
&& Logical “and”
|| Logical “or”
! Logical “not”

Using these, you can combine multiple primitive conditions:

Kotlin
if (planet == "Jupiter" || planet == "Saturn" || planet == "Uranus" || planet == "Neptune") {
  println("Your chosen planet has rings, typically made of rocks and ice boulders")
}

Quiz

Conditional control flow using if

1.

How would you express the condition “the text is empty or at least 3 characters long”?

A.

text == "" || text.length <= 3

B.

text == "" && text.length >= 3

C.

text == "" || text.length >= 3

D.

text != "" || text.length >= 3

1 / 1

Exercise

Complete the following code snippet to check whether…

  1. the age variable is between 18 and 21 (both inclusive).
  2. the username variable equals "admin" or "system"
  3. the number variable is equal to neither 17 nor 42.
Problem
Solution
// Note: In this code widget, you have access to the predefined variables age, username, and number.
// Check age
// Check username
// Check number

Summary

  • Kotlin’s if conditions work the exact same way as in other languages like Java or C.
  • The logical and comparison operators are also known from some other languages.
    • But in contrast to Java, Kotlin offers == for structural equality and === for referential equality checks.

In the next lesson, you will use the when statement for conditional control flow and learn when to prefer it over if.