Control flow: for, while, if, when
This post is for beginners who want to start learning Kotlin language and already know Java. We will show simple examples of for, while, if, when in both languages, in parallel, for an easier understanding. We will use Log.e in order to print string messages, as from our point of view, is easier to read them in the Logcat.
1. for loop
Enhanced For-Loop
Java
- inline
|
1 2 3 4 5 6 |
List<String> names = new ArrayList<>(); names.add("Laura"); names.add("Catalin"); names.add("George"); for (String name : names) Log.e("Names", "My name is " + name); |
- using block
|
1 2 3 4 5 6 7 8 |
List<String> names = new ArrayList<>(); names.add("Laura"); names.add("Catalin"); names.add("George"); for (String name : names) { Log.e("Names", "My name is " + name); } |
Kotlin
- inline
|
1 2 3 |
val names = listOf("Laura", "Catalin", "George") for (name in names) Log.e("Names", "My name is $name") |
Notice that in Kotlin we used a “$” symbol in order to obtain the value of name variable. This is called a template expression.
- using block
|
1 2 3 4 5 |
val names = listOf("Laura", "Catalin", "George") for (name in names) { Log.e("Names", "My name is $name") } |
Index-based loop
Java
|
1 2 3 4 5 6 7 8 |
List<String> names = new ArrayList<>(); names.add("Laura"); names.add("Catalin"); names.add("George"); for (int i = 0; i < names.size(); i++) { Log.e("Names", "My name is " + names.get(i)); } |
Kotlin
|
1 2 3 4 5 |
val names = listOf("Laura", "Catalin", "George") for (i in names.indices) { Log.e("Names", "My name is ${names[i]}") } |
Notice that here, we used beside “$” symbol, curly braces “{“. This was necessary because names[i] is an expression.
withIndex or Ranges loop
Java
|
1 2 3 4 5 6 7 8 |
List<String> names = new ArrayList<>(); names.add("Laura"); names.add("Catalin"); names.add("George"); for (int i = 0; i < names.size(); i++) { Log.e("Names", "The name at " + i + " is " + names.get(i)); } |
Kotlin
|
1 2 3 4 5 |
val names = listOf("Laura", "Catalin", "George") for ((i, value) in names.withIndex()) { Log.e("Names", "The name at $i is $value") } |
OR using Ranges (Kotlin)
|
1 2 3 |
for (i in 1..10) { print(i) } |
2. while loop
The while loop in Kotlin is the same as the while loop in Java.
Both Java and Kotlin
|
1 2 3 4 5 6 |
var count = 1 while (count < 11) { ... count++ } |
3. if expression
Traditional usage
Java
|
1 2 3 4 5 6 7 |
int a = 1; if (a < 0) { Log.e("Log", "a is negative"); } else { Log.e("Log", "a is positive"); } |
Kotlin
|
1 2 3 4 5 6 7 |
val a = 1 if (a < 0) { Log.e("Log", "a is negative") } else { Log.e("Log", "a is positive") } |
As an expression (inline if)
Java
|
1 2 |
int a = 1; String s = (a < 0) ? "a is negative" : "a is positive"; |
Kotlin
|
1 2 |
val a = 1 val s = if (a < 0) "a is negative" else "a is positive" |
In both Java and Kotlin, if we use inline if, the “else” branch is required.
As an expression with blocks
Java
NONE
Kotlin
- 1 statement
|
1 2 3 4 5 6 7 8 9 10 |
val a = 1 val b = 2 val message = if (a < b) { "a is min" } else { "b is min" } Log.e("Log", "message = $message") |
The output for the above code will be:
|
1 |
E/Log: message = a is min |
- 2 statements or more
|
1 2 3 4 5 6 7 8 9 10 11 12 |
val a = 1 val b = 2 val message = if (a < b) { Log.e("Log", "a is min") a } else { Log.e("Log", "b is min") b } Log.e("Log", "min = $message") |
The output of the above code is this:
|
1 2 |
E/Log: a is min E/Log: min = 1 |
What happens in this case? Well, a is indeed lower than b, so the code from the if branch will be executed. First, it will log the “a is min” message, and then a value will be assigned to “message” variable and the latest Log.e will log the message “min = 1” because a = 1.
As the official documentation states: ” if branches can be blocks, and the last expression is the value of a block”.
4. when expression
when expression in Kotlin is the equivalent of switch in Java.
General form
Java
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 |
int a = 1; String month; switch (a) { case 1: month= "January"; break; case 2: month= "February"; break; default: month= "this month is missing"; } Log.e("Log", "month = " + month); |
Kotlin
|
1 2 3 4 5 6 7 8 9 10 11 12 |
val a = 1 val month: String when (a) { 1 -> month = "January" 2 -> month = "February" else -> { // equivalent of "default" in Java. Note that it can be written as a block too! month = "this month is missing" } } Log.e("Log", "month = $month") |
The output is:
|
1 |
E/Log: message = a is 1 |
Many cases that should be handled in the same way
Java
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 |
int a = 1; String message; switch (a) { case 1: case 2: message = "first 2 months"; break; case 3: message = "March"; break; default: message = "a is neither 1 nor 2"; } Log.e("Log", "message = " + message); |
Kotlin
|
1 2 3 4 5 6 7 8 9 10 11 12 |
val a = 1 val month: String when (a) { 1, 2 -> month = "first 2 months" 3 -> month = "March" else -> { // equivalent of "default" in Java. Note that is written as a block! month = "this month is missing" } } Log.e("Log", "month = $month") |
The output is:
|
1 |
E/Log: month = first 2 months |
Arbitrary expressions (not only constants)
Java
In Java we can’t use arbitrary expressions. Constant expressions are required.
Kotlin
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 |
val a = 1 val month: String when (a) { 1, 2 -> month = "first 2 months" getInt(3) -> month = "March" else -> { // equivalent of "default" in Java. Note that is written as a block! month = "this month is missing" } } Log.e("Log", "month = $month") } fun getInt(x: Int): Int { return x } |
Check a value for being in or !in a range or a collection
Java
NONE
Kotlin
|
1 2 3 4 5 6 7 8 9 10 11 12 13 |
val a = 3 val month: String when (a) { in 1..3 -> month = "first trimester" in 3..6 -> month = "second trimester" !in 1..12 -> month = "outside of range" else -> { // equivalent of "default" in Java. Note that is written as a block! month = "this month is missing" } } Log.e("Log", "month = $month") |
The output is this:
|
1 |
E/Log: month = first trimester |
Check that a value is or !is of a particular type
Java
NONE
Kotlin
|
1 2 3 4 5 6 7 8 |
val a = 3 fun isInt (a: Any) = when (a) { is Int -> "a is Int" else -> "is not an Int" } Log.e("Log", "message = ${isInt(a)}") |
Output is:
|
1 |
E/Log: message = a is Int |
Notice that in this scenario we assigned the value of when expression directly to the isInt function (method). This is one of the shorter ways of writing a function.
Replacement for an if–else if chain
Java
NONE
Kotlin
|
1 2 3 4 5 6 7 8 9 10 |
val a = 0 val message: String when { a > 0 -> message = "a is positive" a < 0 -> message = "a is negative" else -> message = "a is equal to 0" } Log.e("Log", "message = $message") |
The output is:
|
1 |
E/Log: message = a is equal to 0 |
Note that in this scenario, when expression is followed directly by a curly brace “{“.
