Migrating from Java to Kotlin: Collections and iterating through them

Operations that are the same in Java and Kotlin

In Kotlin, there are many operations on collections that look absolutely the same as in Java.

Operations differ a bit

Operations that don't exist in Java's standard library

• zip(), unzip() – transform a collection.

• aggregate() – group by some condition.

• takeLast(), takeLastWhile(), dropLast(), dropLastWhile() – take or drop elements by a predicate.

• slice(), chunked(), windowed() – retrieve collection parts.

• Plus (+) and minus (-) operators – add or remove elements.

If you want to dive deep into zip(), chunked(), windowed(), and some other operations, watch a video by Sebastian Aigner about advanced collection operations in Kotlin:

Mutability

In Java, there are mutable collections:

// Java // This list is mutable! public List<Customer> getCustomers() { … } 

Partially mutable:

// Java List<String> numbers = Arrays.asList("one", "two", "three", "four"); numbers.add("five"); // Fails in runtime with `UnsupportedOperationException` 

And immutable:

// Java List<String> numbers = new LinkedList<>(); // This list is immutable! List<String> immutableCollection = Collections.unmodifiableList(numbers); immutableCollection.add("five"); // Fails in runtime with `UnsupportedOperationException` 

If you write the last two pieces of code in IntelliJ IDEA, the IDE will warn you that you're trying to modify an immutable object. This code will compile and fail in runtime with UnsupportedOperationException. You can't say if a collection is mutable looking at its type.

Apart from Java, in Kotlin, you explicitly declare mutable or read-only collections depending on your needs. If you try to modify a read-only collection, the code won't compile:

// Kotlin val numbers = mutableListOf("one", "two", "three", "four") numbers.add("five") // This is OK val immutableNumbers = listOf("one", "two") //immutableNumbers.add("five") // Compilation error - Unresolved reference: add 

Read more about immutability in the Kotlin coding conventions.

Covariance

In Java, you can't pass a collection with a descendant type to a function that takes a collection of the ancestor type. For example, if Rectangle extends Shape, you can't pass a collection of Rectangle elements to a function that takes a collection of Shape elements. To make the code compilable, use the type ? extends Shape, so the function can take collections with any inheritors of Shape:

// Java class Shape {} class Rectangle extends Shape {} public void doSthWithShapes(List<? extends Shape> shapes) { /* If using just List<Shape>, the code won't compile when calling this function with the List<Rectangle> as the argument as below */ } public void main() { var rectangles = List.of(new Rectangle(), new Rectangle()); doSthWithShapes(rectangles); } 

In Kotlin, the read-only collection types are covariant. This means that, if a Rectangle class inherits from the Shape class, you can use the type List<Rectangle> anywhere the List<Shape> type is required. In other words, the collection types have the same subtyping relationship as the element types. Maps are covariant on the value type, but not on the key type. In turn, mutable collections aren't covariant; otherwise, this would lead to runtime failures.

// Kotlin open class Shape(val name: String) class Rectangle(private val rectangleName: String) : Shape(rectangleName) fun doSthWithShapes(shapes: List<Shape>) { println("The shapes are: ${shapes.joinToString { it.name }}") } fun main() { val rectangles = listOf(Rectangle("rhombus"), Rectangle("parallelepiped")) doSthWithShapes(rectangles) } 

Read more details about collection types.

Ranges and progressions

In Kotlin, you can create intervals using ranges. For example, Version(1, 11)..Version(1, 30) stays for all the versions from 1.11 to 1.30. You can check that your version is in the range using the in operator: Version(0, 9) in versionRange.

In Java, you need to manually check if a Version fits both bounds:

// Java class Version implements Comparable<Version> { int major; int minor; Version(int major, int minor) { this.major = major; this.minor = minor; } @Override public int compareTo(Version o) { if (this.major != o.major) { return this.major - o.major; } return this.minor - o.minor; } } public void compareVersions() { var minVersion = new Version(1, 11); var maxVersion = new Version(1, 31); System.out.println( versionIsInRange(new Version(0, 9), minVersion, maxVersion)); System.out.println( versionIsInRange(new Version(1, 20), minVersion, maxVersion)); } public Boolean versionIsInRange(Version versionToCheck, Version minVersion, Version maxVersion) { return versionToCheck.compareTo(minVersion) >= 0 && versionToCheck.compareTo(maxVersion) <= 0; } 

In Kotlin, you operate with a range as a whole object. You don't need to create two variables and compare a Version with them:

// Kotlin class Version(val major: Int, val minor: Int): Comparable<Version> { override fun compareTo(other: Version): Int { if (this.major != other.major) { return this.major - other.major } return this.minor - other.minor } } fun main() { val versionRange = Version(1, 11)..Version(1, 30) println(Version(0, 9) in versionRange) println(Version(1, 20) in versionRange) } 

As soon as you need to exclude one of the bounds, for example, to check if a version is greater or equal (>=) than the minimal version and less (<) than the maximal version, such inclusive ranges won't help.

Comparison by several criteria

In Java, to compare objects by several criteria, you may use the comparing() and thenComparingX() functions from the Comparator interface. For example, to compare people by their name and age:

class Person implements Comparable<Person> { String name; int age; public String getName() { return name; } public int getAge() { return age; } Person(String name, int age) { this.name = name; this.age = age; } @Override public String toString() { return this.name + " " + age; } } public void comparePersons() { var persons = List.of(new Person("Jack", 35), new Person("David", 30), new Person("Jack", 25)); System.out.println(persons.stream().sorted(Comparator .comparing(Person::getName) .thenComparingInt(Person::getAge)).collect(toList())); } 

In Kotlin, you just enumerate by which fields you want to compare:

data class Person( val name: String, val age: Int ) fun main() { val persons = listOf(Person("Jack", 35), Person("David", 30), Person("Jack", 25)) println(persons.sortedWith(compareBy(Person::name, Person::age))) } 

Sequences

In Java, you can generate a sequence of numbers this way:

// Java int sum = IntStream.iterate(1, e -> e + 3) .limit(10).sum(); System.out.println(sum); // Prints 145 

In Kotlin, use sequences. Multi-step processing of sequences is executed lazily when possible: actual computing happens only when the result of the whole processing chain is requested.

fun main() { //sampleStart // Kotlin val sum = generateSequence(1) { it + 3 }.take(10).sum() println(sum) // Prints 145 //sampleEnd } 

Sequences may reduce the number of steps that are needed to perform some filtering operation. See the sequence processing example that shows the difference between Iterable and Sequence.

Removal of elements from a list

In Java, the remove() function accepts an index of an element to remove.

In case of removal an integer element, use the Integer.valueOf() function as the argument for the remove() function:

// Java public void remove() { var numbers = new ArrayList<>(); numbers.add(1); numbers.add(2); numbers.add(3); numbers.add(1); numbers.remove(1); // This removes by index System.out.println(numbers); // [1, 3, 1] numbers.remove(Integer.valueOf(1)); System.out.println(numbers); // [3, 1] } 

In Kotlin, there are two types of element removal: by index with removeAt() and by value with remove().

fun main() { //sampleStart // Kotlin val numbers = mutableListOf(1, 2, 3, 1) numbers.removeAt(0) println(numbers) // [2, 3, 1] numbers.remove(1) println(numbers) // [2, 3] //sampleEnd } 

Traverse a map

In Java, you can traverse a map via forEach:

// Java numbers.forEach((k,v) -> System.out.println("Key = " + k + ", Value = " + v)); 

In Kotlin, use a for loop or similar to Java forEach to traverse a map:

// Kotlin for ((k, v) in numbers) { println("Key = $k, Value = $v") } // Or numbers.forEach { (k, v) -> println("Key = $k, Value = $v") } 

Get the first and the last items of a possibly empty collection

In Java, you can safely get the first and the last items by checking the size of the collection and using indices:

// Java var list = new ArrayList<>(); //... if (list.size() > 0) { System.out.println(list.get(0)); System.out.println(list.get(list.size() - 1)); } 

And with the getFirst() and getLast() functions for Deque and its inheritors:

// Java var deque = new ArrayDeque<>(); //... if (deque.size() > 0) { System.out.println(deque.getFirst()); System.out.println(deque.getLast()); } 

In Kotlin, there are special functions firstOrNull() and lastOrNull(). Using the Elvis operator, you can perform further actions right away depending on the result of a function. For example, firstOrNull():

// Kotlin val emails = listOf<String>() // Might be empty val theOldestEmail = emails.firstOrNull() ?: "" val theFreshestEmail = emails.lastOrNull() ?: "" 

Create a set from a list

In Java, to create a Set from a List, you can use the Set.copyOf function:

// Java public void listToSet() { var sourceList = List.of(1, 2, 3, 1); var copySet = Set.copyOf(sourceList); System.out.println(copySet); } 

In Kotlin, use the function toSet():

fun main() { //sampleStart // Kotlin val sourceList = listOf(1, 2, 3, 1) val copySet = sourceList.toSet() println(copySet) //sampleEnd } 

Group elements

In Java, you may group elements with the Collectors function groupingBy():

// Java public void analyzeLogs() { var requests = List.of( new Request("https://kotlinlang.org/docs/home.html", 200), new Request("https://kotlinlang.org/docs/home.html", 400), new Request("https://kotlinlang.org/docs/comparison-to-java.html", 200) ); var urlsAndRequests = requests.stream().collect( Collectors.groupingBy(Request::getUrl)); System.out.println(urlsAndRequests); } 

In Kotlin, use the function groupBy():

class Request( val url: String, val responseCode: Int ) fun main() { //sampleStart // Kotlin val requests = listOf( Request("https://kotlinlang.org/docs/home.html", 200), Request("https://kotlinlang.org/docs/home.html", 400), Request("https://kotlinlang.org/docs/comparison-to-java.html", 200) ) println(requests.groupBy(Request::url)) //sampleEnd } 

Filter elements

In Java, to filter elements from a collection, you need to use the Stream API. The Stream API has intermediate and terminal operations. filter() is an intermediate operation, which returns a stream. To receive a collection as the output, you need to use a terminal operation, for example, collect(). For example, to leave only those pairs which keys end with 1 and values are greater than 10:

// Java public void filterEndsWith() { var numbers = Map.of("key1", 1, "key2", 2, "key3", 3, "key11", 11); var filteredNumbers = numbers.entrySet().stream() .filter(entry -> entry.getKey().endsWith("1") && entry.getValue() > 10) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue)); System.out.println(filteredNumbers); } 

In Kotlin, filtering is built-in into collections, and filter() returns the same collection type that was filtered. So, all you need to write is the filter() and its predicate:

fun main() { //sampleStart // Kotlin val numbers = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key11" to 11) val filteredNumbers = numbers.filter { (key, value) -> key.endsWith("1") && value > 10 } println(filteredNumbers) //sampleEnd } 

Learn more about filtering maps.

// Java public void objectIsInstance() { var numbers = new ArrayList<>(); numbers.add(null); numbers.add(1); numbers.add("two"); numbers.add(3.0); numbers.add("four"); System.out.println("All String elements in upper case:"); numbers.stream().filter(it -> it instanceof String) .forEach( it -> System.out.println(((String) it).toUpperCase())); } 

// Kotlin fun main() { //sampleStart // Kotlin val numbers = listOf(null, 1, "two", 3.0, "four") println("All String elements in upper case:") numbers.filterIsInstance<String>().forEach { println(it.uppercase()) } //sampleEnd } 

// Java public void testPredicates() { var numbers = List.of("one", "two", "three", "four"); System.out.println(numbers.stream().noneMatch(it -> it.endsWith("e"))); // false System.out.println(numbers.stream().anyMatch(it -> it.endsWith("e"))); // true System.out.println(numbers.stream().allMatch(it -> it.endsWith("e"))); // false } 

fun main() { //sampleStart // Kotlin val numbers = listOf("one", "two", "three", "four") println(numbers.none { it.endsWith("e") }) println(numbers.any { it.endsWith("e") }) println(numbers.all { it.endsWith("e") }) //sampleEnd } 

Collection transformation operations

// Java public void zip() { var colors = List.of("red", "brown"); var animals = List.of("fox", "bear", "wolf"); for (int i = 0; i < Math.min(colors.size(), animals.size()); i++) { String animal = animals.get(i); System.out.println("The " + animal.substring(0, 1).toUpperCase() + animal.substring(1) + " is " + colors.get(i)); } } 

fun main() { //sampleStart // Kotlin val colors = listOf("red", "brown") val animals = listOf("fox", "bear", "wolf") println(colors.zip(animals) { color, animal -> "The ${animal.replaceFirstChar { it.uppercase() }} is $color" }) //sampleEnd } 

// Java public void associate() { var numbers = List.of("one", "two", "three", "four"); var wordAndLength = numbers.stream() .collect(toMap(number -> number, String::length)); System.out.println(wordAndLength); } 

fun main() { //sampleStart // Kotlin val numbers = listOf("one", "two", "three", "four") println(numbers.associateWith { it.length }) //sampleEnd } 

What's next?

• Visit Kotlin Koans – complete exercises to learn the Kotlin syntax. Each exercise is created as a failing unit test and your job is to make it pass.

• Look through other Kotlin idioms.

• Learn how to convert existing Java code to Kotlin with Java to Kotlin converter.

• Discover collections in Kotlin.

If you have a favorite idiom, contribute it by sending a pull request.