List data class kotlin

Collections in Java and Kotlin

Collections are groups of a variable number of items (possibly zero) that are significant to the problem being solved and are commonly operated on. This guide explains and compares collection concepts and operations in Java and Kotlin. It will help you migrate from Java to Kotlin and write your code in the authentically Kotlin way.

The first part of this guide contains a quick glossary of operations on the same collections in Java and Kotlin. It is divided into operations that are the same and operations that exist only in Kotlin. The second part of the guide, starting from Mutability, explains some of the differences by looking at specific cases.

For an introduction to collections, see the Collections overview or watch this video by Sebastian Aigner, Kotlin Developer Advocate.

All of the examples below use Java and Kotlin standard library APIs only.

Operations that are the same in Java and Kotlin

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

Operations on lists, sets, queues, and deques

Add an element or elements

Use the plusAssign ( += ) operator: collection += element , collection += anotherCollection .

Check whether a collection contains an element or elements

Use the in keyword to call contains() in the operator form: element in collection .

Check whether a collection is empty

Use isNotEmpty() to check whether a collection is not empty.

Remove under a certain condition

Leave only selected elements

Remove all elements from a collection

Get a stream from a collection

Kotlin has its own way to process streams: sequences and methods like map() and filter() .

Get an iterator from a collection

Operations on maps

Add an element or elements

In Kotlin, the assignment mapList data class kotlin = value behaves the same as put(key, value) . Also, you may use the plusAssign ( += ) operator: map += Pair(key, value) or map += anotherMap .

Replace an element or elements

Use the indexing operator mapList data class kotlin = value instead of put() and replace() .

Use the indexing operator to get an element: map[index] .

Check whether a map contains an element or elements

Use the in keyword to call contains() in the operator form: element in map .

Check whether a map is empty

Use isNotEmpty() to check whether a map is not empty.

remove(key) , remove(key, value)

Remove all elements from a map

stream() on entries, keys, or values

Operations that exist only for lists

Get an index of an element

Get the last index of an element

Use the indexing operator to get an element: list[index] .

Replace an element or elements

Use the indexing operator instead of set() : list[index] = value .

Operations that differ a bit

Operations on any collection type

Get flat access to nested collection elements

collectionOfCollections.forEach(flatCollection::addAll) or collectionOfCollections.stream().flatMap().collect()

Apply the given function to every element

Apply the provided operation to collection elements sequentially and return the accumulated result

Group elements by a classifier and count them

Check whether collection elements satisfy a condition

stream().noneMatch() , stream().anyMatch() , stream().allMatch()

Take the first N elements

Take elements with a predicate

Skip the first N elements

Skip elements with a predicate

Build maps from collection elements and certain values associated with them

To perform all of the operations listed above on maps, you first need to get an entrySet of a map.

Operations on lists

Sort a list into natural order

Sort a list into descending order

Remove an element from a list

removeAt(index) , remove(element) or collection -= element

Fill all elements of a list with a certain value

Get unique elements from a list

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

• zip() , unzip() – transform a collection.
• aggregate() – group by a 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 take a deep dive into zip() , chunked() , windowed() , and some other operations, watch this video by Sebastian Aigner about advanced collection operations in Kotlin:

Mutability

In Java, there are mutable collections:

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

// Java List numbers = new LinkedList<>(); // This list is immutable! List 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 tell whether a collection is mutable by looking at its type.

Unlike in 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 on the Kotlin coding conventions page.

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 ? extends Shape type so the function can take collections with any inheritors of Shape :

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

In Kotlin, read-only collection types are covariant. This means that if a Rectangle class inherits from the Shape class, you can use the type List anywhere the List 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. Mutable collections aren’t covariant – 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) < println("The shapes are: $>«) > fun main()

Ranges and progressions

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

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

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:

As soon as you need to exclude one of the bounds, like to check whether a version is greater than or equal to ( >= ) the minimum version and less than ( < ) the maximum version, these 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 < 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()

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

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.

Sequences may reduce the number of steps that are needed to perform some filtering operations. See the sequence processing example, which 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.

When removing 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() .

Traverse a map

In Java, you can traverse a map via forEach :

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

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:

You can also use the getFirst() and getLast() functions for Deque and its inheritors:

In Kotlin, there are the 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() // 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:

In Kotlin, use the function toSet() :

Group elements

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

In Kotlin, use the function groupBy() :

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, like collect() . For example, to leave only those pairs whose keys end with 1 and whose values are greater than 10 :

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

Filter elements by type

In Java, to filter elements by type and perform actions on them, you need to check their types with the instanceof operator and then do the type cast:

// 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())); >

In Kotlin, you just call filterIsInstance() on your collection, and the type cast is done by Smart casts:

Test predicates

Some tasks require you to check whether all, none, or any elements satisfy a condition. In Java, you can do all of these checks via the Stream API functions allMatch() , noneMatch() , and anyMatch() :

// 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 >

In Kotlin, the extension functions none() , any() , and all() are available for every Iterable object:

Collection transformation operations

Zip elements

In Java, you can make pairs from elements with the same positions in two collections by iterating simultaneously over them:

If you want to do something more complex than just printing pairs of elements into the output, you can use Records. In the example above, the record would be record AnimalDescription(String animal, String color) <> .

In Kotlin, use the zip() function to do the same thing:

zip() returns the List of Pair objects.

If collections have different sizes, the result of zip() is the smaller size. The last elements of the larger collection are not included in the result.

Associate elements

In Java, you can use the Stream API to associate elements with characteristics:

In Kotlin, use the associate() function:

What’s next?

• Visit Kotlin Koans – complete exercises to learn 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 the Java to Kotlin converter.
• Discover collections in Kotlin.

If you have a favorite idiom, we invite you to share it by sending a pull request.

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