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collections.html
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<!DOCTYPE html>
<html><script async="true" src="https://www.googletagmanager.com/gtag/js?id=UA-127781126-2"></script><script>window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag('js', new Date()); gtag('config', 'UA-127781126-2');window.dataLayer = window.dataLayer || []; function gtag(){dataLayer.push(arguments);} gtag('js', new Date()); gtag('config', 'UA-127781126-2');</script><script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script><script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script><script src="js/highlight.9.16.2.js"></script><script src="js/bootstrap.min.js"></script><script>hljs.initHighlightingOnLoad();</script><link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous"><link rel="stylesheet" href="css/style.css"><link rel="stylesheet" href="css/highlightjs-night-owl.css"><link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png"><link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png"><link rel="icon" type="image/png" sizes="16x16" href="/favicon-16x16.png"><link rel="manifest" href="/site.webmanifest"><link rel="mask-icon" href="/safari-pinned-tab.svg" color="#5bbad5"><title>Kotlin vs Java</title><meta charset="utf-8"><meta name="viewport" content="width=device-width" initial-scale="1"><meta name="msapplication-TileColor" content="#da532c"><meta name="theme-color" content="#ffffff"><body><div class="container-fluid text-center header-title"><div class="row text-center"><div class="col"><p>Kotlin vs Java</p></div></div></div><nav class="navbar navbar-dark navbar-expand-lg"><button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation"><span class="navbar-toggler-icon"></span></button><div id="navbarNav" class="collapse navbar-collapse"><ul class="navbar-nav"><li class="nav-item px-2"><a class="nav-link" href="/index.html">Basics</a></li><li class="nav-item px-2"><a class="nav-link" href="/classes.html">Classes</a></li><li class="nav-item px-2 active"><a class="nav-link" href="/collections.html">Collections</a><span class="sr-only"><current></current></span></li><li class="nav-item px-2"><a class="nav-link" href="/concurrency.html">Concurrency</a></li><li class="nav-item px-2"><a class="nav-link" href="/delegated-properties.html">Delegated Properties</a></li><li class="nav-item px-2"><a class="nav-link" href="/dsl.html">DSL</a></li><li class="nav-item px-2"><a class="nav-link" href="/functions.html">Functions</a></li><li class="nav-item px-2"><a class="nav-link" href="/io.html">I/O</a></li><li class="nav-item px-2"><a class="nav-link" href="/scoping-functions.html">Scoping Functions</a></li><li class="nav-item px-2"><a class="nav-link" href="/spring-framework.html">Spring Framework</a></li><li class="nav-item px-2"><a class="nav-link" href="/error-prone.html">Error prone</a></li></ul></div></nav><a class="github-corner" href="https://github.com/driver733/kotlin-vs-java" aria-label="View source on Github"><svg width="80" height="80" viewBox="0 0 250 250" style="fill:#70B7FD; color:#fff; position: absolute; top: 0; border: 0; right: 0;" aria-hidden="true"><path d="M0,0 L115,115 L130,115 L142,142 L250,250 L250,0 Z"></path><path class="octo-arm" d="M128.3,109.0 C113.8,99.7 119.0,89.6 119.0,89.6 C122.0,82.7 120.5,78.6 120.5,78.6 C119.2,72.0 123.4,76.3 123.4,76.3 C127.3,80.9 125.5,87.3 125.5,87.3 C122.9,97.6 130.6,101.9 134.4,103.2" fill="currentColor" style="transform-origin: 130px 106px;"></path><path class="octo-body" d="M115.0,115.0 C114.9,115.1 118.7,116.5 119.8,115.4 L133.7,101.6 C136.9,99.2 139.9,98.4 142.2,98.6 C133.8,88.0 127.5,74.4 143.8,58.0 C148.5,53.4 154.0,51.2 159.7,51.0 C160.3,49.4 163.2,43.6 171.4,40.1 C171.4,40.1 176.1,42.5 178.8,56.2 C183.1,58.6 187.2,61.8 190.9,65.4 C194.5,69.0 197.7,73.2 200.1,77.6 C213.8,80.2 216.3,84.9 216.3,84.9 C212.7,93.1 206.9,96.0 205.4,96.6 C205.1,102.4 203.0,107.8 198.3,112.5 C181.9,128.9 168.3,122.5 157.7,114.1 C157.9,116.9 156.7,120.9 152.7,124.9 L141.0,136.5 C139.8,137.7 141.6,141.9 141.8,141.8 Z" fill="currentColor"></path></svg><style>.github-corner:hover .octo-arm{animation:octocat-wave 560ms ease-in-out}@keyframes octocat-wave{0%,100%{transform:rotate(0)}20%,60%{transform:rotate(-25deg)}40%,80%{transform:rotate(10deg)}}@media (max-width:500px){.github-corner:hover .octo-arm{animation:none}.github-corner .octo-arm{animation:octocat-wave 560ms ease-in-out}}</style></a><div class="section"><div class="list-group-flush"><div class="list-group-item border-0"><div class="title">ListOf</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var map = Map.of(
1, "One",
2, "Two",
3, "Three"
);
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val numbers = listOf(1, 2, 3)
val map = mapOf(1 to "One",
2 to "Two",
3 to "Three")
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Filter</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">var filtered = numbers
.stream()
.filter(num -> num > 5)
.collect(Collectors.toList())</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val filtered = numbers.filter { it > 5 }
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">GroupBy</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var groups = items.stream()
.collect(
Collectors.groupingBy(
item -> (item % 2) == 0 ? "even" : "odd"
)
);
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val groups = numbers.groupBy {
if (it % 2 == 0) "even" else "odd"
}
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">PartitionBy</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var partitioned = numbers.stream()
.collect(
Collectors.partitioningBy(num -> num % 2 == 0)
);
final var evens = partitioned.get(true);
final var odds = partitioned.get(false);
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val (evens, odds) = numbers.partition { it % 2 == 0 }
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">SortBy</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var users = getUsers();
users.sort(Comparator.comparing(user -> user.lastname));
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val users = getUsers()
users.sortBy { it.lastname }
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Fold</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">private <T> Double calculateTotals(final Grid<T> grid) {
return grid.getRows()
.stream()
.flatMap(e -> e.getColumns().stream())
.collect(Collectors.groupingBy(Column::getTitle))
.entrySet()
.stream()
.flatMap(e -> e.getValue().stream())
.map(Column::getValue)
.reduce(
0.0,
(accumulator, value) ->
value instanceof Number
? accumulator + ((Number) (value)).doubleValue()
: accumulator,
Double::sum
);
@Data // Lombok
class Grid<T> {
private final List<Row<T>> rows;
}
@Data // Lombok
class Row<T> {
private final List<Column<T>> columns;
}
@Data // Lombok
class Column<T> {
private final String title;
private final T value;
}</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">fun <T> calculateTotals(grid: Grid<T>) =
grid.rows
.flatMap(Row<T>::columns)
.groupingBy(Column<T>::title)
.fold(0.0) {
accumulator,
(_, value) ->
accumulator +
when (value) {
is Number -> value.toDouble()
else -> 0.0
}
}
data class Grid<T>(val rows: List<Row<T>>)
data class Row<T>(val columns: List<Column<T>>)
data class Column<T>(val title: String, val value: T)
// https://kotlinexpertise.com/kotlin-productivity/</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">AssociateBy</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var map = currencies.stream()
.collect(
Collectors.toMap(
Currency::getCode,
Function.identity()
)
);</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val map = currencies.associateBy { it.code }</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Zip</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var numbers = List.of(1, 2, 3);
final var names = List.of("Jack", "Michael", "Bob");
IntStream
.range(0, Math.min(names.size(), ages.size()))
.mapToObj(i -> names.get(i) + ":" + ages.get(i))
.collect(Collectors.toList());</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val pairs = listOf(1, 2, 3).zip(listOf("one", "two", "three")) // List<Pair<Int, String>></code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Infix functions</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">var list = List.of("a", "b", "c")
list.add("d")
list.remove("d")</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val list = mutableListOf("a", "b", "c")
list += "d"
list -= "d"</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">CollectingAndThen</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">final var topPaidEmployee = Map.of(
"Alex", 250,
"Boss", 999,
"Chris", 400
).entrySet()
.stream()
.filter(employee -> !employee.getKey().equals("Boss"))
.collect(
Collectors.collectingAndThen(
Collectors.maxBy(Comparator.comparing(Map.Entry::getValue)),
employee -> {
if (employee.isPresent()) {
return employee.get().getKey();
}
throw new IllegalStateException("No employees");
}
)
);
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val topPaidEmployee = listOf("Alex" to 250, "Boss" to 999, "Chris" to 400)
.filter { it.first != "Boss" }
.maxBy { it.second }
?.first
?: throw IllegalStateException("No employees")</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">For</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">for (int i = 1; i < 11; i++) { }
for (int i = 1; i < 11; i += 2) { }
for (String item : collection) { }
for (var entry: map.entrySet()) { }
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">for (i in 1 until 11) { }
for (i in 1..10 step 2) {}
for (item in collection) {}
for ((index, item) in collection.withIndex()) {}
for ((key, value) in map) {}
</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Repeat</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">for (int i = 0; i < 3; i++) {
System.out.println("Hello")
}</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">repeat(3) {
println("Hello")
}</code></pre></div></div></div></div><div class="list-group-item border-0"><div class="title">Comparators</div><div class="container-fluid"><div class="row"><div class="col"><div class="lang">Java</div><pre class="code"><code class="java">var persons = List.of(
new Person("Bob C.", 5), new Person("Alex G.", 10), new Person("Alex G.", 12)
);
persons.sort(
Comparator
.comparing(Person::getScore, Comparator.reverseOrder())
.thenComparing((Person::getName), Comparator.naturalOrder())
);
// [(Alex G., 12), (Alex G., 10), (Bob C., 5)]
@Data // Lombok
class Person {
private final String name;
private final Integer score;
}
</code></pre></div><div class="col"><div class="lang">Kotlin</div><pre class="code"><code class="kotlin">val personsSorted = listOf(
Pair("Bob C.", 5), Pair("Alex G.", 10), Pair("Alex G.", 12)
).sortedWith(
compareByDescending<Pair<String, Int>> { it.second }
.thenBy { it. first }
)</code></pre></div></div></div></div></div></div><footer><div class="container text-center"><div class="row justify-content-center pt-4"><div class="col"><span>Java 13</span></div></div><div class="row justify-content-center"><div class="col"><span>Kotlin 1.3.60</span></div></div><div class="row justify-content-center pt-4 pb-2"><div class="col"><span>Created by: <a href="https://www.driver733.com">@driver733</a></span></div></div><div class="row justify-content-center"><div class="col"><a href="https://github.com/driver733/kotlin-vs-java"><img src="//img.shields.io/github/stars/driver733/kotlin-vs-java.svg?style=social" alt="GitHub stars"></a></div></div><div class="row justify-content-center pt-4 pb-4"><div class="col"><span>Based on: </span><a href="https://github.com/fabiomsr/from-java-to-kotlin">fabiomsr/from-java-to-kotlin</a></div></div></div></footer></body>
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