Java 8 Stream API 轉換到 Kotlin 集合API

Kotlin提供的集合操作的API相對Java 8 Stream的API簡潔很多。

下面是Java 8 Stream API轉換到Kotlin集合API。

對映屬性聚合為列表

Java

List names = users.stream().map(User::getName).collect(Collectors.toList());

kotlin

val list = user.map { it.name }  // 不需要toList(),很簡潔

轉換元素為字串並用逗號連結/

Java

String joined = users.stream().map(User::toString).collect(Collectors.joining(", "));

Kotlin

val joined = users.joinToString(", ")

計算薪資總數

Java

int total = employees.stream()
.collect(Collectors.summingInt(Employee::getSalary)));

Kotlin

val total = employees.sumBy { it.salary }

分組

Java

Map> byDept = employees.stream().collect(Collectors.groupingBy(Employee::getDepartment));

Kotlin

val byDept = employees.groupBy { it.department }

分組統計

Java

Map totalByDept = employees.stream().collect(Collectors.groupingBy(Employee::getDepartment, Collectors.summingInt(Employee::getSalary)));

Kotlin

val totalByDept = employees.groupBy { it.dept }.mapValues { it.value.sumBy { it.salary }}

分片

Java

Map> passingFailing = students.stream().collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

Kotlin

val passingFailing = students.partition { it.grade >= PASS_THRESHOLD }

過濾對映

Java

List namesOfMaleMembers = roster.stream().filter(p -> p.getGender() == Person.Sex.MALE).map(p -> p.getName()).collect(Collectors.toList());

Kotlin

val namesOfMaleMembers = roster.filter { it.gender == Person.Sex.MALE }.map { it.name }

分組元素

Java

Map namesByGender = roster.stream()
                .collect(Collectors.groupingBy(Person::getGender, 
                                Collectors.mapping(Person::getName, Collectors.toList())));

Kotlin

val namesByGender = roster.groupBy { it.gender }.mapValues { it.value.map { it.name } }

過濾列表元素

Java

List filtered = items.stream()
.filter( item -> item.startsWith("o") )
.collect(Collectors.toList());

Kotlin

val filtered = items.filter { it.startsWith('o') }

查詢列表最短的字串

Java

String shortest = items.stream()
.min(Comparator.comparing(item -> item.length()))
.get();

Kotlin

val shortest = items.minBy { it.length }

過濾列表並統計總數

Java

long count = items.stream().filter( item -> item.startsWith("t")).count();

Kotlin

val count = items.filter { it.startsWith('t') }.size

// 不需要filter直接統計
val count = items.count { it.startsWith('t') }

原文連結

kotlin將物件轉換為map_Java 8 Stream API轉換到Kotlin集合API

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