Java 8 lambda to kotlin lambda

Question

public class Main {
    static class Account {
        private Long id;
        private String name;
        private Book book;

        public Account(Long id, String name, Book book) {
            this.id = id;
            this.name = name;
            this.book = book;
        }

        public String getName() {
            return name;
        }
    }
    public static void main(String[] args) {
        List<Account> data1 = new ArrayList<>();
        data1.add(new Account(1L,"name",null));
        List<String> collect = data1.stream().map(account -> account.getName()).collect(Collectors.toList());

        System.out.println(collect);
    }
}

In the above code I am trying to convert the following line

List<String> collect = data1.stream().map(account -> account.getName()).collect(Collectors.toList());

into kotlin code. Kotlin online editor gives me the following code

 val collect = data1.stream().map({ account-> account.getName() }).collect(Collectors.toList())
    println(collect)

which gives compilation error when i try to run it.

how to fix this???

or what is the kotlin way to get list of string from list of Account Object

So the problem is not the lambda. The problem is that Kotlin collections don't have a stream() method. See https://youtrack.jetbrains.com/issue/KT-5175 — JB Nizet, Apr 30 '16 at 09:02
what is the kotlin way to get list of string from list of Account Object — Mahabub, Apr 30 '16 at 09:03
Possible duplicate of [How can I call collect(Collectors.toList()) on a Java 8 Stream in Kotlin?](http://stackoverflow.com/questions/35721528/how-can-i-call-collectcollectors-tolist-on-a-java-8-stream-in-kotlin) — Jayson Minard, May 09 '16 at 22:15

score 12 · Accepted Answer · edited May 23 '17 at 12:02

12

Kotlin collections don't have a stream() method.

As mentioned in https://youtrack.jetbrains.com/issue/KT-5175, you can use

(data1 as java.util.Collection<Account>).stream()...

or you can use one of the native Kotlin alternatives that don't use streams, listed in the answers to this question:

val list = data1.map { it.name }

edited May 23 '17 at 12:02

Community

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answered Apr 30 '16 at 09:04

JB Nizet

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score 1 · Answer 2 · edited May 23 '17 at 12:10

As @JBNizet says, don't use streams at all, if you are converting to Kotlin then convert all the way:

List<String> collect = data1.stream().map(account -> account.getName()).collect(Collectors.toList());

to

val collect = data1.map { it.name } // already is a list, and use property `name`

and in other cases you will find that other collection types can become lists simply with toList() or to a set as toSet() and so on. And everything in Streams has an equivalent in Kotlin runtime already.

There is no need for Java 8 Streams at all with Kotlin, they are more verbose and add no value.

For more replacements to avoid Streams, read: What Java 8 Stream.collect equivalents are available in the standard Kotlin library?

You should read the following as well:

Kotlin API reference for kotlin.collections
Kotlin API reference for kotlin.sequences

And maybe this is a duplicate of: How can I call collect(Collectors.toList()) on a Java 8 Stream in Kotlin?

Java 8 lambda to kotlin lambda

2 Answers2