How to use stream in Java 8 to collect a couple of fields into one list?

Question

For example I have class Person with name and surname fields.

I want to collect a List of String (names and surnames all together) from List of Person, but it seems that I can't use map twice per one list or can't use stream twice per list.

My code is:

persons.stream()
   .map(Person::getName)
   .collect(Collectors.toSet())
   .stream().map(Person::getSurname) 
   .collect(Collectors.toList())

but it keeps telling me that Person::getSurname non-static method can't be referenced from static context.

What am I doing wrong?

Presumably `Person::getName` returns a `String`? Then collecting that yields a `Set`. So, you can't call `Person::getSurname` on the elements of that. — Andy Turner, Feb 03 '17 at 15:55
What is the result supposed to be? Just a `List` with all of them mixed together? — Jorn Vernee, Feb 03 '17 at 15:56
When you say "names and surnames all together", do you mean concatenated? Alternating? — khelwood, Feb 03 '17 at 15:56
You are probably looking for `persons.stream().flatMap(p -> Stream.of(p.getName(), p.getSurname()).collect(Collectors.toSet())` — Alexis C., Feb 03 '17 at 15:57
@user2620644 take a look http://stackoverflow.com/questions/26684562/whats-the-difference-between-map-and-flatmap-methods-in-java-8 — Naman, Feb 03 '17 at 16:07

score 25 · Accepted Answer · answered Feb 03 '17 at 16:02

25

To get both names and surnames in the same list, you could do this:

List<String> set = persons.stream()
  .flatMap(p -> Stream.of(p.getName(),p.getSurname()))
  .collect(Collectors.toList());

answered Feb 03 '17 at 16:02

Landei

54,104
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1

That's it, thanks!!! It works, but could you please answer, what does flatMap mean? You can reuse stream over the same list or what? – user2620644 Feb 03 '17 at 16:03
@user2620644 `flatMap` is one of the two methods that define a "monadic type". Take `Optional`, which has a normal `map` function using a `Function ` to transform the content of the `Optional`. But in some cases you have a function that results in the wrapper type itself, e.g. `Function >`. Using `map`would result in a nested `Optional>`, so you need a method instead which "peels of" or "flattens" one layer - and that's where you use `flatMap`. In our case, every element is converted to a `Stream` itself, and `flatMap` "glues" these `Stream`s together. – Landei Feb 12 '17 at 11:36

score 7 · Answer 2 · answered Feb 03 '17 at 16:04

7

When you're doing :

persons.stream().map(Person::getName).collect(Collectors.toSet())

The result is a Set<String> that contains only the name of the persons. Then you're recreating a stream from this Set and not from your List<Person> persons.

That's why you can not use Person::getSurname to map this Set.

The solution from @Alexis C. : persons.stream().flatMap(p -> Stream.of(p.getName(), p.getSurname()).collect(Collectors.toSet()) must do the job.

answered Feb 03 '17 at 16:04

Matthieu Saleta

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I get it, but what does flatMap means? What does it do? – user2620644 Feb 03 '17 at 16:06
1

FlatMap reduce all the `Stream` produced by `Stream.of(p.getName(), p.getSurname())` into 1 `Stream` and then you collect inside a `Set`. – Matthieu Saleta Feb 03 '17 at 16:09

score 2 · Answer 3 · answered Feb 03 '17 at 16:04

2

Your code should look something like that:

persons.stream()
.map(person -> person.getName() + " " + person.getSurname)
.collect(Collectors.toList());

answered Feb 03 '17 at 16:04

matejetz

520
3
7

It will concatenate the strings into one, I dont want this, I want list to consist of separate string objects. – user2620644 Feb 03 '17 at 16:06
Oh, I mistunderstood your question. Go ahead with Landeis Answer then. – matejetz Feb 03 '17 at 16:07

score 0 · Answer 4 · answered May 15 '19 at 14:27

if person has first name and middle name optional then use below code

return Stream.of(Optional.ofNullable(person)
.map(Person::getFirstName)
.orElse(null),
Optional.ofNullable(person)
.map(Person::getMiddleName)
.orElse(null))
.filter(Objects::nonNull)
.collect(Collectors.joining(SPACE));

How to use stream in Java 8 to collect a couple of fields into one list?

4 Answers4