Collect a Stream to a LinkedList

Question

When streaming through List, how can I collect the output to a linked list?

I have tried the following:

public static void main(String[] args) {
    List<String> firstList = new ArrayList<>();
    firstList.add("pavan");
    firstList.add("kumar");
    LinkedList<String> filtered= new LinkedList<>();
    filtered = (LinkedList<String>) firstList.stream().filter(t->firstList.contains("p")).collect(Collectors.toList());
    System.out.println(filtered);        
}

But this is giving java.util.ArrayList cannot be cast to java.util.LinkedList.

Is there a special reason _why_ you need a `LinkedList` and cannot code against the `List` interface? — Mick Mnemonic, Jul 18 '16 at 09:58
I wanted to use Deque interface methods implemented by LinkedList — nanpakal, Jul 18 '16 at 10:01
Okay, it would then still be best to code against the interface and declare the variable as `Deque`. — Mick Mnemonic, Jul 18 '16 at 10:03
Doing so would make it simpler to later on switch to another implementation (e.g. `ArrayDeque`, `ConcurrentLinkedDeque`, `LinkedBlockingDeque`). More reasons why it's a good idea, in general, in this post: [What does it mean to “program to an interface”?](http://stackoverflow.com/questions/383947/what-does-it-mean-to-program-to-an-interface) — Mick Mnemonic, Jul 18 '16 at 10:07
I am asking about how to name variables like Deque and still code against Interface.Can you please explain — nanpakal, Jul 18 '16 at 10:13
Use `Deque filtered=firstList.stream() .filter(t -> t.contains("p")) .collect(Collectors.toCollection(ArrayDeque::new));` Note that I replaced `LinkedList` with `ArrayDeque`, as there is really no use case for `LinkedList`, even if you need a `Deque`. Further, I replaced your filter expression with something, that is more likely matching your intention. — Holger, Jul 18 '16 at 15:34

Eran · Answer 1 · 2016-07-18T11:20:37.273

21

collect(Collectors.toList()) returns a List. You can't assume which List implementation it will return.

Use Collectors.toCollection(), to specify the actual Collection (or List in your case) implementation you wish to collect the data into :

LinkedList<String> filtered =
    firstList.stream()
             .filter(t->firstList.contains("p"))
             .collect(Collectors.toCollection(LinkedList::new));

edited Jul 18 '16 at 11:20

answered Jul 18 '16 at 09:57

Eran

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Shettyh · Answer 2 · 2016-07-18T10:25:09.837

-1

Use constructor

 filtered = new LinkedList<String>(firstList.stream().filter(t->firstList.contains("p")).collect(Collectors.toList());

Linked list constructor takes Collection as argument so any return from asList() will be compatible

edited Jul 18 '16 at 10:25

answered Jul 18 '16 at 10:00

Shettyh

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3

Use of `.collect(Collectors.toCollection(LinkedList::new))` is more efficient. if you use copy-constructor, 2 lists would be created. – Vladimir Vagaytsev Jul 18 '16 at 10:34

Collect a Stream to a LinkedList

2 Answers2