flatMap() to convert a stream of LinkedList into a stream of String

Question

My goal is exactly what the title say. What I'm doing is:

.stream().flatMap(x -> x.getTitles())

getTitles() returns a LinkedList<String>, and I expected flatMap() to do the job and create a stream of Strings instead of a stream of LinkedList<String>, but Eclipse says:

Type mismatch: cannot convert from LinkedList<String> to Stream<? extends Object>

How can I do that? (I need to do it with streams, it's all part of a bigger stream computation)

Pshemo · Accepted Answer · 2015-06-22T20:14:29.393

10

flatMap expects mapping to stream, not to collection. Use

.stream().flatMap(x -> x.getTitles().stream())
//                                   ^^^^^^^^ add this

edited Jun 22 '15 at 20:14

answered Jun 22 '15 at 20:09

Pshemo

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Awesome, it worked, thanks!! However, why do I have to do that? Aren't `map()` and `flatMap()` supposed to substitute the elements in the original stream with the ones obtained with the lambda expression? I thought that in the way that you wrote I would have created a stream of streams of (a single) string (which I don't think it's possible). – RVKS Jun 22 '15 at 20:25
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`map` is used when you want to have `Stream` and map *each* element from that stream to *one* other element `B` so you would get `Stream`. But `flatMap` is used when you want to map element `A` to possibly more than one elements, which comes from another stream. So if you have relation `a->stream{i,i}, b->stream{k,l}` stream of `a,b` can become stream of `{i,j,k,l}`. – Pshemo Jun 22 '15 at 20:31
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Or lets take this example. Lets assume that you have steam of `List`. If you do `map(list -> list.stream()` you will end up with `Stream>`, but with `flatMap` you are ending up with stream of elements from returned stream, so you are getting `Stream`. – Pshemo Jun 22 '15 at 20:46
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@RVKS You can find more info about map vs flatMap here: http://stackoverflow.com/q/26684562/1393766 – Pshemo Jun 22 '15 at 20:53
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Another way you could do this: say `x` is an instance of `MyClass`, then you could do `.stream().map(MyClass::getTitles).flatMap(Collection::stream)` – Andrew Mairose Aug 24 '15 at 16:14

flatMap() to convert a stream of LinkedList into a stream of String

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