47

How is takeWhile() different from filter() in Java 9. What extra utility does it have?

Stream.of(1,2,3,4,5,6,7,8,9,10).filter(i -> i < 4 )
        .forEach(System.out::println);

This is perhaps what following would do

Stream.of(1,2,3,4,5,6,7,8,9,10).takeWhile(i -> i < 4 )
        .forEach(System.out::println);

What was the need of this new function then?

ZhekaKozlov
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Rajeev Ranjan
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    A quick google search came up with [this](https://blog.codefx.org/java/java-9-stream/): "Called on an ordered stream it will return a new one that consists of those element that passed the predicate until the first one failed. It’s a little like filter but it cuts the stream off as soon as the first element fails the predicate. _In its parlance, it takes elements from the stream while the predicate holds and stops as soon as it no longer does._" - If you'd change your predicate to get odd elements only `filter()` would result in `1,3,5,7,9` while `takeWhile()` would result in just `1`. – Thomas Oct 20 '17 at 13:59
  • Okay. So perhaps it is a more narrowed down form of `filter`. – Rajeev Ranjan Oct 20 '17 at 14:02
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    In fact this is why there is a `while` in the function name indicating `break` if predicate does not hold. – Rajeev Ranjan Oct 20 '17 at 14:07
  • You got it :) .. – Thomas Oct 20 '17 at 14:11

5 Answers5

92

filter will remove all items from the stream that do not satisfy the condition.

takeWhile will abort the stream on the first occurrence of an item which does not satisfy the condition.

e.g.

Stream.of(1,2,3,4,5,6,7,8,9,10,9,8,7,6,5,4,3,2,1)
    .filter(i -> i < 4 )
    .forEach(System.out::print);

will print

123321

but

Stream.of(1,2,3,4,5,6,7,8,9,10,9,8,7,6,5,4,3,2,1)
    .takeWhile(i -> i < 4 )
    .forEach(System.out::print);

will print

123

OldCurmudgeon
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14

The difference

takeWhile

Takes values while the filter is true, then stops

filter

Takes all the values that match the filter

Example

Stream.of(1,2,3,4,5,6,7,8,9,10).filter(i -> i % 2 == 0 )
    .forEach(System.out::println);

TIO

This outputs all of the even numbers from 2 to 10 inclusive.

Stream.of(1,2,3,4,5,6,7,8,9,10).takeWhile(i -> i % 2 == 0 )
    .forEach(System.out::println);

TIO

This outputs nothing, as 1 is not even, so it stops before outputing anything.

jrtapsell
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3

According to this blogpost: https://blog.codefx.org/java/java-9-stream/

Called on an ordered stream it will return a new one that consists of those element that passed the predicate until the first one failed. It’s a little like filter but it cuts the stream off as soon as the first element fails the predicate. In its parlance, it takes elements from the stream while the predicate holds and stops as soon as it no longer does.

i marked the important stuff in bold.

The example from the post:

Stream.of("a", "b", "c", "", "e")
    .takeWhile(s -> !String.isEmpty(s));
    .forEach(System.out::print);

Outputs abc. Because it filters until the first one doesn't match the predicate (in the example the 4th element). So you could say it's shortcircuiting the stream.

Lino
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2

It could be used to get the index of the first null value e.g.

Stream.of("Test", "Test2", null, "Test2", null).takeWhile(Objects:isNull).count();

You could do the same with filter but in that case you would need to have a condition to break the filter as soon as you get the first null value. So in practice filter is not suited for that kind of work while takeWhile is exactly for that case.

Murat Karagöz
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-1

The operations takeWhile and dropWhile are more efficient than a filter if you know that the source is sorted. These operations make use of short-circuiting and that means: a computation stops as a result is found, there is no need to process the whole stream with filter operation

Robertas Tutlys
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