Java util list stream

Retrieving a List from a java.util.stream.Stream in Java 8

I was playing around with Java 8 lambdas to easily filter collections. But I did not find a concise way to retrieve the result as a new list within the same statement. Here is my most concise approach so far:

List sourceLongList = Arrays.asList(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L); List targetLongList = new ArrayList<>(); sourceLongList.stream().filter(l -> l > 100).forEach(targetLongList::add); 

Examples on the net did not answer my question because they stop without generating a new result list. There must be a more concise way. I would have expected, that the Stream class has methods as toList() , toSet() , … Is there a way that the variables targetLongList can be directly be assigned by the third line?

15 Answers 15

What you are doing may be the simplest way, provided your stream stays sequential—otherwise you will have to put a call to sequential() before forEach .

[later edit: the reason the call to sequential() is necessary is that the code as it stands ( forEach(targetLongList::add) ) would be racy if the stream was parallel. Even then, it will not achieve the effect intended, as forEach is explicitly nondeterministic—even in a sequential stream the order of element processing is not guaranteed. You would have to use forEachOrdered to ensure correct ordering. The intention of the Stream API designers is that you will use collector in this situation, as below.]

targetLongList = sourceLongList.stream() .filter(l -> l > 100) .collect(Collectors.toList()); 

Addition: I think this codes gets a little shorter, clearer and prettier if you use a static import of toList . This is done by placing the following among the imports of the file: static import java.util.stream.Collectors.toList; . Then the collect call reads just .collect(toList()) .

In Eclipse it is possible to make the IDE add a static import for methods. This is done by adding the Collectors class in Preferences -> Java -> Editor -> Content Assist -> Favorites. After this, you only have to type toLi at hit Ctr+Space to have the IDE fill in toList and add the static import.

One thing to keep in mind is that IntStream and some other almost-but-not-quite- Stream s do not have the collect(Collector) method and you will have to call IntStream.boxed() to convert them to a regular Stream first. Then again, maybe you just want toArray() .

@amarnathharish Because forEach doesn’t guarantee the order of operation execution for a parallel stream. The JavaDoc says «The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does not guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of parallelism.» (The first sentence of this quote actually means that order is not guaranteed for sequential streams either, although in practice it is preserved.)

Another approach is to use Collectors.toList :

targetLongList = sourceLongList.stream(). filter(l -> l > 100). collect(Collectors.toList()); 

Another approach is to use Collectors.toCollection :

targetLongList = sourceLongList.stream(). filter(l -> l > 100). collect(Collectors.toCollection(ArrayList::new)); 

Despite beeing recommended to code against interfaces, there are clear cases (one of them being GWT) when you have to code against concrete implementations (unless you want all List implementations compiled and delivered as javascript).

Another pro for this method, from the Collectors::toList javadoc: «There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection(Supplier) .»

I like to use a util method that returns a collector for ArrayList when that is what I want.

I think the solution using Collectors.toCollection(ArrayList::new) is a little too noisy for such a common operation.

ArrayList result = sourceLongList.stream() .filter(l -> l > 100) .collect(toArrayList()); public static Collector toArrayList()

With this answer I also want to demonstrate how simple it is to create and use custom collectors, which is very useful generally.

If you declare result as List you don’t need to use this util method. Collectors.toList will do. Also, using specific classes instead of interfaces is a code smell.

@LluisMartinez: «Collectors.toList will do.»: No, not in many situations. Because it’s not a good idea to use toList if you for example want to modify the list later in the program. The toList documentation says this: «There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned; if more control over the returned List is required, use toCollection .». My answer demonstrates a way to make it more convenient to do that in a common case.

There is a new method Stream.toList() in Java 16:

List targetLongList = sourceLongList .stream() .filter(l -> l > 100) .toList(); 

This is the call which you can use to convert any Stream to List.

 List myList = stream.collect(Collectors.toList()); 

If you have an array of primitives, you can use the primitive collections available in Eclipse Collections.

LongList sourceLongList = LongLists.mutable.of(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L); LongList targetLongList = sourceLongList.select(l -> l > 100); 

If you can’t change the sourceLongList from List :

List sourceLongList = Arrays.asList(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L); List targetLongList = ListAdapter.adapt(sourceLongList).select(l -> l > 100, new ArrayList<>()); 

If you want to use LongStream :

long[] sourceLongs = new long[]; LongList targetList = LongStream.of(sourceLongs) .filter(l -> l > 100) .collect(LongArrayList::new, LongArrayList::add, LongArrayList::addAll); 

Note: I am a contributor to Eclipse Collections.

A little more efficient way (avoid the creating the source List and the auto-unboxing by the filter):

List targetLongList = LongStream.of(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L) .filter(l -> l > 100) .boxed() .collect(Collectors.toList()); 

If you don’t mind using 3rd party libraries, AOL’s cyclops-react lib (disclosure I am a contributor) has extensions for all JDK Collection types, including List. The ListX interface extends java.util.List and adds a large number of useful operators, including filter.

ListX sourceLongList = ListX.of(1L, 10L, 50L, 80L, 100L, 120L, 133L, 333L); ListX targetLongList = sourceLongList.filter(l -> l > 100); 

ListX also can be created from an existing List (via ListX.fromIterable)

There is an another variant of collect method provided by LongStream class and similarly by IntStream and DoubleStream classes too .

 R collect(Supplier supplier, ObjLongConsumer accumulator, BiConsumer combiner) 

Performs a mutable reduction operation on the elements of this stream. A mutable reduction is one in which the reduced value is a mutable result container, such as an ArrayList, and elements are incorporated by updating the state of the result rather than by replacing the result. This produces a result equivalent to:

R result = supplier.get(); for (long element : this stream) accumulator.accept(result, element); return result; 

Like reduce(long, LongBinaryOperator), collect operations can be parallelized without requiring additional synchronization. This is a terminal operation.

And answer to your question with this collect method is as below :

 LongStream.of(1L, 2L, 3L, 3L).filter(i -> i > 2) .collect(ArrayList::new, (list, value) -> list.add(value) , (list1, list2) -> list1.addAll(list2)); 

Below is the method reference variant which is quite smart but some what tricky to understand :

 LongStream.of(1L, 2L, 3L, 3L).filter(i -> i > 2) .collect(ArrayList::new, List::add , List::addAll); 

Below will be the HashSet variant :

 LongStream.of(1L, 2L, 3L, 3).filter(i -> i > 2) .collect(HashSet::new, HashSet::add, HashSet::addAll); 

Similarly LinkedList variant is like this :

 LongStream.of(1L, 2L, 3L, 3L) .filter(i -> i > 2) .collect(LinkedList::new, LinkedList::add, LinkedList::addAll); 

To collect in a mutable list:

targetList = sourceList.stream() .filter(i -> i > 100) //apply filter .collect(Collectors.toList()); 

To collect in a immutable list:

targetList = sourceList.stream() .filter(i -> i > 100) //apply filter .collect(Collectors.toUnmodifiableList()); 

Explanation of collect from the JavaDoc:

Performs a mutable reduction operation on the elements of this stream using a Collector. A Collector encapsulates the functions used as arguments to collect(Supplier, BiConsumer, BiConsumer), allowing for reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning. If the stream is parallel, and the Collector is concurrent, and either the stream is unordered or the collector is unordered, then a concurrent reduction will be performed (see Collector for details on concurrent reduction.)

This is a terminal operation.

When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to maintain isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe data structures (such as ArrayList), no additional synchronization is needed for a parallel reduction.

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