This question already has answers here:
Collection to stream to a new collection
(4 answers)
Closed 6 years ago.
I have the following code:
Queue<Reward> possibleRewards =
Stream.of(Reward.values())
.flatMap(reward -> IntStream.range(0, reward.getOccurencies()).mapToObj(i -> reward))
.collect(Collectors.toList());
As you can see, I need to collect the elements of the Stream into a Queue, not a List. However, there's no Collectors.toQueue() method. How can I collect the elements into a Queue?
You can use Collectors.toCollection(), which lets you choose whatever Collection implementation you wish to produce:
Queue<Reward> possibleRewards =
Stream.of(Reward.values())
.flatMap(reward -> IntStream.range(0, reward.getOccurencies()).mapToObj(i -> reward))
.collect(Collectors.toCollection(PriorityQueue::new)); // use whatever Queue
// implementation you want
Queue<Reward> possibleRewards = new LinkedBlockingQueue<>(); //Or whichever type of queue you would like
possibleRewards.addAll(Stream.of(Reward.values())
.flatMap(reward -> IntStream.range(0, reward.getOccurencies()).mapToObj(i -> reward))
.collect(Collectors.toList()));
Related
This question already has answers here:
Find all indexes of a value in a List [duplicate]
(3 answers)
Closed 12 days ago.
I have a list of strings and I want to add to a set all indexes from array where the string is not empty,
I tried doing this:
columnNum.addAll((Collection<? extends Integer>) IntStream.range(0, row.size()).filter(i-> StringUtils.isNotEmpty(row.get(i))));
but I get an exception
You have to use boxed:
var list = List.of("","a","","b");
var set = IntStream.range(0, list.size())
.filter(i ->
!list.get(i).isEmpty()).boxed().collect(Collectors.toSet());
Collect the stream to a List first. An IntStream is not a Collection.
columnNum.addAll(IntStream.range(0, row.size())
.filter(i-> StringUtils.isNotEmpty(row.get(i)))
.boxed().collect(Collectors.toList())); // or .toList() with Java 16+
This question already has answers here:
How to force max to return ALL maximum values in a Java Stream?
(9 answers)
Closed 2 years ago.
Is it possible (or even viable) to merge these two stream operations into a single pass solution?
int max = locations.stream()
.map(location -> location.getAvailableScooters().size())
.max(Comparator.naturalOrder())
.orElse(-1);
return locations.stream()
.filter(location -> location.getAvailableScooters().size() == max)
.collect(Collectors.toSet());
The only way I see the merging is possible is like this:
return locations.stream()
.filter(location -> location.getAvailableScooters().size() ==
locations.stream()
.map(location -> location.getAvailableScooters().size())
.max(Comparator.naturalOrder())
.orElse(-1) )
.collect(Collectors.toSet());
which is a much worse solution
max and collect are both terminal operations which makes the merging impossible
This question already has answers here:
Chaining Optionals in Java 8
(10 answers)
Closed 2 years ago.
public static BigDecimal calculateSomething(List<Type> myList, Optional<Type> secondOne) {
return myList.stream()
.findFirst()
.map(x -> x.getBalance().subtract(x.getAmount()))
.orElse(secondOne.map(x -> x.getBalance().subtract(x.getAmount()))
.orElse(BigDecimal.ZERO));
}
I want to do some mapping on firstOne from myList if it's present. If it's not I want to do same thing on the secondOne. If it's not present either then return ZERO.
Is there a way to write this inside of one stream and reduce code duplication and stream inside of the stream on Optional?
Yep:
return myList.stream().findFirst()
.or(() -> secondOne)
.map(x -> x.getBalance().subtract(x.getAmount()))
.orElse(BigDecimal.ZERO));
This question already has answers here:
Convert List of List into list in java
(5 answers)
Closed 5 years ago.
I have a multimap Map<T,List<L>> map and I need a list with all the values of the values from the map, namely List<L>. With map.values() I get a List<List<L>>, but thats not what I want.
Does someone know a clean solution without looping?
If you are using Java 8, you could collect all L values from all List<L>s in a single List<L> by Stream#flatMap:
final List<L> list = map
// get a Collection<List<L>>
.values()
// make a stream from the collection
.stream()
// turn each List<L> into a Stream<L> and merge these streams
.flatMap(List::stream)
// accumulate the result into a List
.collect(Collectors.toList());
Otherwise, a for-each approach with Collection#addAll can be applied:
final List<L> list = new ArrayList<>();
for (final List<L> values : map.values()) {
list.addAll(values);
}
This question already has an answer here:
how to keep the unfiltered data in the collection in Java 8 Streaming API?
(1 answer)
Closed 7 years ago.
I am playing with Java 8 and trying to work with stream(). What I want to achieve is say, I have a list [1,2,3,4], I want to double the even numbers 2 and 4 and then return the list with the updated values. So, I will get [1,4,3,8] as a result. I tried the following but it only returned [4,8].
List<Integer> myList =
Arrays.asList(1,2,3,4);
myList = myList
.stream()
.filter(n -> n%2==0)
.map(n -> n*2)
.collect(Collectors.toList());
System.out.println(myList);
Is there a way to do what I want?
The filter method removes those elements that don't fit the condition. It does not make the rest of the operations not applicable. Move the condition inside your map method.
myList = myList
.stream()
.map(n -> (n % 2 == 0) ? n*2 : n)
.collect(Collectors.toList());