How to get a List from a HashMap<String,List>
14
I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().
I want a simple one-line method using java 8's stream.
What I have tried until now :
HashMap<String,List<E>> map = new HashMap<>();
List<E> list = map.values(); // does not compile
list = map.values().stream().collect(Collectors.toList()); // does not compile
java collections java-8 java-stream collectors
edited yesterday
Nicholas K
5,89951031
asked yesterday
Yassine Ben Hamida
300112
add a comment |
14
I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().
I want a simple one-line method using java 8's stream.
What I have tried until now :
HashMap<String,List<E>> map = new HashMap<>();
List<E> list = map.values(); // does not compile
list = map.values().stream().collect(Collectors.toList()); // does not compile
java collections java-8 java-stream collectors
edited yesterday
Nicholas K
5,89951031
asked yesterday
Yassine Ben Hamida
300112
add a comment |
14
14
14
4
I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().
I want a simple one-line method using java 8's stream.
What I have tried until now :
HashMap<String,List<E>> map = new HashMap<>();
List<E> list = map.values(); // does not compile
list = map.values().stream().collect(Collectors.toList()); // does not compile
java collections java-8 java-stream collectors
edited yesterday
Nicholas K
5,89951031
asked yesterday
Yassine Ben Hamida
300112
I want to extract a List<E> from a Map<String,List<E>> (E is a random Class) using stream().
I want a simple one-line method using java 8's stream.
What I have tried until now :
HashMap<String,List<E>> map = new HashMap<>();
List<E> list = map.values(); // does not compile
list = map.values().stream().collect(Collectors.toList()); // does not compile
java collections java-8 java-stream collectors
java collections java-8 java-stream collectors
edited yesterday
Nicholas K
5,89951031
asked yesterday
Yassine Ben Hamida
300112
edited yesterday
Nicholas K
5,89951031
asked yesterday
Yassine Ben Hamida
300112
edited yesterday
Nicholas K
5,89951031
edited yesterday
Nicholas K
5,89951031
edited yesterday
Nicholas K
5,89951031
5,89951031
asked yesterday
Yassine Ben Hamida
300112
asked yesterday
Yassine Ben Hamida
300112
asked yesterday
Yassine Ben Hamida
300112
300112
add a comment |
add a comment |
6 Answers
6
active
oldest
votes
25
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values()
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
answered yesterday
Aomine
41k74071
add a comment |
10
Or use forEach
map.forEach((k,v)->list.addAll(v));
or as Aomine commented use this
map.values().forEach(list::addAll);
answered yesterday
Hadi J
9,95731743
3
good idea in showing a non-stream version. btw it would be better to iterate over thevaluessince you're not doing anything with theki.e. you can doList<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
– Aomine
yesterday
1
@Aominevalues()may involve a construction of an expensive intermediate object, thus simply ignoringkmay be advantageous from a performance perspective.
– oakad
yesterday
@oakad You're correct in the sense thatvaluesmay involve creating a new object but note that the returned collection byvaluesis lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which meansmap.values().forEach(list::addAll).
– Aomine
yesterday
add a comment |
8
Here's an alternate way to do it with Java-9 and above:
List<E> result = map.values()
.stream()
.collect(Collectors.flatMapping(List::stream, Collectors.toList()));
edited yesterday
nullpointer
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
6
Just the apiNote :- TheflatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy.
– nullpointer
yesterday
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
add a comment |
5
In addition to other answers:
List<E> result = map.values()
.stream()
.collect(ArrayList::new, List::addAll, List::addAll);
This could also do the trick.
answered yesterday
ETO
1,973422
add a comment |
4
You can use Collection.stream with flatMap as:
Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
or use a non-stream version as:
List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)
answered yesterday
nullpointer
43.8k1093180
add a comment |
2
Simply use :-
map.values().stream().flatMap(List::stream).collect(Collectors.toList());
answered yesterday
Nicholas K
5,89951031
add a comment |
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6 Answers
6
active
oldest
votes
6 Answers
6
active
oldest
votes
active
oldest
votes
active
oldest
votes
25
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values()
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
answered yesterday
Aomine
41k74071
add a comment |
25
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values()
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
answered yesterday
Aomine
41k74071
add a comment |
25
25
25
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values()
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
answered yesterday
Aomine
41k74071
map.values() returns a Collection<List<E>> not a List<E>, if you want the latter then you're required to flatten the nested List<E> into a single List<E> as follows:
List<E> result = map.values()
.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
answered yesterday
Aomine
41k74071
answered yesterday
Aomine
41k74071
answered yesterday
Aomine
41k74071
answered yesterday
Aomine
41k74071
41k74071
add a comment |
add a comment |
10
Or use forEach
map.forEach((k,v)->list.addAll(v));
or as Aomine commented use this
map.values().forEach(list::addAll);
answered yesterday
Hadi J
9,95731743
3
good idea in showing a non-stream version. btw it would be better to iterate over thevaluessince you're not doing anything with theki.e. you can doList<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
– Aomine
yesterday
1
@Aominevalues()may involve a construction of an expensive intermediate object, thus simply ignoringkmay be advantageous from a performance perspective.
– oakad
yesterday
@oakad You're correct in the sense thatvaluesmay involve creating a new object but note that the returned collection byvaluesis lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which meansmap.values().forEach(list::addAll).
– Aomine
yesterday
add a comment |
10
Or use forEach
map.forEach((k,v)->list.addAll(v));
or as Aomine commented use this
map.values().forEach(list::addAll);
answered yesterday
Hadi J
9,95731743
3
good idea in showing a non-stream version. btw it would be better to iterate over thevaluessince you're not doing anything with theki.e. you can doList<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
– Aomine
yesterday
1
@Aominevalues()may involve a construction of an expensive intermediate object, thus simply ignoringkmay be advantageous from a performance perspective.
– oakad
yesterday
@oakad You're correct in the sense thatvaluesmay involve creating a new object but note that the returned collection byvaluesis lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which meansmap.values().forEach(list::addAll).
– Aomine
yesterday
add a comment |
10
10
10
Or use forEach
map.forEach((k,v)->list.addAll(v));
or as Aomine commented use this
map.values().forEach(list::addAll);
answered yesterday
Hadi J
9,95731743
Or use forEach
map.forEach((k,v)->list.addAll(v));
or as Aomine commented use this
map.values().forEach(list::addAll);
answered yesterday
Hadi J
9,95731743
edited yesterday
answered yesterday
Hadi J
9,95731743
answered yesterday
Hadi J
9,95731743
answered yesterday
Hadi J
9,95731743
9,95731743
3
good idea in showing a non-stream version. btw it would be better to iterate over thevaluessince you're not doing anything with theki.e. you can doList<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
– Aomine
yesterday
1
@Aominevalues()may involve a construction of an expensive intermediate object, thus simply ignoringkmay be advantageous from a performance perspective.
– oakad
yesterday
@oakad You're correct in the sense thatvaluesmay involve creating a new object but note that the returned collection byvaluesis lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which meansmap.values().forEach(list::addAll).
– Aomine
yesterday
add a comment |
3
good idea in showing a non-stream version. btw it would be better to iterate over thevaluessince you're not doing anything with theki.e. you can doList<String> result = new ArrayList<>(); map.values().forEach(result::addAll);
– Aomine
yesterday
1
@Aominevalues()may involve a construction of an expensive intermediate object, thus simply ignoringkmay be advantageous from a performance perspective.
– oakad
yesterday
@oakad You're correct in the sense thatvaluesmay involve creating a new object but note that the returned collection byvaluesis lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which meansmap.values().forEach(list::addAll).
– Aomine
yesterday
3
3
good idea in showing a non-stream version. btw it would be better to iterate over the
values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);– Aomine
yesterday
good idea in showing a non-stream version. btw it would be better to iterate over the
values since you're not doing anything with the k i.e. you can do List<String> result = new ArrayList<>(); map.values().forEach(result::addAll);– Aomine
yesterday
1
1
@Aomine
values() may involve a construction of an expensive intermediate object, thus simply ignoring k may be advantageous from a performance perspective.– oakad
yesterday
@Aomine
values() may involve a construction of an expensive intermediate object, thus simply ignoring k may be advantageous from a performance perspective.– oakad
yesterday
@oakad You're correct in the sense that
values may involve creating a new object but note that the returned collection by values is lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which means map.values().forEach(list::addAll).– Aomine
yesterday
@oakad You're correct in the sense that
values may involve creating a new object but note that the returned collection by values is lazily created once, thereafter, the same instance will be returned. So it's not so expensive after all. Further, I'd favour readability here which means map.values().forEach(list::addAll).– Aomine
yesterday
add a comment |
8
Here's an alternate way to do it with Java-9 and above:
List<E> result = map.values()
.stream()
.collect(Collectors.flatMapping(List::stream, Collectors.toList()));
edited yesterday
nullpointer
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
6
Just the apiNote :- TheflatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy.
– nullpointer
yesterday
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
add a comment |
8
Here's an alternate way to do it with Java-9 and above:
List<E> result = map.values()
.stream()
.collect(Collectors.flatMapping(List::stream, Collectors.toList()));
edited yesterday
nullpointer
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
6
Just the apiNote :- TheflatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy.
– nullpointer
yesterday
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
add a comment |
8
8
8
Here's an alternate way to do it with Java-9 and above:
List<E> result = map.values()
.stream()
.collect(Collectors.flatMapping(List::stream, Collectors.toList()));
edited yesterday
nullpointer
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
Here's an alternate way to do it with Java-9 and above:
List<E> result = map.values()
.stream()
.collect(Collectors.flatMapping(List::stream, Collectors.toList()));
edited yesterday
nullpointer
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
edited yesterday
nullpointer
43.8k1093180
edited yesterday
nullpointer
43.8k1093180
edited yesterday
nullpointer
43.8k1093180
43.8k1093180
answered yesterday
Ravindra Ranwala
8,56031634
answered yesterday
Ravindra Ranwala
8,56031634
answered yesterday
Ravindra Ranwala
8,56031634
8,56031634
6
Just the apiNote :- TheflatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy.
– nullpointer
yesterday
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
add a comment |
6
Just the apiNote :- TheflatMapping()collectors are most useful when used in a multi-level reduction, such as downstream of agroupingByorpartitioningBy.
– nullpointer
yesterday
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
6
6
Just the apiNote :- The
flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.– nullpointer
yesterday
Just the apiNote :- The
flatMapping() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.– nullpointer
yesterday
8
8
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
It decreases indeed the readability here. The Java 8 way is clearly more relevant here.
– davidxxx
yesterday
add a comment |
5
In addition to other answers:
List<E> result = map.values()
.stream()
.collect(ArrayList::new, List::addAll, List::addAll);
This could also do the trick.
answered yesterday
ETO
1,973422
add a comment |
5
In addition to other answers:
List<E> result = map.values()
.stream()
.collect(ArrayList::new, List::addAll, List::addAll);
This could also do the trick.
answered yesterday
ETO
1,973422
add a comment |
5
5
5
In addition to other answers:
List<E> result = map.values()
.stream()
.collect(ArrayList::new, List::addAll, List::addAll);
This could also do the trick.
answered yesterday
ETO
1,973422
In addition to other answers:
List<E> result = map.values()
.stream()
.collect(ArrayList::new, List::addAll, List::addAll);
This could also do the trick.
answered yesterday
ETO
1,973422
answered yesterday
ETO
1,973422
answered yesterday
ETO
1,973422
answered yesterday
ETO
1,973422
1,973422
add a comment |
add a comment |
4
You can use Collection.stream with flatMap as:
Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
or use a non-stream version as:
List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)
answered yesterday
nullpointer
43.8k1093180
add a comment |
4
You can use Collection.stream with flatMap as:
Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
or use a non-stream version as:
List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)
answered yesterday
nullpointer
43.8k1093180
add a comment |
4
4
4
You can use Collection.stream with flatMap as:
Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
or use a non-stream version as:
List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)
answered yesterday
nullpointer
43.8k1093180
You can use Collection.stream with flatMap as:
Map<String, List<E>> map = new HashMap<>(); // program to interface
List<E> list = map.values()
.stream()
.flatMap(Collection::stream)
.collect(Collectors.toList());
or use a non-stream version as:
List<E> list = new ArrayList<>();
map.values().forEach(list::addAll)
answered yesterday
nullpointer
43.8k1093180
edited yesterday
answered yesterday
nullpointer
43.8k1093180
answered yesterday
nullpointer
43.8k1093180
answered yesterday
nullpointer
43.8k1093180
43.8k1093180
add a comment |
add a comment |
2
Simply use :-
map.values().stream().flatMap(List::stream).collect(Collectors.toList());
answered yesterday
Nicholas K
5,89951031
add a comment |
2
Simply use :-
map.values().stream().flatMap(List::stream).collect(Collectors.toList());
answered yesterday
Nicholas K
5,89951031
add a comment |
2
2
2
Simply use :-
map.values().stream().flatMap(List::stream).collect(Collectors.toList());
answered yesterday
Nicholas K
5,89951031
Simply use :-
map.values().stream().flatMap(List::stream).collect(Collectors.toList());
answered yesterday
Nicholas K
5,89951031
edited yesterday
answered yesterday
Nicholas K
5,89951031
answered yesterday
Nicholas K
5,89951031
answered yesterday
Nicholas K
5,89951031
5,89951031
add a comment |
add a comment |
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