Hashmap with Streams in Java 8 Streams to collect value of Map

Question

Let consider a hashmap

Map<Integer, List> id1 = new HashMap<Integer,List>();

I inserted some values into both hashmap.

For Example,

List<String> list1 = new ArrayList<String>();

list1.add("r1");
list1.add("r4");

List<String> list2 = new ArrayList<String>();
list2.add("r2");
list2.add("r5");

List<String> list3 = new ArrayList<String>();
list3.add("r3");
list3.add("r6");

id1.put(1,list1);
id1.put(2,list2);
id1.put(3,list3);
id1.put(10,list2);
id1.put(15,list3);

Q1) Now I want to apply a filter condition on the key in hashmap and retrieve the corresponding value(List).

Eg: Here My query is key=1, and output should be 'list1'

I wrote

id1.entrySet().stream().filter( e -> e.getKey() == 1);

But I don't know how to retrieve as a list as output of this stream operation.

Q2) Again I want to apply a filter condition on the key in hashmap and retrieve the corresponding list of lists.

Eg: Here My query is key=1%(i.e key can be 1,10,15), and output should be 'list1','list2','list3'(list of lists).

Don’t use half-baked Generic types. Most probably, your map ought to be `Map>` rather than `Map` — Holger, Apr 14 '15 at 13:06

Eran · Accepted Answer · 2015-04-14T11:01:06.697

If you are sure you are going to get at most a single element that passed the filter (which is guaranteed by your filter), you can use findFirst :

Optional<List> o = id1.entrySet()
                      .stream()
                      .filter( e -> e.getKey() == 1)
                      .map(Map.Entry::getValue)
                      .findFirst();

In the general case, if the filter may match multiple Lists, you can collect them to a List of Lists :

List<List> list = id1.entrySet()
                     .stream()
                     .filter(.. some predicate...)
                     .map(Map.Entry::getValue)
                     .collect(Collectors.toList());

fge · Answer 2 · 2015-04-14T11:25:39.370

What you need to do is create a Stream out of the Map's .entrySet():

// Map<K, V> --> Set<Map.Entry<K, V>> --> Stream<Map.Entry<K, V>>
map.entrySet().stream()

From the on, you can .filter() over these entries. For instance:

// Stream<Map.Entry<K, V>> --> Stream<Map.Entry<K, V>>
.filter(entry -> entry.getKey() == 1)

And to obtain the values from it you .map():

// Stream<Map.Entry<K, V>> --> Stream<V>
.map(Map.Entry::getValue)

Finally, you need to collect into a List:

// Stream<V> --> List<V>
.collect(Collectors.toList())

If you have only one entry, use this instead (NOTE: this code assumes that there is a value; otherwise, use .orElse(); see the javadoc of Optional for more details):

// Stream<V> --> Optional<V> --> V
.findFirst().get()

@user2336315 yes, this code assumes that the value exists; OP isn't very clear about this. Ahwell — fge, Apr 14 '15 at 11:24

user2336315 · Answer 3 · 2015-04-14T11:27:56.263

4

For your Q2, there are already answers to your question. For your Q1, and more generally when you know that the key's filtering should give a unique value, there's no need to use Streams at all.

Just use get or getOrDefault, i.e:

List<String> list1 = id1.getOrDefault(1, Collections.emptyList());

edited Apr 14 '15 at 11:27

answered Apr 14 '15 at 11:21

user2336315

15,697
10
46
64

Basit · Answer 4 · 2015-06-10T11:43:29.503

You can also do it like this

public Map<Boolean, List<Student>> getpartitionMap(List<Student> studentsList) {

    List<Predicate<Student>> allPredicates = getAllPredicates();
    Predicate<Student> compositePredicate =  allPredicates.stream()
                             .reduce(w -> true, Predicate::and)
     Map<Boolean, List<Student>> studentsMap= studentsList
                .stream()
                .collect(Collectors.partitioningBy(compositePredicate));
    return studentsMap;
}

public List<Student> getValidStudentsList(Map<Boolean, List<Student>> studentsMap) throws Exception {

    List<Student> validStudentsList =  studentsMap.entrySet()
             .stream()
             .filter(p -> p.getKey() == Boolean.TRUE)
             .flatMap(p -> p.getValue().stream())
             .collect(Collectors.toList());

    return validStudentsList;
}

public List<Student> getInValidStudentsList(Map<Boolean, List<Student>> studentsMap) throws Exception {

    List<Student> invalidStudentsList = 
             partionedByPredicate.entrySet()
             .stream()
             .filter(p -> p.getKey() == Boolean.FALSE)
             .flatMap(p -> p.getValue().stream())
             .collect(Collectors.toList());

    return invalidStudentsList;

}

With flatMap you will get just List<Student> instead of List<List<Student>>.

Thanks

score 2 · Answer 5 · edited Oct 28 '19 at 13:21

2

Using keySet-

id1.keySet().stream()
        .filter(x -> x == 1)
        .map(x -> id1.get(x))
        .collect(Collectors.toList())

edited Oct 28 '19 at 13:21

Nikolai Shevchenko

7,083
8
33
42

answered Oct 28 '19 at 13:02

Shreya Mour

21
3

score 0 · Answer 6 · answered Feb 21 '22 at 12:07

0

why all that !!!

List list1 = id1.get(1);

answered Feb 21 '22 at 12:07

YoHaRo

142
2
7

While this code snippet may solve the problem, it doesn't explain why or how it answers the question. Please [include an explanation for your code](//meta.stackexchange.com/q/114762/269535), as that really helps to improve the quality of your post. – Luca Kiebel Feb 21 '22 at 12:29

score 0 · Answer 7 · answered Oct 04 '22 at 07:14

    HashMap<Integer, String> hashmap = new HashMap<>();
    hashmap.put(1, "a");
    hashmap.put(2, "b");
    List<String> collect = hashmap.keySet().stream()
                            .map(k -> "key=" + k + " value=" + hashmap.get(k))
                            .collect(Collectors.toList());
    System.out.println(collect);

score -1 · Answer 8 · answered Jan 19 '21 at 19:02

-1

Maybe the sample is oversimplified, but you don't need the Java stream API here. Just use the Map directly.

 List<String> list1 = id1.get(1); // this will return the list from your map

answered Jan 19 '21 at 19:02

vbg

613
6
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Hashmap with Streams in Java 8 Streams to collect value of Map

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