[java] Hashmap with Streams in Java 8 Streams to collect value of Map

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).

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());

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

Using keySet-

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

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()

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