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).

Answer 1

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());