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

Question

Let consider a hashmap

Map id1 = new HashMap();

I inserted some values into both hashmap.

For

Answer 1

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 list1 = id1.getOrDefault(1, Collections.emptyList());