How to flatten this list of wrapped maps into a different Java 8 map view?
I'm having a hard time figuring out how to deal with cleaning up some of my code in order to do this:
I have a list of objects Context
. Everyone Context
has String userId
a Map<String, SomeObject> someObjects
.
I want to flatten this in Map<String, SomeObjects>
, where kep is userId
. To be more specific:
class Context {
String userId;
Map<String, List<SomeObject>> // the String here is something other than userId
// other stuff, getters/setters
}
Considering List<Context>
I want to get Map<String, List<SomeObject>
but where the String is actually the userId.
Is there a clean way to do this?
source to share
Assuming a Context
has String userid
and a List<SomeObject> someObjects
:
Map<String, Set<SomeObject>> map = contexts.stream()
.collect(Collectors.toMap(Context::getUserid,
c -> c.getSomeObjects().values().stream()
.flatMap(Collection::stream)
.collect(Collectors.toSet())
));
The key point here is to use toMap()
for collecting userid
and flatmap()
to turn a Stream<List<SomeObject>>
into Stream<SomeObject>
so that you can collect them into one collection.
source to share
Creating the Context class for storage String
and Map
datatypes
class Context {
String userId;
Map<String, List<Integer>> map;
public Context(String s, List<Integer> list) {
userId = s;
map = new HashMap<>();
map.put(userId, list);
}
public void setValues(String s, List<Integer> list) {
map.put(s, list);
}
}
Now let's create a Solution class with List<Context>
public class Solution {
public static void main(String[] args) {
List<Integer> list;
// Context c1
list = new ArrayList<>(Arrays.asList(1, 2, 3));
Context c1 = new Context("dev", list);
list = new ArrayList<>(Arrays.asList(-1, -3));
c1.setValues("dev2", list);
list = new ArrayList<>(Arrays.asList(-6, -3));
c1.setValues("dev3", list);
// Context c2
list = new ArrayList<>(Arrays.asList(12, 15, 18));
Context c2 = new Context("macy", list);
list = new ArrayList<>(Arrays.asList(-12, -13));
c2.setValues("macy2", list);
list = new ArrayList<>(Arrays.asList(-8, -18));
c2.setValues("macy3", list);
// Context c3
list = new ArrayList<>(Arrays.asList(20, 30));
Context c3 = new Context("bob", list);
list = new ArrayList<>(Arrays.asList(-31, -32));
c3.setValues("bob2", list);
// Context List
List<Context> contextList = new ArrayList<>();
contextList.addAll(Arrays.asList(c1, c2, c3));
retrieveRecords(contextList);
}
private static void retrieveRecords(List<Context> contextList) {
// regular way of retrieving map values
for (Context c : contextList) {
System.out.println(c.map);
}
System.out.println();
// Retrieving only those records which has String key as userID
for (Context c : contextList) {
System.out.println(c.userId + "\t=>\t" + c.map.get(c.userId));
}
}
}
source to share