How to convert a Map to a List in java

Question

I have a map in the following format that is populated at runtime.

  Map<String, List<VerificationDetail>>

I want to convert this to a List so that all values in Map are populated in the list. I tried with few approached but unable to populate the List with all the details available in the map.

What did you try? What did not work? What do you want in the list? — Burkhard, Apr 29 '16 at 04:06
@Burkhard . I tried the following List list = new ArrayList((Collection extends Value>) finalMap.values()); — Stalwart, Apr 29 '16 at 04:16
`for (Map.Entry> entry : map.entrySet()) al.addAll(entry.getValue());` remember each `value` in your `map` is a `List`! — Elliott Frisch, Apr 29 '16 at 04:24
@Stalwart this isn't going to be a one-liner with base Java 6. — Louis Wasserman, Apr 29 '16 at 04:27
So you want to create a `List` of `List` objects out of the `map` ? — Som Bhattacharyya, Apr 29 '16 at 04:37
@ElliottFrisch Should use `values()`, not `entrySet()`. ;-) `List al = new ArrayList(); for (List values : map.values()) al.addAll(values);` — Andreas, Apr 29 '16 at 04:45
Thanks Andreas. If I want to associate the key of the map to a String in the VerificationDetail and populate the values in the map to a List within VerificationDetail? (I dont want to have the same keys repeated again within the list) My List will be like below VerificationDetail.java ************************ String key; (Map's key) List - This will contain the map.values() associated with that particular key. — Stalwart, Apr 29 '16 at 06:46

score 0 · Answer 1 · answered May 20 '16 at 21:21

Well, it isn't a one-liner but I guess it does the job!

// Map that has the details
Map<String, List<VerificationDetail>> map = new HashMap<>(1);

// Meanwhile some values are added to the map . . .

// The list where all your map values going to be inserted
List<VerificationDetail> allDetails = new ArrayList<>(1);

for (Map.Entry<String, List<VerificationDetail>> entry : map.entrySet()) {
    allDetails.addAll(entry.getValue());
}

score 0 · Answer 2 · answered Aug 23 '18 at 07:09

I might be answering late but this could help someone else, there is better way in java 8 to do this,

Map<String, List<VerificationDetail>> map = new HashMap<>();

List<VerificationDetail> combinedList = new ArrayList<VerificationDetail>();

map.values().forEach(o-> combinedList.addAll(o));

System.out.println(combinedList);

How to convert a Map to a List in java

2 Answers2