how to convert map keys to uppercase in using java 8 streams?

Question

I have a method as below

private Map<String,List<String>>  createTableColumnListMap(List<Map<String,String>> nqColumnMapList){

        Map<String, List<String>> targetTableColumnListMap = 
                nqColumnMapList.stream()
                     .flatMap(m -> m.entrySet().stream())
                     .collect(groupingBy(Map.Entry::getKey, mapping(Map.Entry::getValue, toList())));
        return targetTableColumnListMap;
    }

I want to uppercase the map keys but couldn't find a way to do it. is there a java 8 way to achieve this?

Answer 1

This doesn't require any fancy manipulation of Collectors. Lets say you have this map

Map<String, Integer> imap = new HashMap<>();
imap.put("One", 1);
imap.put("Two", 2);

Just get a stream for the keySet() and collect into a new map where the keys you insert are uppercased:

Map<String, Integer> newMap = imap.keySet().stream()
        .collect(Collectors.toMap(key -> key.toUpperCase(), key -> imap.get(key)));

// ONE - 1
// TWO - 2

Edit:

@Holger's comment is correct, it would be better (and cleaner) to just use an entry set, so here is the updated solution

Map<String, Integer> newMap = imap.entrySet().stream()
            .collect(Collectors.toMap(entry -> entry.getKey().toUpperCase(), entry -> entry.getValue()));