Convert Java Map Enum keys to Map String keys

Question

I have the following map:

Map<DataFields, String> myMap;

But I need to convert it to the following:

Map<String, String> myMap;

My best feeble attempt which doesn't even compile is:

myMap.keySet().stream().map(k -> k.name()).collect(Collectors.toMap(k, v)

I think you might need to use `entrySet()` instead of `keySet()`. — clinomaniac, Feb 27 '18 at 18:43
not sure if FP reduces any efficiency. Another answer https://stackoverflow.com/questions/17208346/how-to-convert-mapobject-object-to-mapstring-string-in-java — Adesh Kumar, Feb 27 '18 at 19:10
Also, not sure why you needed such a map. Wherever you have to use this map you could have effectively converted the key to string and then used. — Adesh Kumar, Feb 27 '18 at 19:11

score 9 · Accepted Answer · answered Feb 27 '18 at 18:44

9

You need to stream the entrySet() (so you have both the keys and the values), and collect them to a map:

Map<String, String> result =
    myMap.entrySet()
         .stream()
         .collect(Collectors.toMap(e -> e.getKey().name(), e -> e.getValue()));

answered Feb 27 '18 at 18:44

Mureinik

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`e -> e.getValue()` replace with `Entry::getValue` – Eugene Feb 27 '18 at 19:03
Eugene it's just a syntactic sugar. Any performance advantage of using `Entry::getValue` – Adesh Kumar Feb 27 '18 at 19:08
1

@AdeshKumar first, unless you tag me with `@`, I can't tell you commented, second is that a question or an affirmation? – Eugene Feb 27 '18 at 19:35
1

Yes, it's advantageous for both performance and readability to use method references where possible. – Louis Wasserman Feb 27 '18 at 19:43
2

@AdeshKumar a lambda expression will create one more synthetic method, while method references will not, that's about the only difference (besides readability) – Eugene Feb 27 '18 at 19:56

kasopey · Answer 2 · 2018-02-27T18:56:02.027

2

Map<String, String> result = myMap
    .entrySet() // iterate over all entries (object with tow fields: key and value)
    .stream() // create a stream
    .collect(Collectors.toMap(e -> e.getKey().toString(), e -> e.getValue()));
        // collect to map: convert enum Key value toString() and copy entry value

edited Feb 27 '18 at 18:56

answered Feb 27 '18 at 18:50

kasopey

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I'd prefer to use `name()` over `toString()` which is the reason I'll accept the other answer. – jiveturkey Feb 27 '18 at 19:01

score 2 · Answer 3 · answered Feb 27 '18 at 19:04

Another way of doing same without Collectors helper. Using entryset will make it very easy to map.

  map.entrySet()
                .stream()
                .collect(
                        () -> new HashMap<String, String>(),
                        (Map newMap, Map.Entry<DataFields, String> entry) -> {
                            newMap.put(entry.getKey().name(), entry.getValue());
                        }
                        ,
                        (Map map1, Map map2) -> {
                            map.putAll(map2);
                        }
                );

score 2 · Answer 4 · answered Feb 27 '18 at 20:36

2

A Java 8, succint way to do it (without streams):

Map<String, String> result = new HashMap<>();
myMap.forEach((k, v) -> result.put(k.name(), v));

answered Feb 27 '18 at 20:36

fps

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Convert Java Map Enum keys to Map String keys

4 Answers4