Why does Gson's toJson return null when using one-liner map

Asked Jun 22 '17 at 11:56

Active Jun 22 '17 at 12:03

Viewed 1,162 times

Somehow new Gson().toJson() returns null when I give it an one-liner map:

Map map = new HashMap() {{ put("hei", "sann"); }};
new Gson().toJson(map);  // returns null!

All other implementations I can find works as expected:

new JSONObject(map).toString();             // returns {"hei":"sann"}
JsonOutput.toJson(map);                     // returns {"hei":"sann"}
new ObjectMapper().writeValueAsString(map); // returns {"hei":"sann"}

It works if I wrap the map:

new Gson().toJson(new HashMap(map));  // returns {"hei":"sann"}

A regular map works too:

map = new HashMap();
map.put("hei", "sann");
new Gson().toJson(map);  // returns {"hei":"sann"}

Is it a bug or a feature?

I've created a test project at https://github.com/henrik242/map2json

Relevant Gson issue: https://github.com/google/gson/issues/1080

edited Jun 22 '17 at 12:03

asked Jun 22 '17 at 11:56

neu242

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Could you try with `Map` ? I don't remember having issues with it but I never use `Map` without type – AxelH Jun 22 '17 at 12:02
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See this answer: https://stackoverflow.com/a/1958961/4899193 – Usagi Miyamoto Jun 22 '17 at 12:02
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Did you not read those github comments? Don't try to serialize an anonymous inner class – OneCricketeer Jun 22 '17 at 12:11
@cricket_007 I did. My point is that every other json serializer works. – neu242 Jun 22 '17 at 12:33

Why does Gson's toJson return null when using one-liner map

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