Do JSON keys need to be unique?

Question

The following question is related to a question that I had asked earlier: Help parsing simple JSON (using JSON for JAVA ME)

Do JSON keys need to be unique? For example, I was having trouble parsing the following XML (with JSON ME):

{
  "name" : "JACK",
  "name" : "JILL",
  "name" : "JOHN",
  "name" : "JENNY",
  "name" : "JAMES",
  "name" : "JIM"
}

And, apparently, its because the keys must be unique. I'm just wondering if thats true in all cases or not. For example, if I were using something other than JSON ME, would I be able to parse all of these names?

Thanks.

The [JSON RFC](http://www.ietf.org/rfc/rfc4627.txt) says "the names within an object SHOULD be unique" so implementations will differ.You may find one that has more relaxed rules. — samplebias, Mar 15 '11 at 02:15

JeanK · Accepted Answer · 2015-03-13T19:07:54.723

50

There is no "error" if you use more than one key with the same name, but in JSON, the last key with the same name is the one that is going to be used.

In your case, the key "name" would be better to contain an array as it's value, instead of having a number of keys "name". It doesn't make much sense the same object or "thing" to have two names, or two of the same properties that are in conflict.

E.g.:

{
  "name" : [ "JOHN", "JACK", "...", ... ]
}

edited Mar 13 '15 at 19:07

answered Mar 15 '11 at 02:16

JeanK

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Your answer refers to behaviour of one particular implementation and not to the JSON standard. – Karol S Mar 22 '17 at 19:42
3

`but in JSON, the last key with the same name is the one that is going to be used.` Reference please? – Koray Tugay May 06 '18 at 18:12

score 33 · Answer 2 · answered Jun 03 '13 at 10:19

33

From RFC 4627:

An object structure is represented as a pair of curly brackets surrounding zero or more name/value pairs (or members). A name is a string. A single colon comes after each name, separating the name from the value. A single comma separates a value from a following name. The names within an object SHOULD be unique.

answered Jun 03 '13 at 10:19

jhanschoo

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RFC4627 is obsoleted by RFC7159 and says almost the same. http://stackoverflow.com/questions/21832701/does-json-syntax-allow-duplicate-keys-in-an-object/23195243#23195243 – user454322 Apr 22 '14 at 03:57

score 5 · Answer 3 · answered Mar 15 '11 at 02:15

5

jQuery is able to parse it. But if you try to access it, it's just giving back the last value.

Check out http://jsfiddle.net/MQmM4/2/

So, it's parsable, I guess, but the value gets overridden if you use the same key.

answered Mar 15 '11 at 02:15

Bala R

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score 4 · Answer 4 · answered May 17 '12 at 12:16

4

here is a possible solution using array. just use array index

the_json_array.getJSONObject(0);


{"nameList":
[{"name" : "JACK"},
{"name" : "JILL"},
{"name" : "JOHN"},
{"name" : "JENNY"},
{"name" : "JAMES"},
{"name" : "JIM"}]}

answered May 17 '12 at 12:16

jeet.chanchawat

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score 1 · Answer 5 · answered Jun 25 '16 at 19:03

A JSON Object looks like the following

public JSONObject(Map<?, ?> map) {
    this.map = new HashMap<String, Object>();
    if (map != null) {
        for (final Entry<?, ?> e : map.entrySet()) {
            final Object value = e.getValue();
            if (value != null) {
                this.map.put(String.valueOf(e.getKey()), wrap(value));
            }
        }
    }
}

A JSON Object is basically a hashmap containing key value pair.

This is why you are getting overwritten each time. To avoid this

Either you must have unique key values

Or you should wrap the key value pair as individual objects into an array

Have a look at this JSON Object java implementation to know in depth about JSON.

Do JSON keys need to be unique?

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