Java-8 JSONArray to HashMap

Question

I'm trying to convert JSONArray to a Map<String,String> via streams and Lambdas. The following isn't working:

org.json.simple.JSONArray jsonArray = new org.json.simple.JSONArray();
jsonArray.add("pankaj");
HashMap<String, String> stringMap = jsonArray.stream().collect(HashMap<String, String>::new, (map,membermsisdn) -> map.put((String)membermsisdn,"Error"), HashMap<String, String>::putAll);
HashMap<String, String> stringMap1 = jsonArray.stream().collect(Collectors.toMap(member -> member, member -> "Error"));

To Avoid typecasting in Line 4, I'm doing Line 3

Line 3 gives the following errors:

Multiple markers at this line
- The type HashMap<String,String> does not define putAll(Object, Object) that is applicable here
- The method put(String, String) is undefined for the type Object
- The method collect(Supplier, BiConsumer, BiConsumer) in the type Stream is not applicable for the arguments (HashMap<String, String>::new, (<no type> map, <no type> membermsisdn) 
 -> {}, HashMap<String, String>::putAll)

And Line 4 gives the following error:

Type mismatch: cannot convert from Object to HashMap<String,String>

I'm trying to learn Lambdas and streams. Can somebody help me out?

I don't think this is necessarily a duplicate. This is converting a JSONArray, and the problem arises from the use of lambdas — samczsun
– samczsun, Commented Jan 7, 2016 at 14:19
@ozOli READ/THINK twice before you downvote or reacting on something you think is wrong — Pankaj Singhal
– Pankaj Singhal, Commented Jan 7, 2016 at 14:23

samczsun · Accepted Answer · 2016-01-07 15:06:39Z

4

It would appear that json-simple's JSONArray extends an ArrayList without providing any generic types. This causes stream to return a Stream that doesn't have a type either.

Knowing this, we can program on the interface of List instead of JSONArray

List<Object> jsonarray = new JSONArray();

Doing this will allow us to stream properly like so:

Map<String, String> map = jsonarray.stream().map(Object::toString).collect(Collectors.toMap(s -> s, s -> "value"));

answered Jan 7, 2016 at 15:06

samczsun

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7 Comments

VGR Over a year ago

That assignment generates a compiler warning. It should be List<?>, not List<Object>.

Pankaj Singhal Over a year ago

@VGR It doesn't generate any warning. I just tried.

samczsun Over a year ago

It generates a warning depending on your settings. However, it would also generate a warning on .add so I didn't think too much of it.

samczsun Over a year ago

It calls toString on each element of the JSONArray

samczsun Over a year ago

JSONArray's toString will be called

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