How to convert a collection/ array into JSONArray using stream in java 8

Question

Im having a double array , I need to convert the array into a JSONArray using java streams. I tried using forEach (shared mutability) which leads to loss of data.

public static JSONArray arrayToJson(double[] array) throws JSONException{
    JSONArray jsonArray = new JSONArray();
    
    Arrays.stream(array)
         .forEach(jsonArray::put);  

    return jsonArray;
}

Is there any way I could to create JSONArray using streams?

Answer 1

Your code works, but you can write something like this (jdk 8+):

return Arrays.stream(array)
             .collect(Collector.of(
                          JSONArray::new, //init accumulator
                          JSONArray::put, //processing each element
                          JSONArray::put  //confluence 2 accumulators in parallel execution
                     ));

one more example (create a String from List<String>):

List<String> list = ...
String str = list.stream()
                 .collect(Collector.of(
                    StringBuilder::new,
                    StringBuilder::append,
                    StringBuilder::append,
                    StringBuilder::toString //last action of the accumulator (optional)  
                 ));

Looks nice, but compiler complaints: error: incompatible thrown types JSONException in method reference .collect(Collector.of(JSONArray::new, JSONArray::put, JSONArray::put)

I checked this on jdk 13.0.1 and JSON 20190722 and didn't find problems except of Expected 3 arguments, but found 1 in .collect(...).

(Gradle : implementation group: 'org.json', name: 'json', version: '20190722')

---

Fix:

public static JSONArray arrayToJson(double[] array) throws JSONException {
    return Arrays.stream(array).collect(
            JSONArray::new,
            JSONArray::put,
            (ja1, ja2) -> {
                for (final Object o : ja2) {
                    ja1.put(o);
                }
            }
    );
}

Note: The combiner cannot be a method reference to just JSONArray::put because this will just put one array into the other (e.g. [[]]) instead of actually combining them as is the desired behavior.