Java 8: How to write lambda stream to work with JsonArray?

Question

I\'m very new to Java 8 lambdas and stuff... I want to write a lambda function that takes a JsonArray, goes over its JsonObjects and creates a list of values of certain fiel

Answer 1

JSONArray is a sub-class of java.util.ArrayList and JSONObject is a sub-class of java.util.HashMap.

Therefore, new JSONObject().put("name", "John") returns the previous value associated with the key (null), not the JSONObject instance. As a result, null is added to the JSONArray.

This, on the other hand, works:

    JSONArray jsonArray = new JSONArray();
    JSONObject j1 = new JSONObject();
    j1.put ("name", "John");
    JSONObject j2 = new JSONObject();
    j2.put ("name", "David");
    jsonArray.add(j1);
    jsonArray.add(j2);
    Stream ss = jsonArray.stream().map (json->json.toString ());
    List list = ss.collect (Collectors.toList ());
    System.out.println(list);

For some reason I had to split the stream pipeline into two steps, because otherwise the compiler doesn't recognize that .collect (Collectors.toList()) returns a List.

The output is:

[{"name":"John"}, {"name":"David"}]