Java 8: How to write a lambda stream to work with JsonArray?
I'm very new to Java 8 lambdas and stuff ... I want to write a lambda function that takes a JsonArray, iterates over the JsonObjects and creates a list of the values of a specific field.
For example, a function that takes a JsonArray: [{name: "John"}, {name: "David"}] and returns a list ["John", "David"].
I wrote the following code:
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
public class Main {
public static void main(String[] args) {
JSONArray jsonArray = new JSONArray();
jsonArray.add(new JSONObject().put("name", "John"));
jsonArray.add(new JSONObject().put("name", "David"));
List list = (List) jsonArray.stream().map(json -> json.toString()).collect(Collectors.toList());
System.out.println(list);
}
}
However, I am getting the error:
Exception on thread "main" java.lang.NullPointerException
Do you know how to solve it?
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JSONArray is subclass java.util.ArrayList
and JSONObject is subclass java.util.HashMap
.
Hence, it new JSONObject().put("name", "John")
returns the previous value associated with key ( null
), not the instance JSONObject
. The result is null
added to 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<String> ss = jsonArray.stream().map (json->json.toString ());
List<String> 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 won't recognize what it .collect (Collectors.toList())
returns List
.
Output:
[{"name":"John"}, {"name":"David"}]
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Go to the list first, then you can use the stream in the usual way
private class MyClass{
public MyClass(JSONObject json) {
}
}
private List<MyClass> jsonArrayToMyClassList(JSONArray jsonArray){
List<JSONObject> jsonList = (List<JSONObject>) jsonArray
.stream()
.map(JSONObject.class::cast)
.collect(Collectors.toList());
return jsonList
.stream()
.map(MyClass::new)
.collect(Collectors.toList());
}
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