如何在JsonObject中存储嵌套的ArrayList

Question

So I have this ArrayList existing of school classes and each class has an ArrayList containing ArrayLists of additional values. I would have to declare it like this

ArrayList<ArrayList<String>>

I'll be looping through this rota of classes and will add this in a JsonObject using the JsonObjectBuilder. Here's an example.

JsonArrayBuilder arrBuilder = Json.createArrayBuilder();
for (Rota class : rotaArray) {
  JsonObjectBuilder builderRota = Json.createObjectBuilder();
  String example = class.getExample();
  //This is where I get the ArrayList of each class.
  ArrayList<ArrayList<String>> arrExample = class.getArrExample();
  builderRota
      .add("example", example)
      //This is where I'm stuck since the builder expects it to have a String as value instead of an ArrayList.
      .add("arr", arrExample);
   arrBuilder.add(builderRota);
}

Now, how would I go about storing that ArrayList in the JsonObject so that I can use this later on? (I'll be retrieving this JsonObject with Polymer, but being able to store the array first would be the first step.

Thank in advance!

Answer 1

You can use jackson library to do this easily. That library will not need you to mess around with Json* classes that you have right now. Try this -

@org.junit.jupiter.api.Test
public void testJson() throws JsonProcessingException {
    ArrayList<ArrayList<String>> input = new ArrayList<>();
    for(int i = 1; i<=10; i++) {
        input.add(new ArrayList<>(Arrays.asList(String.valueOf(2*i),String.valueOf(3*i))));
    }

    // This converts the Array to Json
    // This class is part of jackson library - https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind/2.9.8
    ObjectMapper objectMapper = new ObjectMapper();
    System.out.println(objectMapper.writeValueAsString(input));
}

For me, it printed [["2","3"],["4","6"],["6","9"],["8","12"],["10","15"],["12","18"],["14","21"],["16","24"],["18","27"],["20","30"]]