如何在 Java 中快速读取大型 JSON 文件到 ArrayLists

Question

I'm trying to read a large set of data structured in the .json file format in Java. I am currently using org.json.simple to open the .json files. I can successfully open the .json file from a local directory and read single elements as so:

public static void main(String[] args) throws Exception {

        JSONParser parser = new JSONParser();

        Object obj = parser.parse(new FileReader("res/data.json"));

        JSONObject jsonObject = (JSONObject) obj;

        for(Iterator iterator = jsonObject.keySet().iterator(); iterator.hasNext();) {
            String key = (String) iterator.next();
            System.out.println(jsonObject.get(key));
            JSONArray k = (JSONArray) jsonObject.get(key);
            JSONObject p = (JSONObject) k.get(1);
            JSONArray b = (JSONArray) p.get("corners");
            System.out.println(b);
            JSONArray z = (JSONArray) p.get(1);
            System.out.println(z);
        }
    }

The .json file is of this structure:

{
    "buildings": [
        {
            "height": 5,
            "corners": [
                [
                    [
                        579.0,
                        13.0
                    ]
                ],
                [
                    [
                        538.0,
                        39.0
                    ]
                ],
                [
                    [
                        610.0,
                        12.0
                    ]
                ],
                [
                    [
                        558.0,
                        44.0
                    ]
                ],
                [
                    [
                        583.0,
                        29.0
                    ]
                ],
                [
                    [
                        561.0,
                        24.0
                    ]
                ],
                [
                    [
                        596.0,
                        21.0
                    ]
                ]
            ]
        },

where each object in 'buildings' has a height attribute which is an int and an float array of corners. My question is how I can convert these JSONObjects and JSONArrays into primitive Java datatypes and ArrayLists of such. In this case, the desired end result would be an array or ArrayList of integers representing the heights of each instance and an ArrayList of Arrays of floats for the corners.

Answer 1

You can use jackson for JSON parsing to convert to ArrayList. I have written a sample program with the assumption that the JSON is stored in file named test.json

     ObjectMapper mapper=new ObjectMapper();
         ArrayList heightList=new ArrayList<Integer>(); //list of heights
         ArrayList cornerList=new ArrayList<float[]>(); //list of corners
         JsonNode buildings=mapper.readTree(new File("test.json")).get("buildings");
         for (JsonNode item:buildings) {
            heightList.add(item.get("height"));
            cornerList.add(item.get("corners"));
         }