How to convert hashmap to JSON object in Java

Question

How to convert or cast hashmap to JSON object in Java, and again convert JSON object to JSON string?

Answer 1

First convert all your objects into valid Strings

HashMap<String, String> params = new HashMap<>();
params.put("arg1", "<b>some text</b>");
params.put("arg2", someObject.toString());

Then insert the entire map into a org.json.JSONObject

JSONObject postData = new JSONObject(params);

Now you can get the JSON by simply calling the object's toString

postData.toString()
//{"arg1":"<b>some text<\/b>" "arg2":"object output"}

Create a new JSONObject

JSONObject o = new JSONObject(postData.toString());

Or as a byte array for sending over HTTP

postData.toString().getBytes("UTF-8");

Answer 2

You can use:

new JSONObject(map);

Caution: This will only work for a Map<String, String>!

Other functions you can get from its documentation
http://stleary.github.io/JSON-java/index.html

Answer 3

This solution works with complex JSONs:

public Object toJSON(Object object) throws JSONException {
    if (object instanceof HashMap) {
        JSONObject json = new JSONObject();
        HashMap map = (HashMap) object;
        for (Object key : map.keySet()) {
            json.put(key.toString(), toJSON(map.get(key)));
        }
        return json;
    } else if (object instanceof Iterable) {
        JSONArray json = new JSONArray();
        for (Object value : ((Iterable) object)) {
            json.put(toJSON(value));
        }
        return json;
    }
    else {
        return object;
    }
}

Answer 4

This is typically the work of a Json library, you should not try to do it yourself. All json libraries should implement what you are asking for, and you can find a list of Java Json libraries on json.org, at the bottom of the page.

Answer 5

Gson way for a bit more complex maps and lists using TypeToken.getParameterized method:

We have a map that looks like this:

Map<Long, List<NewFile>> map;

We get the Type using the above mentioned getParameterized method like this:

Type listOfNewFiles = TypeToken.getParameterized(ArrayList.class, NewFile.class).getType();
Type mapOfList = TypeToken.getParameterized(LinkedHashMap.class, Long.class, listOfNewFiles).getType();

And then use the Gson object fromJson method like this using the mapOfList object like this:

Map<Long, List<NewFile>> map = new Gson().fromJson(fileContent, mapOfList);

The mentioned object NewFile looks like this:

class NewFile
{
    private long id;
    private String fileName;

    public void setId(final long id)
    {
        this.id = id;
    }

    public void setFileName(final String fileName)
    {
        this.fileName = fileName;
    }
}

The deserialized JSON looks like this:

{
  "1": [
    {
      "id": 12232,
      "fileName": "test.html"
    },
    {
      "id": 12233,
      "fileName": "file.txt"
    },
    {
      "id": 12234,
      "fileName": "obj.json"
    }
  ],
 "2": [
    {
      "id": 122321,
      "fileName": "test2.html"
    },
    {
      "id": 122332,
      "fileName": "file2.txt"
    },
    {
      "id": 122343,
      "fileName": "obj2.json"
    }
  ]
}