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

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

We have a map that looks like this:

Map> 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> 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"
    }
  ]
}