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

Late to the party but here is my GSON adhoc writer for serializing hashmap. I had to write map of key-value pairs as json string attributes, expect one specific to be integer type. I did not want to create custom JavaBean wrapper for this simple usecase.

GSON JsonWriter class is easy to use serializer class containing few strongly typed writer.value() functions.

// write Map as JSON document to http servlet response
Map sd = DAO.getSD(123);
res.setContentType("application/json; charset=UTF-8");
res.setCharacterEncoding("UTF-8");
JsonWriter writer = new JsonWriter(new OutputStreamWriter(res.getOutputStream(), "UTF-8"));
writer.beginObject();
for(String key : sd.keySet()) {
    String val = sd.get(key);
    writer.name(key);
    if (key.equals("UniqueID") && val!=null)
        writer.value(Long.parseLong(val));
    else
        writer.value(val);
}
writer.endObject();
writer.close();

If none of the custom types be needed I could have just use toJson() function. gson-2.2.4.jar library is just under 190KB without any brutal dependencies. Easy to use on any custom servlet app or standalone application without big framework integrations.

Gson gson = new Gson(); 
String json = gson.toJson(myMap);