I know this is old (and I am new to java), but I ran into the same problem. And the answers were not as clear to me as a newbie... so I thought I would add what I learned.

I used a third-party library to aid in the endeavor: org.codehaus.jackson All of the downloads for this can be found here.

For base JSON functionality, you need to add the following jars to your project's libraries: jackson-mapper-asl and jackson-core-asl

Choose the version your project needs. (Typically you can go with the latest stable build).

Once they are imported in to your project's libraries, add the following import lines to your code:

 import org.codehaus.jackson.JsonGenerationException;
 import org.codehaus.jackson.map.JsonMappingException;
 import com.fasterxml.jackson.databind.ObjectMapper;

With the java object defined and assigned values that you wish to convert to JSON and return as part of a RESTful web service

User u = new User();
u.firstName = "Sample";
u.lastName = "User";
u.email = "sampleU@example.com";

ObjectMapper mapper = new ObjectMapper();
    
try {
    // convert user object to json string and return it 
    return mapper.writeValueAsString(u);
}
catch (JsonGenerationException | JsonMappingException  e) {
    // catch various errors
    e.printStackTrace();
}

The result should looks like this: {"firstName":"Sample","lastName":"User","email":"sampleU@example.com"}

And instead of handling errors in every mapper.readValue(json, Class) you can write a helper class which has another Generic method.

and use

String jsonString = "{\"id\":100,\"firstName\":\"Adam\"}";
Student student = JSONUtils.convertToObject(jsonString, Student.class);

I'm returning null and fancying printing trace & checking null later on. You can handle error cases on your own way.

public class JSONUtils {
    public static String convertToJSON(Object object) {
        ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
        String json;
        try {
            json = ow.writeValueAsString(object);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
            return convertToJSON(e);
        }
        return json;
    }
    
    
    public static <T> T convertToObject(Class<T> clazz, String jsonString) {
        try {
            ObjectMapper mapper = new ObjectMapper();
            return (T) mapper.readValue(jsonString, clazz);
        } catch(Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}