import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
import org.json.simple.parser.ParseException;

public class FastTest {
    public static void main (String[] args) {
        String j1 = "{\"param1\":\"value1\",\"param2\":\"value2\"}";
        String j2 = "{\"param1\":\"value1\",\"param2\":\"value2\",\"list\":[{\"param3\":\"value3\",\"param4\":\"value4\"}]}";
        String j3 = "{\"param1\":\"value1\"}";

        parseAndOutput(j1);
        parseAndOutput(j2);
        parseAndOutput(j3);
    }


    private static void parseAndOutput (String json) {
        String p1 = "param1";
        String p2 = "param2";
        String l = "list";
        String p3 = "param3";
        String p4 = "param4";

        System.out.println("");

        try {
            JSONObject jsonRequest = (JSONObject) new JSONParser().parse(json);

            System.out.println("parameter: " + p1 + " value: " + jsonRequest.get(p1));
            System.out.println("parameter: " + p2 + " value: " + jsonRequest.get(p2));

            JSONArray ja = (JSONArray) jsonRequest.get(l);

            if (ja == null) {
                System.out.println("No subparameters");
                return;
            }

            for (Object jo : ja) {
                JSONObject j = (JSONObject) jo;

                System.out.println("parameter: " + p3 + " value: " + j.get(p3));
                System.out.println("parameter: " + p4 + " value: " + j.get(p4));
            }

        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

Exception in thread "main" java.lang.NullPointerException
	at FastTest.parseAndOutput(FastTest.java:40)
	at FastTest.main(FastTest.java:12)