Object-to-JSON string in Java

ObjectToJSON.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class ObjectToJSON {
    public static void main(String[] args) {
        Map<Object, Object> birthday = new HashMap<Object, Object>();
        birthday.put("day", 12);
        birthday.put("month", 8);
        birthday.put("year", 1986);

        List<String> languages = new ArrayList<String>();
        languages.add("Objective-C");
        languages.add("JavaScript");
        languages.add("Java");

        Map<Object, Object> object = new HashMap<Object, Object>();
        object.put("firstName", "Bryan");
        object.put("lastName", "Irace");
        object.put("birthday", birthday);
        object.put("languages", languages);
        object.put("worksAtTumblr", true);
        object.put("null", null);

        System.out.println(toJSON(object));
    }

    private static String toJSON(Object object) {
        if (object instanceof Map) {
            Map map = (Map) object;

            StringBuilder builder = new StringBuilder("{");

            boolean first = true;

            for (Object key : map.keySet()) {
                if (!first)
                    builder.append(",");

                builder.append(String.format("\"%s\": %s", key, toJSON(map.get(key))));

                first = false;
            }

            builder.append("}");

            return builder.toString();

        } else if (object instanceof List) {
            List list = (List) object;

            StringBuilder builder = new StringBuilder("[");

            for (int i = 0; i < list.size(); i++) {
                if (i > 0)
                    builder.append(",");

                builder.append(toJSON(list.get(i)));
            }

            builder.append("]");

            return builder.toString();

        } else if (object instanceof Number || object instanceof Boolean) {
            return object.toString();

        } else if (object instanceof String) {
            return String.format("\"%s\"", ((String) object).replaceAll("\"", "\\\""));

        } else if (object == null) {
            return "null";

        } else {
            throw new IllegalArgumentException("ORLY?");
        }
    }
}

ObjectToJSONWithGuava.java

import com.google.common.base.Function;
import com.google.common.base.Joiner;
import com.google.common.collect.*;

import java.util.List;
import java.util.Map;

public class ObjectToJSON {
    public static void main(String[] args) {
        System.out.println(toJSON(ImmutableMap.of(
                "firstName", "Bryan",
                "lastName", "Irace",
                "birthday", ImmutableMap.of("day", 12, "month", 8, "year", 1986),
                "languages", ImmutableList.of("Objective-C", "Java", "JavaScript"),
                "worksAtTumblr", true)));
    }

    private static String toJSON(Object object) {
        if (object instanceof Map) {
            final Map map = (Map) object;
            
            return String.format("{%s}", Joiner.on(",").join(Collections2.transform(map.keySet(), new Function() {
                @Override
                public Object apply(Object key) {
                    return String.format("\"%s\" : %s", key, toJSON(map.get(key)));
                }
            })));

        } else if (object instanceof List) {
            return String.format("[%s]", Joiner.on(",").join(Lists.transform((List) object, new Function() {
                @Override
                public Object apply(Object o) {
                    return toJSON(o);
                }
            })));

        } else if (object instanceof Number || object instanceof Boolean) {
            return object.toString();

        } else if (object instanceof String) {
            return String.format("\"%s\"", ((String) object).replaceAll("\"", "\\\""));

        } else {
            throw new IllegalArgumentException("ORLY?");
        }
    }
}

good