“uses unchecked or unsafe operations” [duplicate]

Why am I getting “uses unchecked or unsafe operations” error everytime i compile? What’s wrong with the code? I copied the exact same code from this tutorial http://www.mkyong.com/java/json-simple-example-read-and-write-json/

import java.io.FileWriter;
import java.io.IOException;

import org.json.JSONArray;
import org.json.JSONObject;

public class JsonSimpleExample {
    public static void main(String[] args) {

        JSONObject obj = new JSONObject();
        obj.put("name", "mkyong.com");
        obj.put("age", new Integer(100));

        JSONArray list = new JSONArray();
        list.add("msg 1");
        list.add("msg 2");
        list.add("msg 3");

        obj.put("messages", list);

        try {
            FileWriter file = new FileWriter("c:\\test.json");
            file.write(obj.toJSONString());
            file.flush();
            file.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.print(obj);
    }
}