Java JSON connection and parsing

Question

I want to get some values from a JSON using this url:http://api.bitcoincharts.com/v1/markets.json. Im using quick-json to parse it the problem is i don't know how to do it since JSON is very confusing to me in JAVA. I want to get the values that you get here http://api.bitcoincharts.com/v1/markets.json and put them in Hashmap... so i can retrieve them by keys using quick-json for parsing ... can anyone help me tell me how should i proceed using the following code?

import com.json.parsers.JSONParser;
import com.json.parsers.JsonParserFactory;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import sun.org.mozilla.javascript.internal.json.JsonParser;



public class Restfultest { 

    public static float [] lastprice = new float[5000];
    public static float [] amounttraded = new float[5000];
    public static String  allvalue;
    public static int value = 0;
    public static String [] s = new String[4];

    public static void main(String[] args) {

      try {

        URL url = new URL("http://api.bitcoincharts.com/v1/markets.json");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Accept", "application/json");

        if (conn.getResponseCode() != 200) {
            throw new RuntimeException("Failed : HTTP error code : "
                    + conn.getResponseCode());
        }
                /*
        BufferedReader br = new BufferedReader(new InputStreamReader(
            (conn.getInputStream())));*/

                 JsonParserFactory factory=JsonParserFactory.getInstance();
                 JSONParser parser=factory.newJsonParser();
                 Map jsonData=parser.parse(conn.getInputStream()); /*Gives error because of conn.getInputStream*/


               /*

                  parsing...


                */
        conn.disconnect();

      } catch (MalformedURLException e) {

        e.printStackTrace();

      } catch (IOException e) {

        e.printStackTrace();

      }

    }

the error i get in that line its because the object i put in parse is of type JSONParser how do i use JSON at Java at ALL? this is very confusing... cant find any good examples that show how to connect to an Web API and retrieve some values ...

Really would appreciate some help ... thanks ...

Answer 1

Your first problem is unrelated to JSON entirely and simply not understanding Java.

JSONParser.parse() requires an character set name along with the InputStream:

Map<String,String> jsonData=parser.parseJson(conn.getInputStream(), "UTF-8");

After that change, it works fine. However, that's not going to be all that useful.

Looking at the response from the URL you supply, the answer is surprisingly simple: The JSON isn't anything that could be converted to a Map. So what you get is a Map with exactly one entry (root) and the entire JSON response as the value (and mangled to boot).

In short, you're using a really limited, barely documented JSON parser, and from I can find digging through it, it can't parse a JSON response that is an array at the top level.

Edit to add: Here's a complete solution using the Google Gson library to retrieve this data:

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class App
{
   public static void main(String[] args) throws IOException  
    {
        URL url = new URL("http://api.bitcoincharts.com/v1/markets.json");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        conn.setRequestProperty("Accept", "application/json");

        if (conn.getResponseCode() != 200) {
            throw new RuntimeException("Failed : HTTP error code : "
                    + conn.getResponseCode());
        }

        InputStreamReader isr = new InputStreamReader(conn.getInputStream());
        TypeToken<List<MyJsonClass>> token = new TypeToken<List<MyJsonClass>>(){};
        List<MyJsonClass> list = new Gson().fromJson(isr, token.getType());

        for (MyJsonClass mjc : list)
        {
            System.out.println(mjc.symbol + " : " + mjc.latest_trade);
        }
    } 
}

// No getters / setters for simple example
class MyJsonClass
{

    /*
    {"volume": 8.621800000000, "latest_trade": 1389906326,
        "bid": 11149.760000000000, "high": 10794.930000000000, 
        "currency": "ZAR", "currency_volume": 88097.489637000000, 
        "ask": 9722.590000000000, "close": 10350.000000000000, 
        "avg": 10217.99272042960866640376719, "symbol": "localbtcZAR", 
        "low": 9262.100000000000}
     */

    public double volume;
    public int latest_trade;
    public double bid;
    public double high;
    public String currency;
    public double currency_volume;
    public double ask;
    public double close;
    public double avg;
    public String symbol;
    public double low;
}

Answer 2

The piece you are missing is how to get the data from the InputStream into a String you can parse: see here: Read/convert an InputStream to a String for how to do that - once you know you have a 200 OK response to your request, suck the data from the connection's InputStream into a String and pass it to your parse method.

Answer 3

You should take a look into Google's GSON

You can basically make a POJO that holds primiative types and GSON will automatically populate your class it the JSON data.

Take a look at this basic example