JAVA Retrive current bitcoin price with JSON

Question

I want to get current/historical bitcoin price by using JSON..

However, the code shows a following error

Exception in thread "main" java.lang.NullPointerException at RwithJlab.Basic.main(Basic.java:19)

---------------------------------code---------------------------------

package RwithJlab;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.nio.charset.Charset;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

public class Basic 
{
    public static void main(String[] args) throws MalformedURLException, IOException, JSONException
    {
        JSONObject data = getJSONfromURL("https://blockchain.info/charts/market-price?format=json");
        JSONArray data_array = data.getJSONArray("values");

        for (int i = 0; i < ((CharSequence) data_array).length(); i++)
        {
            JSONObject price_point = (JSONObject) data_array.get(i);

            //  Unix time
            int x = price_point.getInt("1364062505");

            //  Bitcoin price at that time
            double y = price_point.getDouble("y");

            //  Do something with x and y.
            System.out.println(x);

        }

    }

    public static JSONObject getJSONfromURL(String URL) throws JSONException
    {
        try
        {
            URLConnection uc;
            URL url = new URL(URL);
            uc = url.openConnection();
            uc.setConnectTimeout(10000);
            uc.addRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)");
            uc.connect();

            BufferedReader rd = new BufferedReader(
                    new InputStreamReader(uc.getInputStream(), 
                    Charset.forName("UTF-8")));

            StringBuilder sb = new StringBuilder();
            int cp;
            while ((cp = rd.read()) != -1)
            {
                sb.append((char)cp);
            }

            String jsonText = (sb.toString());            

            return new JSONObject(jsonText.toString());
        } catch (IOException ex)
        {
            return null;
        }
    }
}

please help

Answer 1

You should at least log Exceptions when you catch them, otherwise you are out of clue when that exception occurs.

In your case you are catching an IOException and returning null. This results in a NullPointerException later, but you cannot see the root cause.

Log that IOException (at least call ex.printStackTrace()) and you will see the real reason.

Answer 2

You get JSONException at line int x = price_point.getInt("1364062505");

First of all, look at the source JSON from the URL you are trying to retrieve.

It's structure is:

values: [ {x : timestamp, y : value}, ... ]

Where timestamp is a representaion of the date in millis and value is the price of BTC in USD.

You're trying to getInt("1364062505") while such key in values JSON array does not exist.

JSONException is thrown at getInt(key) if the key is not found or if the value cannot be converted to an integer (look here).

You need to write int x = price_point.getInt("x"), or even better - replace your getInt() to optInt() and getDouble() to optDouble(). In this case you won't get any exceptions, only a zero, if such key in JSONObject does not exist. Then you should check your values in if (value != 0) block