PKIX path building failed: sun.security.provider: unable to find valid certification path to requested target

When I can change Http url into Https i got this Exception

Exceptionjavax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Here is my code

String userName = ""; String password = "";

String userencode = URLEncoder.encode(userName, "utf-8");

String passencode = URLEncoder.encode(password, "utf-8");

String joint = "userName=" + userencode + "&" + "password=" + passencode;

byte[] postData = joint.toString().getBytes("UTF-8");

URL url = new URL("https:");

HttpsURLConnection.setDefaultHostnameVerifier(new NullHostNameVerifier());

HttpURLConnection con = (HttpsURLConnection) url.openConnection();

con.setRequestMethod("POST");

con.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");

con.setRequestProperty("Connection", "keep-alive");

con.setDoOutput(true);

OutputStreamWriter wr = new OutputStreamWriter(con.getOutputStream());

System.out.println("url load");

wr.write(joint);

wr.flush();

String inputLine;

int res=con.getResponseCode();

System.out.println(res);

BufferedReader in = new BufferedReader(new

InputStreamReader(con.getInputStream()));

StringBuffer responsedata = new StringBuffer();

while ((inputLine = in.readLine()) != null) {

responsedata.append(inputLine);

}

System.out.println(responsedata);

}

catch (Exception e) {

System.out.println("Exception"+e);

}