public class MainActivity extends AppCompatActivity {

    private TextView tv;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = (Button) findViewById(R.id.btnhit);
        tv = (TextView) findViewById(R.id.tv);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v){
                new JSONtask().execute("http://api.nbp.pl/api/exchangerates/tables/A?format=json");
            }
        });
    }
    public class JSONtask extends AsyncTask<String, String, String>{

        @Override
        protected String doInBackground(String... params) {
            HttpURLConnection connection = null;
            BufferedReader reader = null;
            try {
                URL url = new URL(params[0]);
                connection = (HttpURLConnection) url.openConnection();
                connection.connect();
                InputStream stream = connection.getInputStream();
                reader = new BufferedReader(new InputStreamReader(stream));
                StringBuffer buffer = new StringBuffer();
                String line = "";
                while ((line = reader.readLine()) != null) {
                    buffer.append(line);
                }
                String jsonData = buffer.toString();
                JSONObject object  = new JSONObject(jsonData);
                StringBuffer newBuffer = new StringBuffer();
                JSONArray currencyArray = object.getJSONArray("rates");
                for(int i = 0; i<currencyArray.length(); i++) {
                    JSONObject currencyObject = currencyArray.getJSONObject(i);
                    String code = currencyObject.getString("code");
                    newBuffer.append(code + "\n");
                }

                return newBuffer.toString();
            } catch (MalformedURLException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (JSONException e) {
                e.printStackTrace();
            } finally {
                if (connection != null) {
                    connection.disconnect();
                }
                try {
                    if (reader != null) {
                        reader.close();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        return null;
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            tv.setText(result);
        }
    }
}