Bunch of Java classes that allow you to read a Google Calender and output all user's calenders/events in a structured way as json

APIManager.java

package project1;

import java.io.UnsupportedEncodingException;

import java.text.SimpleDateFormat;

import java.util.Date;
import java.util.HashMap;
import java.net.URLEncoder;

import org.json.JSONObject;

public class APIManager {
    private static final String API_KEY = "YOUR-API-KEY";
    private static final String CLIENT_ID = "YOUR-CLIENT-ID";
    private static final String CLIENT_SECRET = "YOUR-CLIENT-SECRET";
    
    // url for checking token info
    private static final String TOKEN_INFO_URL = "https://www.googleapis.com/oauth2/v1/tokeninfo";
    // url for refresh token
    private static final String REFRESH_TOKEN_URL = "https://accounts.google.com/o/oauth2/token";
    // url for retrieving calendar lists
    private static final String CALENDARLIST_URL = "https://www.googleapis.com/calendar/v3/users/me/calendarList";
    // url for retrieving event lists
    private static final String EVENTLIST_URL = "https://www.googleapis.com/calendar/v3/calendars";
    // utf-8 encoding
    private static final String ENCODING = "UTF-8";
    
    public APIManager() {
        super();
    }

    /**
     * Checks whether accessToken is valid or not.
     * If it is not valid, then send request with refresh token to update access token.
     * Otherwise, return access token.
     * 
     * @param accessToken
     * @param refreshToken
     * @return
     */
    public static String obtainAccessToken(String accessToken, String refreshToken) {
        String urlString = TOKEN_INFO_URL + "?access_token=" + accessToken;
        try {
            // get token info
            String result = NetworkManager.retriveDataFromUrl(urlString, NetworkManager.GET, null, null);
            JSONObject tokenJson = new JSONObject(result);
            
            // if result includes :"error" we try to get access token by using refresh token
            if (tokenJson.has("error")) {
                HashMap<String, String> params = new HashMap<String, String>();
                params.put("client_id", CLIENT_ID);
                params.put("client_secret", CLIENT_SECRET);
                params.put("refresh_token", refreshToken);
                params.put("grant_type", "refresh_token");
                    
                String repeat = NetworkManager.retriveDataFromUrl(REFRESH_TOKEN_URL, NetworkManager.POST, null, params);
                JSONObject json = new JSONObject(repeat);
                return json.getString("access_token");
            } else {
                return accessToken;
            }
        } catch (Exception e) {
            return "";
        }
    }

    /**
     * Returns json string as String, representing calendars for particular access token.
     * 
     * @param accessToken
     * @return
     */
    public static String getCalendarLists(String accessToken) {
        String urlString = CALENDARLIST_URL + "?" +
            "fields=items(id%2Csummary)" +
            "&key=" + API_KEY +
            "&access_token=" + accessToken;
        try {
            String result = NetworkManager.retriveDataFromUrl(urlString, NetworkManager.GET, null, null);
            return result;
        } catch (Exception e) {
            return "{\"error\" : \"" + e.getMessage() + "\" }";
        }   
    }

    /**
     * Returns events for particular calendar.
     * Day offset provides range of timeMin and timeMax for events.
     * 
     * @param accessToken
     * @param calendarId
     * @param dayOffset
     * @return
     */
    public static String getEventsForCalendar(String accessToken, String calendarId, int dayOffset) {
        String dateFormat = "yyyy-MM-dd'T'00:00:00'Z'";
        
        long currentTimeStamp = System.currentTimeMillis() / 1000L;
        long offsetTimeStamp = currentTimeStamp + dayOffset*24*3600;
        
        String timeMin = new SimpleDateFormat(dateFormat).format(new Date(currentTimeStamp * 1000));
        String timeMax = new SimpleDateFormat(dateFormat).format(new Date(offsetTimeStamp * 1000));
        
        // try encoding calendar id, as it can contain symbols like "#" etc.
        try {
            calendarId = URLEncoder.encode(calendarId, ENCODING);
        } catch (UnsupportedEncodingException e) {
            calendarId = calendarId.replace("#", "%23");
        }
        
        String urlString = EVENTLIST_URL + "/" + calendarId + "/events?" +
            "timeMax=" + timeMax +
            "&timeMin=" + timeMin +
            "&fields=items(description%2Cid%2Ckind%2Clocation%2Csummary)" +
            "&key="+API_KEY +
            "&access_token=" + accessToken;
        
        try {
            String result = NetworkManager.retriveDataFromUrl(urlString, NetworkManager.GET, null, null);
            return result;
        } catch (Exception e) {
            return "{\"error\" : \"" + e.getMessage() + "\" }";
        }   
    }
}

Main.java

package project1;

import java.io.IOException;
import java.util.Date;

import java.net.MalformedURLException;

import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;

import java.text.SimpleDateFormat;

import org.json.*;

import java.util.HashMap;

public class Main {
    public Main() {
        super();
    }
    
    public static void main(String[] args) throws NoSuchAlgorithmException,
                                                  KeyManagementException,
                                                  MalformedURLException,
                                                  IOException {
        String accessToken = "ya29.XwB1ey41cXk9QRwAAAD7eqvBAexybHZOyanLFwfud0Wn9408TgjX1iNC5ZpuGA";
        String refreshToken = "1/EAy446VlsEoGD190XAcMxB6T4S8NbTNh9SUr6cefxvk";
        
        System.out.println(getCalendarsAndEvents(accessToken, false, refreshToken, 10));
    }
    
    /**
     * Returns json with calendar data and events.
     * If "checkToken" is true, method will call "obtainAccessToken" method to make sure that access token is valid.
     * dayOffse is offset between max event date and min event date.
     * 
     * @param accessToken
     * @param checkToken
     * @param refreshToken
     * @param dayOffset
     * @return
     */
    private static String getCalendarsAndEvents(String accessToken, boolean checkToken, String refreshToken, int dayOffset) {
        if (checkToken)
            accessToken = APIManager.obtainAccessToken(accessToken, refreshToken);
        
        String calendarsString = APIManager.getCalendarLists(accessToken);
        JSONObject json = new JSONObject(calendarsString);
        
        if (json.has("error"))
            return calendarsString;
        
        String result = "{ \"calendars\" : [";
        
        JSONArray calendars = json.getJSONArray("items");
        for (int i=0; i<calendars.length(); i++) {
            String id = calendars.getJSONObject(i).getString("id");
            String summary = calendars.getJSONObject(i).getString("summary");
            result += "{ \"id\" : \"" + id + "\", \"summary\" : \"" + summary + "\", ";
            String eventsString = APIManager.getEventsForCalendar(accessToken, id, dayOffset);
            result += eventsString.substring(1, eventsString.length()-1) + " }" + ((i<calendars.length()-1)?",":"");
        }
        
        result += " ] }";
        
        return result;
    }
}

NetworkManager.java

package project1;

import java.io.BufferedReader;
import java.io.IOException;

import java.io.InputStreamReader;
import java.io.DataOutputStream;

import java.net.MalformedURLException;
import java.net.URL;
import javax.net.ssl.HttpsURLConnection;

import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

import java.util.HashMap;

import javax.net.ssl.*;

public class NetworkManager {
    public static final String GET = "GET";
    public static final String POST = "POST";
    
    public NetworkManager() {
        super();
    }

    /**
     * Sends HTTP request and receives json data (preferably).
     * If error happened, would return json: {error: "response error message", code : "http response code"}
     * 
     * @param urlString
     * @param method
     * @param headerParams
     * @param params
     * @return
     * @throws NoSuchAlgorithmException
     * @throws KeyManagementException
     * @throws MalformedURLException
     * @throws IOException
     */
    public static String retriveDataFromUrl(String urlString, String method, HashMap<String, String> headerParams, HashMap<String, String> params) throws NoSuchAlgorithmException,
                                                  KeyManagementException,
                                                  MalformedURLException,
                                                  IOException {
        // configure the SSLContext with a TrustManager
        // I hate this part, though necessary to send https requests
        SSLContext ctx = SSLContext.getInstance("TLS");
        ctx.init(new KeyManager[0], new TrustManager[] {new DefaultTrustManager()}, new SecureRandom());
        SSLContext.setDefault(ctx);

        // prepare connection
        URL url = new URL(urlString);
        HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
        conn.setRequestMethod(method);
        
        // set header params
        if (headerParams != null)
            for (String key : headerParams.keySet()) 
                conn.setRequestProperty(key, headerParams.get(key));
        
        // check if it is post, and set up request body
        if (method.equals(POST)) {
            String postParams = "";
            if (params != null)
                for (String key : params.keySet()) 
                    postParams += "&" + key + "=" + params.get(key);
                                          
            conn.setDoOutput(true);
            DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
            
            // substring is simply to eliminate leading "&". I know, should have done better, though lack of time...
            wr.writeBytes(postParams.substring(1)); 
            wr.flush();
            wr.close();
        }
        
        // still ssl...
        conn.setHostnameVerifier(new HostnameVerifier() {
            @Override
            public boolean verify(String arg0, SSLSession arg1) {
                return true;
            }
        });
        
        // get response body
        if (conn.getResponseCode() == HttpsURLConnection.HTTP_OK) {
            String line;
            StringBuffer result = new StringBuffer();
            BufferedReader bf = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            
            while (true) {
                line = bf.readLine();
                if (line == null)
                    break;
                
                result.append(line);
            }
            conn.disconnect();
            
            return result.toString();
        } else {
            return "{\"code\" : \"" + conn.getResponseCode() + "\", \"error\" : \"" + conn.getResponseMessage() + "\" }";
        }
    }
    
    private static class DefaultTrustManager implements X509TrustManager {

        @Override
        public void checkClientTrusted(X509Certificate[] arg0, String arg1) throws CertificateException {}

        @Override
        public void checkServerTrusted(X509Certificate[] arg0, String arg1) throws CertificateException {}

        @Override
        public X509Certificate[] getAcceptedIssuers() {
            return null;
        }
    }
}

I wrote classes with a simple output, as System.Out.println(). To somehow figure out how it works, you need to run Main.java (method "getCalendarsAndEvents" in particular). I did not know, whether I was allowed to use framework for Google Calendar API, so I did not use it. Notice, that it uses Json library (http://www.json.org/java/index.html) to work with json objects correctly (it would take me a lot of time to code that).

Output for "getCalendarsAndEvents(accessToken, false, refreshToken, 10)":
{
"calendars": [{
"id": "[email protected]",
"summary": "[email protected]",
"items": []
}, {
"id": "en-gb.australian#[email protected]",
"summary": "Holidays in Australia",
"items": [{
"kind": "calendar#event",
"id": "20140813_60o30chj6go30e1g60o30dr4ck",
"summary": "Royal National Agricultural Show Day Queensland",
"description": "Public holiday in: Queensland"
}]
}, {
"id": "p#[email protected]",
"summary": "Weather",
"items": [{
"kind": "calendar#event",
"id": "20140812_weatherP",
"summary": "Forecast for Christchurch (12° | 2°)"
}, {
"kind": "calendar#event",
"id": "20140813_weatherP",
"summary": "Forecast for Christchurch (11° | 1°)"
}, {
"kind": "calendar#event",
"id": "20140814_weatherP",
"summary": "Forecast for Christchurch (8° | 3°)"
}, {
"kind": "calendar#event",
"id": "20140815_weatherP",
"summary": "Forecast for Christchurch (15° | 3°)"
}]
}]
}