Unit 3: Network communication and publishing

• Typical flow: the app sends an HTTP request to a REST API, the server returns JSON, and the app parses and displays it.

java// Network calls must run off the main (UI) thread; INTERNET permission required
ExecutorService ex = Executors.newSingleThreadExecutor();
ex.execute(() -> {
    try {
        URL url = new URL("https://api.example.com/courses");
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod("GET");
        BufferedReader br = new BufferedReader(new InputStreamReader(con.getInputStream()));
        StringBuilder sb = new StringBuilder(); String line;
        while ((line = br.readLine()) != null) sb.append(line);
        String json = sb.toString();
        runOnUiThread(() -> showCourses(json));          // update UI on main thread
    } catch (IOException e) { e.printStackTrace(); }
});

• Libraries: OkHttp and Retrofit (with Gson or Moshi) simplify requests, Volley handles request queues; the old Apache HttpClient was removed from Android.

java// JSON: {"courses":[{"name":"BCA","seats":60},{"name":"B.Sc IT","seats":60}]}
JSONArray arr = new JSONObject(json).getJSONArray("courses");
for (int i = 0; i < arr.length(); i++) {
    JSONObject c = arr.getJSONObject(i);
    Log.d("API", c.getString("name") + " " + c.getInt("seats"));
}

java// XML pull parsing: <course><name>BCA</name></course>
XmlPullParser p = Xml.newPullParser();
p.setInput(inputStream, null);
int ev = p.getEventType();
while (ev != XmlPullParser.END_DOCUMENT) {
    if (ev == XmlPullParser.START_TAG && p.getName().equals("name")) Log.d("XML", p.nextText());
    ev = p.next();
}

• A foreground service performs work the user notices (music playback, navigation, fitness tracking) and must show an ongoing notification; it is less likely to be killed by the system.

javaNotification n = new NotificationCompat.Builder(this, "player")
        .setContentTitle("Playing lecture audio").setSmallIcon(R.drawable.ic_play).build();
startForeground(1, n);        // inside the service, soon after it starts

• Recent Android versions require declaring a foreground service type and permission; for deferrable background work, WorkManager is recommended.

• Signing: every APK or bundle must be digitally signed. Create an upload key in a keystore (Build → Generate Signed Bundle/APK); with Play App Signing, Google holds the app signing key and you sign uploads with the upload key. Losing the key without Play App Signing means you cannot update the app.

• Android Market was renamed Google Play in 2012; apps can also be distributed through other stores or direct APK downloads.