public class ChatActivity extends Activity {

  // ...
  
  protected WifiManager wifiManager = null;

  @Override
  protected void onCreate(Bundle state) {
    super.onCreate(state);
  
    // ...

    WifiManager wifiManager = (WifiManager)getSystemService(Context.WIFI_SERVICE);
    multicastLock = wifiManager.createMulticastLock(Constants.TAG);
    multicastLock.setReferenceCounted(true);
    multicastLock.acquire();
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
  
    // ...
  
    if (multicastLock != null) {
      multicastLock.relase();
      multicastLock = null;
    }
  }
  
  // ...
  
}