public class MainActivity extends AppCompatActivity {
    ...
    class NetworkChangeReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
            NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
            if (networkInfo != null && networkInfo.isAvailable()){
                Toast.makeText(context, "网络可用", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(context, "网络不可用", Toast.LENGTH_SHORT).show();
            }
        }
    }
}