Add authority

   <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

Create an instance of IntentFilter and add a value to it, action of "Android. Net. Conn.connectivity" change
When the network status changes, the system issues a broadcast with the value of android.net.connectivity'change. If the broadcast receiver wants to listen to any broadcast, add the corresponding action here.

 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        intentFilter = new IntentFilter();
        intentFilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");
        networkChangeReceiver = new NetworkChangeReceiver();
        registerReceiver(networkChangeReceiver,intentFilter);

    }

All dynamically registered broadcast receivers must be deregistered

 @Override
    protected void onDestroy() {
        super.onDestroy();
        unregisterReceiver(networkChangeReceiver);
    }

Define an internal class NetworkChangeReceiver in MainActivity, inherit BroadcastReceiver, and override onReceive method, whenever the network changes
onReceive will execute

   class NetworkChangeReceiver extends BroadcastReceiver{
        @Override
        public void onReceive(Context context, Intent intent) {
            ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(context.CONNECTIVITY_SERVICE);
            NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
            if (activeNetworkInfo!=null && activeNetworkInfo.isAvailable()){
                Toast.makeText(context, "network is available", Toast.LENGTH_SHORT).show();
            }else{
                Toast.makeText(context, "network is unavailable", Toast.LENGTH_SHORT).show();
            }
        }
    }