public class NetChangeReceiver extends BroadcastReceiver {

  private static final String ANDROID_NET_CHANGE_ACTION = "android.net.conn.CONNECTIVITY_CHANGE";

  @Override
  public void onReceive(Context context, Intent intent) {
    if(intent.getAction().equalsIgnoreCase(ANDROID_NET_CHANGE_ACTION)){
      Toast.makeText(context, "Net Changed", Toast.LENGTH_SHORT).show();
    }
  }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.outman.example.androidtest">

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

  <application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true">

    <activity android:name=".MainActivity">
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>

    <receiver android:name=".NetChangeReceiver">
      <intent-filter >
        <action android:name="android.net.conn.CONNECTIVITY_CHANGE"/>
      </intent-filter>
    </receiver>
  </application>

</manifest>

public class MainActivity extends Activity {
  private NetChangeReceiver netChangeReceiver;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    netChangeReceiver = new NetChangeReceiver();
    registerReceiver(netChangeReceiver, new IntentFilter("android.net.conn.CONNECTIVITY_CHANGE"));
  }

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

public class MainActivity extends Activity {
  private ConnectivityManager.NetworkCallback networkCallback;
  private ConnectivityManager connectivityManager;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    networkCallback = new NetworkCallbackImpl();
    NetworkRequest.Builder builder = new NetworkRequest.Builder();
    NetworkRequest request = builder.build();
    connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    connectivityManager.registerNetworkCallback(request, networkCallback);
  }

  @Override
  protected void onDestroy() {
    super.onDestroy();
    connectivityManager.unregisterNetworkCallback(networkCallback);
  }
  private class NetworkCallbackImpl extends ConnectivityManager.NetworkCallback {
    @Override
    public void onAvailable(Network network) {
      super.onAvailable(network);
      Toast.makeText(getBaseContext(), "onAvailable", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onLosing(Network network, int maxMsToLive) {
      super.onLosing(network, maxMsToLive);
      Toast.makeText(getBaseContext(), "onLosing", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onLost(Network network) {
      super.onLost(network);
      Toast.makeText(getBaseContext(), "onLost", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onCapabilitiesChanged(Network network, NetworkCapabilities networkCapabilities) {
      super.onCapabilitiesChanged(network, networkCapabilities);
      Toast.makeText(getBaseContext(), "onCapabilitiesChanged", Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onLinkPropertiesChanged(Network network, LinkProperties linkProperties) {
      super.onLinkPropertiesChanged(network, linkProperties);
      Toast.makeText(getBaseContext(), "onLinkPropertiesChanged", Toast.LENGTH_SHORT).show();
    }
  }
}