public class ViewActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_conect);

        WebView webView = (WebView) findViewById(R.id.webView);

        webView.setWebViewClient(new CustomWebViewClient());

        webView.loadUrl("http://www.google.com");
    }

    private boolean isDeviceOnline() {
        ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();

        return (netInfo != null && netInfo.isConnectedOrConnecting());
    }

    private class CustomWebViewClient extends WebViewClient {
        @Override
        public boolean shouldOverrideUrlLoading(WebView view, String url) {
            if (!isDeviceOnline()) {
                webView.loadUrl("file:///android_asset/not-found.html");
            } else {
                view.loadUrl(url);
            }

            return true;
        }
    }
}