load file_a.html with internet, load file_b.html if there is no Internet

Question

I am making an App in Android Studio. I am using webview. When someone opens the app, I want

load file_a.html with internet from assets folder

load file_b.html if there is no Internet from assets folder

How to do this? I have written this code in MainActivity.java

Where is the mistake? Even Back button is not functioning peoperly.

import androidx.appcompat.app.AppCompatActivity;

import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.os.Bundle;
import android.view.View;
import android.webkit.WebView;
import android.webkit.WebViewClient;

public class MainActivity extends AppCompatActivity {

    private boolean isNetworkAvailable () {
        ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
        NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
        return activeNetworkInfo != null && activeNetworkInfo.isConnected();
    }

    private WebView webView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        webView = findViewById(R.id.webview);
        webView.getSettings().setJavaScriptEnabled(true);

        if (!isNetworkAvailable()) {
            webView.loadUrl("file:///android_asset/file_a.html");
        } else {
            webView.loadUrl("file:///android_asset/file_b.html");

        }

        @Override
        public void onBackPressed ()
        {
            if (webView.canGoBack()) {
                webView.goBack();
            } else {
                super.onBackPressed();
            }
        }
        
    }
}