public class WebAuthProvider {
    
    private static WebAuthProvider providerInstance;

    private WebAuthProvider(@NonNull Auth0 account) {
        this.account = account;
        //Set default values
    }

    public static class Builder {
        //private constructor. Can only be instantiated from the init() method.
        Builder(Auth0 account) {
            this.account = account;
        }

        //Builder methods to configure the instance
        //...
        ///

        /**
         * Begins the authentication flow.
         */
        public void start(@NonNull Activity activity, @NonNull AuthCallback callback, int requestCode) {
            providerInstance = new WebAuthProvider(account)
                    .useBrowser(useBrowser)
                    .useFullscreen(useFullscreen)
                    .withState(state)
                    .withScope(scope)
                    .useCodeGrant(useCodeGrant)
                    .withParameters(parameters)
                    .withConnection(connectionName);

            providerInstance.start(activity, callback, requestCode);
        }
    }

    // Public methods (User interface)

    // Inits the instance. Can be configured, the user will finally call start().
    public static Builder init(@NonNull Auth0 account) {
        return new Builder(account);
    }

    //Finish the authentication if an instance is found
    public static boolean resume(int requestCode, int resultCode, Intent data) {
        if (providerInstance == null) {
            Log.w(TAG, "There is no previous instance of this provider.");
            return false;
        }
        return providerInstance.authorize(requestCode, resultCode, data);
    }
    
    public static boolean resume(Intent data) {
        if (providerInstance == null) {
            Log.w(TAG, "There is no previous instance of this provider.");
            return false;
        }
        return providerInstance.authorize(data);
    }

    // End Public methods

    
    // Private methods to handle the authentication
    //...
}