how to create an URL field that automatically shortenes the entered URL?

/**
 * Simple service to make http callout to
 * Bitly url shortener service.
 */
public class BitlyService {
    // reusable access token for oauth,
    // required when making API requests
    private String accessToken;
    public BitlyService() {
        this.accessToken = getAccessToken();
    }
    /**
     * Given a long URL will return the shortened version.
     * http://dev.bitly.com/links.html#.v3_shorten
     */
    public String shorten( String url ) {
        HttpRequest req = new HttpRequest();
        req.setEndpoint(
            'callout:Bitly/v3/shorten' +
            '?access_token=' + this.accessToken +
            '&longUrl=' + EncodingUtil.urlEncode( url, 'UTF-8' ) +
            '&format=txt'
        );
        req.setMethod('GET');
        Http http = new Http();
        HttpResponse res = http.send(req);
        return res.getBody();
    }
    /**
     * Get the access token to make authenticated oauth calls.
     * The actual username/password credentials are stored in
     * Named Credentials so that the password is stored securely.
     * 
     * This does require an extra http request when instantiating
     * the service which adds to latency. Alternatively, you could
     * store the generated access token in a custom setting and simply
     * reference it from your code, but then anyone who can view
     * custom settings can view the access token and use the API.
     * Trade-offs.
     */
    private String getAccessToken() {
        HttpRequest req = new HttpRequest();
        req.setEndpoint('callout:Bitly/oauth/access_token');
        req.setMethod('POST');
		req.setHeader('Content-Length', '0');
        Http http = new Http();
        HttpResponse res = http.send(req);
        return res.getBody();
    }
}

public class BitlyShortenURLInvocable {
    @InvocableMethod(
        label = 'shorten'
        description = 'Given case IDs then generates a bitly short url for them'
    )
    public static List<String> shorten( List<ID> oppsIds ) {
        // You can't invoke http callouts from Process Builder or Flow
        // because the database transaction has not committed yet, you will get error:
        // "System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out"
        // To get around this then we'll actually make the call in a @Future method asynchronously.
        shortenAsync( oppsIds );
	return new List<String>();
    }
    @Future( callout = true )
    private static void shortenAsync( List<ID> oppIds ) {
        // Fetch your data and a field where you want to store the generated short url
        List<Opportunity> opps = new List<Opportunity>([ SELECT Id, Long_URL__c, Short_URL__c FROM Opportunity WHERE id IN :oppIds ]);
        // Service to actually call out to bitly and get a shortened url
	BitlyService service = new BitlyService();
        // Bitly does not support bulk url shortening
        // so you must make each call individually.
        // Do be aware of Bitly's rate limiting if you try
        // to mass create a bunch of records in short succession
        // http://dev.bitly.com/rate_limiting.html
        for ( Opportunity opp : opps ) {
            // in this trivial example, we're just creating short urls to the record itself
            opp.Short_URL__c = service.shorten( opp.Long_URL__c );
        }
        // update the records with their short urls
        // use workflow or trigger to fire off the short url field being populated
        // to then send email alerts, etc. including the short url
        if ( opps.size() > 0 ) {
            update opps;
        }
    }
}