[SalesForce] Custom sObject sorting with ‘implements comparable’

I'm trying to perform a sort on a list of custom sObjects using the 'implements comparable' wrapper method. I'm running into some error that it seems isn't affecting anyone else. My custom object looks like so:

gAlert:{
    title: text,
    published: date
}

I'm trying to sort by published.

I tried implementing the following:

public class gAlertWrapper implements Comparable
{
    public Integer compareTo(Object compareTo) 
    {
        gAlertWrapper gAlertWrapper = (gAlertWrapper) compareTo;
        if (gAlert__c.published__c == gAlertWrapper.gAlert__c.published__c) return 0;
        if (gAlert__c.published__c > gAlertWrapper.gAlert__c.published__c) return 1;
        return -1;        
    }
}

I get the following error:

Variable does not exist: gAlert__c

from the following line:

if (gAlert__c.published__c == gAlertWrapper.gAlert__c.published__c) return 0;

The problem seems to be in gAlertWrapper.gAlert__c.published_c

What am I doing incorrectly?

I tried implementing a constructor, like so:

public gAlert__c gA;
public gAlertWrapper(gAlert__c g) {
        gA = g;
    }

But that didn't seem to work either… How can I piece this together?

Thanks!

EDIT: Everything seemed to be working fine, as there were no compiling errors. But at run time there was a

System.NullPointerException: Attempt to de-reference a null object

I know it's not the xml I'm parsing in – it's the same stuff from before.

Here is the code that is trying to save values to entry.gA:

public static list<gAlert__c> getAlerts(string url){
        HttpRequest req = new HttpRequest();
        req.setEndpoint(url);
        req.setMethod('GET');

        Http http = new Http();
        HttpResponse res = http.send(req);
                Dom.Document doc = res.getBodyDocument();
                Dom.XMLNode address = doc.getRootElement();

                string title = '';
                integer count = 1;

        list<gAlert__c> results = new list<gAlert__c>();
        gAlertWrapper entry = new gAlertWrapper();      
        for(Dom.XMLNode child:address.getChildElements()){
            string childElement = child.getName();
            if(childElement == 'title'){
                title = child.getText();
            }
            else if(childElement == 'entry'){
                for(Dom.XmlNode grandChild : child.getChildElements()){
                        string grandChildElement = grandChild.getName();
                    if(grandChildElement == 'title'){
                        entry.gA.title__c = grandChild.getText();
                    }
                    else if(grandChildElement == 'link'){
                        entry.gA.link__c = grandChild.getAttributeValue('href', null);
                    }
                    else if(grandChildElement == 'published'){
                        entry.gA.published__c = grandChild.getText().replace('Z','').replace('T',' ');
                    }
                    else if(grandChildElement == 'content'){
                        entry.gA.content__c = grandChild.getText();
                    }
                    else if(grandChildElement == 'author'){ //this is the last element in the tree
                        results.add(entry.gA.clone(false,false,false,false));
                        entry.gA.clear();
                    }
                }
            }
        }
        return results;
    }
    public class gAlertWrapper implements Comparable{
        public gAlert__c gA;
        //public gAlertWrapper(gAlert__c g) {
        //gA = g;
        //}
        public Integer compareTo(Object compareTo) {
            gAlertWrapper gAlertWrapper = (gAlertWrapper) compareTo;
            if (gA.published__c == gAlertWrapper.gA.published__c) return 0;
            if (gA.published__c > gAlertWrapper.gA.published__c) return 1;
            return -1; 
        }
    }

I'm getting the error at the line:

entry.gA.title__c = grandChild.getText();

I know it isn't the grandVhild.getText(), since that hasn't changed. What is strange is that I would imagine the null pointer would reflect the right side of the equation no?

I guess I'm really fumbling my way around learning how to use this comparable class..

Thank you so much for your patience and help. Happy to upvote everything you've written – it all deserves it.