How to retrieve data from lookup?

How to retrieve data from lookup?

Example below shows that Case_Account__c contain lookup on Case and Account.
 

SELECT Case__r.caseNumber,Account__r.Name FROM Case_Account__c

Put __r at the behind of lookup object.

FATAL_ERROR|System.StringException: no protocol

I received this error :
03:10:47.139 (1139188000)|FATAL_ERROR|System.StringException: no protocol: mypersonaldomain.com

I believed I already tested it before and it should work fine.But for sudden I got this error.The snippet of the code is like below :

   String str =String)this.controller.getRecord().get('User_Referrer__c');  
    if(str != null){  
       URL newURL= new URL(str);  
       System.debug('newURL:::::: host '+newURL.getHost() );  
     }   

Actually the code break when it want to create new URL.

       URL newURL= new URL(str);  

The flow is start like this.User_Referrer__c is URL datatype that being inserted by the user.The code will capture the value , create new URL from it.Previously it work fine until one day, I got report that suddenly it break.

So I try to investigate and found out that the value for User_Referrer__c should be http://mypersonaldomain.com instead of mypersonaldomain.com . See how simple http:// will cause the trouble. I updated the value to http://mypersonaldomain.com  and it work successfully.

After this, I need to think how to validate the value that entered by user. :)