test objects design

I was making changes at my Electricity provider GreenMountain.com and kaboom I got an exception (below). As a tester I wonder how I could have tested this scenario. Where in here are we blowing up and why? The bug is somewhere in MyAccountHandler.getBilingType method. I wonder what kind of a user I am set up as that I blow up on accessing this option. Hmm…

Error: 500

Location: /servlet/MyAccountHandler

Internal Servlet Error:

java.util.ConcurrentModificationException

at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:445)

at java.util.AbstractList$Itr.next(AbstractList.java:422)

at com.gmec.web.servlets.MyAccountHandler.getBillingType(MyAccountHandler.java:1394)

at com.gmec.web.servlets.MyAccountHandler.navigate(MyAccountHandler.java:246)

at com.gmec.web.servlets.MyAccountHandler.doGet(MyAccountHandler.java:90)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)

at org.apache.tomcat.core.ServletWrapper.doService(ServletWrapper.java:405)

at org.apache.tomcat.core.Handler.service(Handler.java:287)

at org.apache.tomcat.core.ServletWrapper.service(ServletWrapper.java:372)

at org.apache.tomcat.core.ContextManager.internalService(ContextManager.java:812)

at org.apache.tomcat.core.ContextManager.service(ContextManager.java:758)

at org.apache.tomcat.service.connector.Ajp13ConnectionHandler.processConnection(Ajp13ConnectionHandler.java:160)

at org.apache.tomcat.service.TcpWorkerThread.runIt(PoolTcpEndpoint.java:416)

at org.apache.tomcat.util.ThreadPool$ControlRunnable.run(ThreadPool.java:501)

at java.lang.Thread.run(Thread.java:484)