This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
check that your listener.ora and tnsnames.ora files have the correct and matching settings for your database server. AND if the db server is on a different machine, (and you're on tcp/ip network) ensure that your HOSTS file has the machine ip address in there so that the oracle drivers can tell to which machine your connection string is referring. hope this helps.