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.
you can try DatabaseMetaData.getUserName this will give you user name used in connection. If you need all users in system Not sure what level of previliges you have but can try Oracle specific query "select * from DBA_USERS".
Check this link for more details on DBA_USERS table.