It not a problem whether you are full time employee or over time. Just make some time and prepare for it. How much Java you know? 1) If you are very new then buy Head First Java, 2nd Edition by Kathy Sierra & Bert Bates first and study that througly then Steps 2 2) If you know Java then buy SCJP Sun Certified Programmer for Java 5 Study Guide by Katherine Sierra & Bert Bates 3) Keep on visiting Javaranch and try to get involve in the forum 4) Read, Read, Read 5) Try every Mock exam you can.
Then I am sure you will pass by Aug! even you working Full Time! Good Luck