Sun Certified Java Programmer(SCJP) heps to recognize you as an expert in Java.It is tool that IT managers use to identify those who have a solid grasp of the core language concepts.
I want to share some of the resources which I found online could greatly aid the preparation of SCJP aspirants.
SCJP e-BOOK:

SCJP Sun Certified Programmer for Java 6 Exam 310-065