Shifting between Java and SAP!!.
I would suggest working in both SAP and Java.
Currently SAP has lots of support for Java since its main product, SAP Netweaver has a Java Stack. It would be easy to learn SAP Portal or SAP XI with Java knowledge.
IMHO: Build multiple skills and build a stable future.
SAP Certified Developer Consultant -Netweaver 2004
SUN Certified Java Programmer 1.4
SUN Certified Java Associate 1.0
IBM Certified Database Associate 700