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.
They are not really compatible. Jikes is a compiler, which takes source code and generates bytecode ready to give to a JVM. Hotspot is a JVM (or at least a speed-up module for one) which takes bytecode and runs it.