Working through the deitel book and I'm back in chapter 2 reviewing the questions. With regards to variable names I said all these were valid _under_bar_, m928134, t5, j7, her_sales$, his_$account_total, a, b$, c, z, z2, h22
I said all these were invalid 3g, 87, 67h2, h22, 2h.
While you're reading that book, I recommend that you frequently take a look at The Java Language Specification. Section 3.8 on identifiers answers your question. Another strategy to answer such questions, that I strongly recommend, is that you don't fully believe what the book or anybody here says, and you write a quick test program to answer the question(s). Good Luck.