Java is used for some games, and for some game tools. It's also used fairly extensively outside of the games industry. If you're a good Java programmer there will be jobs available, and other factors (your education, any experience you can demonstrate, how well you interview, who your competition is, etc.) will determine whether or not you're able to actually get those jobs. If you're looking to make games independently or as a hobbyist you can use any language you like, so Java is a fine choice there. It's sometimes used in professional games, so there are some jobs available, but you'll probably find that the majority are using other languages; often a mixture of C++ and various scripting languages.
It's very rare for a programmer to only ever use one programming language, and most professionals will learn and use many throughout their career. You still have plenty of time to learn additional languages before you'll even be considering professional work, so as long as you're able to learn it isn't hugely important which language you're using right now.
The best way to learn programming is to write programs! Use your books, online resources, video tutorials, etc. to learn the basics of how the language works, and how any libraries you want to learn work, and then set your own goals and try to write your own programs. Don't worry about doing things "properly" or whether or not you're doing things the most efficiently, etc.; you are going to write some really bad code, some pretty average code, and maybe a tiny bit of good code, but this is how you'll get the experience to write great code in future!
I have a java book but it's mostly on C++ and only the last chapter is on java.
What book is that? I wouldn't say you have a Java book at all there if the majority of the text covers a different language!
If you just want to work by yourself as an independent developer then Java is just fine and you should absolutely keep going with it -- some great games have been made with Java (Minecraft, Puzzle Pirates, Spiral Knights, and others), and there's no reason you can't learn to do the same.
If you want a job in the industry, I would probably recommend eventually learning C++ as well, but there's no reason you need to worry about that at this stage, as you still have plenty of time before you'll be looking for jobs -- you'll need to finish school and go to university first! In this case, I would suggest sticking with Java until you're comfortable with at least creating some smaller-to-medium complexity games (you should definitely be able to create Pong, Breakout, and a platforming game at the least) with Java, and then perhaps consider picking up C++ as a second language.
Hope that helps! 
