How can I learn Java ?

If you're dead set on learning Java from a book I'd recommend Head First Java

It will be a long time before you're making any RTS games though if you've never programmed before.

http://docs.oracle.com/javase/tutorial/

I'm an experienced Java programmer and this is my advice:

You really just need to sit down and write code. If you want to make an RTS then go for it, if you want to make an MMO, go for it. Odds are you'll tear through tons of projects before you finally actually finish one so it doesn't really matter. The important thing is to keep writing code. Another thing: you don't need to reinvent the wheel and create your own game engine. Although you will learn a lot it will take a very, very long time. If games are what you want to do and you want to make them in Java then I would suggest finding a good Java game engine. Personally, I use the Java Monkey Engine (http://jmonkeyengine.org/) because it comes with its own IDE (built off of Netbeans) and performance wise it's pretty good. It also makes Android deployment way easier than using Google's Android APIs. It also has somewhat decent documentation and they have a decent sized community if you need help.

For a beginner, just follow an online tutorial. Like what @Kilabit said, you just need to write code. I usually just use books as reference to look up some stuff I don't understand. Or when I do have a lot of time, that's when I sit down and dig deep on the concepts. Most of the time, I don't use books. I just google my immediate needs. That is enough most of the time. Bottom line, follow an online tutorial.

if you want to learn java I would recommend this book

Starting Out with Java: From Control Structures through Objects

http://www.amazon.com/Starting-Out-Java-Control-Structures/dp/0132855836/ref=sr_1_4?s=books&ie=UTF8&qid=1403295913&sr=1-4&keywords=Starting+Out+with+Java%3A+From+Control+Structures+through+Data+Structures

It is a good book. we used it in our first year of college. this is also assuming you have no previous background in programming.

Then when you're conformable programming, you could try something on a smaller scale, such as something like pong, pacman, tetris or a space invaders. these games will help you understand the fundamentals of 2d game programming.

Don't go rushing into something that will be over your head right away, like an RTS. The problems you will face in the above games will be more than enough to keep you busy.

Good luck, Happy coding!

http://www.indiegameprogramming.com/Resources.php

this worked for me http://docs.oracle.com/javase/tutorial/reallybigindex.html

i learn from an app, just look for java apps. there are tons

i learn from an app, just look for java apps. there are tons

Just because there are apps for it doesn't mean they properly teach you or are accurate. I would recommend books that are acknowledged as being accurate and sites that are known for being accurate in order for a beginner in whatever language to learn from them. Apps would be like blowing 0.99 to get one of the sad Amazon books to learn a language.