And also, I know that Greenfoot requires Java, but say I wanted to code something for java, could I just code in the same style I learned to by using Greenfoot?
You don't strictly speaking need the book, but it will help you to understand the fundamentals. You could use another Java book also, or try to find a comprehensive tutorial online.
but say I wanted to code something for java, could I just code in the same style I learned to by using Greenfoot?
You don't code something for Java, you code in Java - Java is the programming language. In Greenfoot you write programs in Java. You can use Java outside Greenfoot as well (but the Greenfoot specific API, such as the World and Actor classes, won't be available).