You cannot use variables that have not been initialized. If you try, the compiler will give you an error message.
However, class member variables are initialized implicitly if you don't initialize them yourself, whereas local variables (variables inside methods) are not - they must be initialized explicitly. For example:
the best way to learn this stuff is to try it out!!! write small, simple programs that do almost nothing (a "Hello, world!!" program would work for many cases), and see what happens!!! it's fast, easy, and you will learn it faster and remember it better if you do.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors