emacs questions

[Pedro Sa da Costa]

I'm trying to have code completion in emacs, but i don't know what to 
do. In eclipse, when we writing a java code line, for example:
System.out., we do C^SPACE to show a window with several methods 
associated (printl, print,etc).
I would like to have something similar in emacs. Can anybody help me? 
And, if the solution passes through etags, can anybody give me a simple 
example, because i don't understand why etags are useful.

Can we browse java api through emacs? I've installed jde, but 
jde-browse-jdk-jdk doesn't work. It says that browse-url-new-window-p is 
null.

It's possible to see, while we are programming, if we did a mistake. In 
eclipse, when we do an error, for example, forget a ; , an underline 
appears in the line indicating that something is wrong. It's possible to 
have something like this in emacs?

Why, sometimes, when we do copy from another application, and we try to 
do paste in the emacs, emacs only yanks the last thing we cut or copy in 
emacs?

Thanks,
Pedro