Warnings and observations- need some explications and opinions

Question

Hallo,
I'm new to Java and using Eclipse for coding. I copied the following code from a book, but Eclipse gives me some yellow flagged warnings (not critical). There are few I can't understand. This would help me a lot in understanding Java better, as I couldn't find answers. Thank you.
warnings I get:
1) The serializable class Window does not declare a static final serialVersionUID field of type long     demo.java     teste/src     line 10     Java Problem
2) The static field BorderLayout.CENTER should be accessed in a static way     demo.java     teste/src     line 29     Java Problem
interesting observations:
3) If I delete setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); and setContentPane(ca);, the program works as before. Are they necessary?
4) Which Layout do you use? Border layout looks for me very unuseful if you can only add 5 buttons, you can' arrange tham ore leave spaces between. Do you use Gridbag Layout in application ore what other, so that I know which is the best
5) I also get the warning : The local variable win is never read     demo.java     teste/src     line 6     Java Problem. Is there any problem IF I have warnings in my code? I don't need to do anything else with the win variable? Should I add some parameters to the variable or leave the warning message.
Thank you very much
import java.awt.*;
import javax.swing.*;
public class demo {
     public static void main (String [] args){
          Window win=new Window();
class Window extends JFrame {
     public Window(){
          super ("Title Bar");
          setSize(400,100);
          //setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          setVisible(true);
          Container ca = getContentPane();
          ca.setBackground(Color.lightGray);
          BorderLayout blm = new BorderLayout();
          ca.setLayout(blm);
          JButton bt1=new JButton ("Start test 1");
          ca.add(bt1,blm.WEST);
          JButton bt2=new JButton("Start test 2");
          ca.add(bt2,blm.EAST);
          JButton bt3=new JButton("Start test 3");
          ca.add(bt3,blm.NORTH);
          JButton bt4=new JButton("Start test 4");
          ca.add(bt4,blm.SOUTH);
          JButton bt5=new JButton("Start test 5");
          ca.add(bt5,blm.CENTER);
          //JButton bt6=new JButton("Start test 6");
          //ca.add(bt6,blm.CENTER);
          //setContentPane(ca);
}

Answer 1

1. You simply haven't defined the static long serialVersionId variable (not really necessary).
2. blm.CENTER should be BorderLayout.CENTER (No biggie but should definately be changed).
3.
part a. The default action is dispose on close, but since your program does nothing other than this simple GUI it will also exit at that point. So, in this case, it doesn't matter, but is better to set it, if that is what you want to have happen.
part b. since ca is defined with getContentPane(), using setContentPane() is not needed.
Judging by the stuff shown, if this is a true copy of the example from the book you used, and not edited by yourself, then I would say to find a new book. These things may only be warnings, and no real big thing, but in a book they are signs of disaster. As it will either cause bad habits in the person using them, or even cause real problems in other areas. If they are this sloppy in such a simple example, I would hate to see anything more complicated from them.