Seems like you created the Jpanel using command
JPanel asd = new asd();
instead of
JPanel asd = new JPanel();
EDIT: based on the code in the comments, its because you're calling methods globally, you can only instantiate objects globally.
If you want to generate entire panel in the constructor, your code should look like this:
public class kompiouteraki{
// attributes here
public kompiouteraki(){
// your code here
}
}
For more information, about how to set up your class, visit class structure