How do classes talk to each other - newb question

Question

Hey, im fairly new to java and have a problem
If i have my main class (main) instantiate a new object of a class (ftp)
how does ftp return information to main after it has been created,
i might need the information at any time so i can't use the constructor again, can i?
i realise main could call a method of ftp to return some information but i cant have it doing it every second when it might only need the information every 10 minutes
can i create my own eventlistener in main to listen for something i can start in ftp?
i can up some code if you like cos i'm sure that makes no sense but any help is very welcome :)

Answer 1

A listener/observer pattern can help, as you have noted.
public class DataClass {
    private int field1;
    //private String field2;
    private List<DataChangedListener> listenerList = new ArrayList<DataChangedListener>();
    public void setField1(int newVal) {
       field1 = newVal;
       fireUpdate();
    public void getField1() { return field1; }
    private void fireUpdate() {
       for ( DataChangedListener listener: listenerList )  {
          listener.dataChanged(this);
    public void addListener( DataChangedListener listener) {
       listenerList.add(listener);
public interface DataChangedListener {
   public void dataChanged(DataClass source);
}