Translating Visual Basic Events into Java Events

Question

I'm in the process of porting a Visual Basic application to Java. Now, most of it is pretty straight forward, since good OO practice is evident in the original VB source. With a good dose of refactoring along the way the Java version is looking pretty good too.
However, I'm having a problem with events. Here's a snuppet of one of a VB class that generates events and one that uses it:
(Invoices.cls)
-=-=-=-=-=-=-=-=-
Event AddItem(ItemID as String)
Event RemoveItem(ItemID as String)
Private m_Items as Collection
Public Sub Add(ItemID as String)
   m_Items.Add ItemID
   RaiseEvent AddItem(ItemID)
End Sub
Public Sub Remove(ItemID as String)
   m_Items.Remove ItemID
   RaiseEvent RemoveItem(ItemID)
End Sub
-=-=-=-=-=-=-=-=-and
(OrderBook.cls)
-=-=-=-=-=-=-=-=-
Private WithEvents m_Invoices as Invoices
Private Sub m_Invoices_AddItem(ItemID as String)
   CalculateTotals
End Sub
Private Sub m_Invoices_RemoveItem(ItemID as String)
   CalculateTotals
End Sub
-=-=-=-=-=-=-=-=-Can anyone give me a few pointers on what the Java equivalent to this VB code is?
Thanks,
Vince.

Answer 1

Hi,
There are few main things involved in a Java event delivery
1. The event source
2. The event listener
3. The even data
4. Registration
5. The event delivery process
I don't know the VB syntax, but any way it gives a good idea of what you wish to achieve.
Here the Invoice class is basically the event source. And the OrderBook class is the event listener. To proceed further you will have to specify (define) the event listener first. Secondly you will have to create an event object which contains the data to be delvered via the event.
1. Specifying the event listener interface
public Interface ItemOperationListener
public void itemAdded (ItemOperationEvent ioe);
public void itemRemoved (ItemOperationEvent ioe);
2. The event class
public class ItemOperationEvent extends java.util.EventObject
String item = null;
public ItemOperationevent(Object source, String Item)
super(source);
this.item = item;
public String getItem()
return item;
3. Creating and firing the event from the event source.
public class Invoice
private collection items; // any collection
// to hold all the event listeners , this way u can define > 1
// listeners
javax.swing.event.EventListenerList listenerList = new javax.swing.event.EventListenerList();
public void addItem(String item)
items.add(item);
// update the listeners
fireItemAdded(item);
public void removeItem(String item)
items.add(item);
// update the listeners
fireItemRemoved(item);
public void addItemOperationListener(itemOperationListener ioe)
listenerList.add(ioe);
public void removeItemOperationListener(itemOperationListener ioe)
listenerList.remove(ioe);
public void fireItemAdded(String item)
ItemOperationEvent e = null;
Object[] listeners = listenerList.getListenerList();
// Process the listeners last to first, notifying
// those that are interested in this event
for (int i = listeners.length-2; i>=0; i-=2) {
if (listeners== ItemOperationListener.class) {
if (e == null)
e = new ItemOperationEvent(this,item);
((ItemOperationlistener)listeners[i+1]).itemAdded(e);
public void fireItemRemoved(String item)
ItemOperationEvent e = null;
Object[] listeners = listenerList.getListenerList();
// Process the listeners last to first, notifying
// those that are interested in this event
for (int i = listeners.length-2; i>=0; i-=2) {
if (listeners[i] == ItemOperationListener.class) {
if (e == null)
e = new ItemOperationEvent(this,item);
((ItemOperationlistener)listeners[i+1]).itemRemoved(e);
4. The listener and the registration .
public class OrderBook implements ItemOperationListener
private Invoice invoice; // invoice instance.
public OrderBook()
// register for event updates .
invoice.additemOperationListener(this);
public void itemAdded(ItemOperationEvent ioe)
String tobeAdded = ioe.getItem();
calculate();
public void itemRemoved(ItemOperationEvent ioe)
String tobeRemoved = ioe.getItem();
calculate();
public void calculate()
I hope this gives a picture of the standard method of event delivery in java . However you can even follow the Observer, Observable pattern (see the documentation for java.util.Observer, Observable).
Hope this helps,
Manish.