How to implement this pluggable mechanism in C# and java?

Question

Let's say I have three object A, B, C.
B is the implementation of following interface:

interface D
{
    event EventHandler<OrderEventArgs> OrderCreated;
    event EventHandler<OrderEventArgs> OrderShipped;
}

I would add two functions in B which will raise the events in the interface.

class B
{
  RaiseOrderCreated();
  RaiseOrderShipped();
}

A is the hosting class and will call B's functions to raised the specified events. I hope in C I can listen to the interface B or class C, whenever the events in B are raised, C can get notified, then C can have reactive to the events.

I am thinking about how to implement this mechanism in C# and Java.
Any help or hints will be great appreciated !

Answer 1

So you'd have an EventListener called C, some object called A which has a field of type B that is some kid of implementation of D.

Typically in Java you design D differently:

interface D {
   static final String ORDER_PROPERTY="ORDER";
   void setOrder(Order order);
   Order getOrder();
}

class B implements D {
   // delegate object to implement support for event listeners and event firing
   private java.beans.PropertyChangeSupport propertyChangeSupport = new java.beans.PropertyChangeSupport(this);
   private Order order = null; // field that holds current order
   @Override
   public void setOrder(Order order) {
      Order old = this.order;
      this.order = order;
      propertyChangeSupport.firePropertyChange(ORDER_PROPERTY, old, order);
   }
   // more B code here
}

Answer 2

Implement this pattern: http://en.wikipedia.org/wiki/Observer_pattern