First, create an object that represents the event. The class name of the event is based on the event name, which in this case is stock. Thus, the event is called StockEvent:

public class StockEvent extends java.util.EventObject {
  String symbol;
  double price;

  public StockEvent (Object source,
                     String symbol,
                     double price) {
    super (source);
    this.symbol = symbol;
    this.price = price;
  }
  public String getSymbol () { return symbol; }
  public double getPrice () { return price; }
}