The StockPriceChangeEvent class contains the ticker of the stock, the old price of the stock, and the new prices of the stock:

    public class StockPriceChangeEvent {
      private final String stockTicker;
      private final BigDecimal oldPrice;
      private final BigDecimal newPrice;
      //Setter, Getters and toString()
    }

The StockPriceChangeEventWithHoldings class extends StockPriceChangeEvent. It has one additional property--holdings. The holdings variable is used to store the number of stocks the trader currently owns:

    public class StockPriceChangeEventWithHoldings 
    extends StockPriceChangeEvent {
      private Integer holdings;
      //Setter, Getters and toString()
    }

The StockTicker enum stores list of stocks that the application supports:

    public enum StockTicker {
      GOOGLE, FACEBOOK, TWITTER, IBM, MICROSOFT
    }