Order.java

import java.io.Serializable;

public class Order implements Serializable {
    private static final long serialVersionUID = 1L;

    public enum Side { BUY, SELL, CANCEL }

    public static final Order POISON_PILL = new Order(-1, Side.BUY, 0, 0, -1);

    private final int    sequenceId;
    private final Side   side;
    private final double price;
    private int          quantity;
    private final long   timestamp;
    private final int    cancelTargetId;

    public Order(int sequenceId, Side side, double price, int quantity, int cancelTargetId) {
        this.sequenceId     = sequenceId;
        this.side           = side;
        this.price          = price;
        this.quantity       = quantity;
        this.cancelTargetId = cancelTargetId;
        this.timestamp      = System.nanoTime();
    }

    public int    getSequenceId()    { return sequenceId; }
    public Side   getSide()          { return side; }
    public double getPrice()         { return price; }
    public int    getQuantity()      { return quantity; }
    public long   getTimestamp()     { return timestamp; }
    public int    getCancelTargetId(){ return cancelTargetId; }

    public void setQuantity(int quantity) { this.quantity = quantity; }

    @Override
    public String toString() {
        if (side == Side.CANCEL)
            return String.format("[#%d] CANCEL target=#%d", sequenceId, cancelTargetId);
        return String.format("[#%d] %s $%.2f x %d qty", sequenceId, side, price, quantity);
    }
}