Implement hashCode method

src/Order.java

@@ -43,6 +43,13 @@ public abstract class Order {
 	
 	void setQty(int qty) { this.qty = qty; }
 	
+	@Override
+	public int hashCode() {
+		// in current implementation, time is assured to 
+		// be unique	
+		return this.time;
+	}
+
 	@Override
 	public boolean equals(Object o) {
 		// self check