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HOMEWORK: OBSERVER PATTERN

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HOMEWORK OBSERVER PATTERN
OBJECTIVE: Apply what we learned in lecture and lab about the Observer Pattern to a toy
stock market application.
INTRODUCTION: Please remember the coding standards here.
Imagine that you are an agent for a brokerage firm. There are some stocks that
you trade in consistently, so you want to know the minute that anyone makes a
buy or sell order in any of those stocks. So you subscribe as an Observer to all of
those stocks of interest. Meanwhile, you have colleagues who also trade in
various stocks. Some of those stocks you are interested in, some of them you are
not. Each of your colleagues subscribe as an Observer to a set of stocks.
When you or some other agent places a buy or sell order on a given stock, that
stock alerts all of the agents who have subscribed to that stock so that they know
immediately about the trade. The information content of each trade is: the stock
being traded, the number of shares in the trade, the dollar amount for the whole
trade (not price/share but the whole amount) and the transaction type.
The Stock that you pass to the Bid constructor should be an instance of the Stock
class, not just the stock symbol. That helps with data integrity in your stock
market so that users do not put in just any old string for the stock symbol. Also,
you will want to use an enumeration for the transaction type. The two types of
transactions that you will want to work with are simply BUY and SELL.
PROCEDURE:
1. Either implement your own Subject and Observer interface, or use the
java.util.Observable (AKA Subject) and java.util.Observer interfaces.
2. Build the Bid object with the necessary member variables and a toString() method. You will
not need getters and setters for this since our little application will just rely on toString to
return data from a given Bid instance.
3. Build the Stock class that implements the Subject/Observable interface.
a. Give Stock a member method called trade. The trade method is how we update
the Stock instance. The trade method accepts an instance of Bid as its only
parameter, and stores that Bid instance locally.
b. Give Stock a member method called getBid() that will return a copy of the Bid that
happened most recently to that instance of Stock.
i. Your update() method in your Agent class (more on that in a moment) will
need to call getBid() to find out just what changed about the Stock that the
Agent subscribes to.
4. Build the Agent class that implements the Observer interface.
a. You will want to store the name of the agent as a member variable.
b. The update() method needs to accept an instance of Subject so that the agent knows
which Stock it was that had the bid placed.
CECS 277 Homework Observer.docx 8/23/2019 8:38:00 PM 2
CECS 277 HOMEWORK OBSERVER PATTERN
5. Write a tester, called ObserverRunner, that will create several Stock instances, several
Agent instances, have the agents subscribe to a few Stocks, and then place some Bids on the
Stocks.
WHAT TO TURN IN:
• Agent.java
• Bid.java
• Observer.java (if you roll your own)
• ObserverRunner.java (the test routine)
• Stock.java
• Subject.java (if you roll your own)
• TransactionType.java (for the Buy/Sell enumeration)
• Sample console output named console.txt.
• The UML class diagram.
SAMPLE OUTPUT:
Agent – name: Tom Clancey alerted to Bid- Symbol:ORCL Buying 20 shares for the amount: $3421.0
Agent – name: Robert Mitchner alerted to Bid- Symbol:ORCL Buying 20 shares for the amount: $3421.0
Agent – name: Noah ben Shea alerted to Bid- Symbol:ORCL Buying 20 shares for the amount: $3421.0
Agent – name: Richard Rohr alerted to Bid- Symbol:ORCL Buying 20 shares for the amount: $3421.0
Agent – name: Tom Clancey alerted to Bid- Symbol:BA Selling 10 shares for the amount: $50.0
Agent – name: Robert Mitchner alerted to Bid- Symbol:BA Selling 10 shares for the amount: $50.0
Agent – name: Noah ben Shea alerted to Bid- Symbol:TRKX Buying 30 shares for the amount: $204.36
Agent – name: Richard Rohr alerted to Bid- Symbol:TRKX Buying 30 shares for the amount: $204.36

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