Grouping objects - Objects First with Java: A Practical Introduction Using BlueJ - page 135

4.14.6 Chaining method calls

In our introduction to the auction project, we noted a chain of object references: Auction
objects store Lot objects; each Lot object can store a Bid object; each Bid object stores a
Person object. If the Auction object needs to identify who currently has the highest bid on a
Lot , then it would need to ask the Lot to return the Bid object for that lot and then ask the Bid
object for the Person who made the bid.

Ignoring the possibility of null object references, we might see something like the following
sequence of statement in order to print the name of a bidder:

Bid bid = lot.getHighestBid();
Person bidder = bid.getBidder();
String name = bidder.getName();
System.out.println(name);

Because the bid , bidder , and name variables are being used here simply as staging posts to
get to the bidder's name, it is common to see sequences like this compressed through the use of
anonymous object references. For instance, we can achieve the same effect with the following:

System.out.println(lot.getHighestBid().getBidder().getName());

This looks as if methods are calling methods, but that is not how this must be read. Bearing
in mind that the two sets of statements are equivalent, the chain of method calls must be read
strictly from left to right:

lot.getHighestBid().getBidder().getName()

The call to getHighestBid returns an anonymous Bid object, and the getBidder method
is then called on that object. Similarly, getBidder returns an anonymous Person object, so
getName is called on that person.

Such chains of method calls can look complicated, but they can be unpicked if you understand
the underlying rules. Even if you choose not to write your code in this more concise fashion,
you should learn how to read it, because you may come across it in other programmers' code.

Exercise 4.48 Add a close method to the Auction class. This should iterate over the
collection of lots and print out details of all the lots. Use a for-each loop. Any lot that has had at
least one bid for it is considered to be sold, so what you are looking for is Lot objects whose
highestBid field is not null . Use a local variable inside the loop to store the value returned
from calls to the getHighestBid method, and then test that variable for the null value.

For lots with a bidder, the details should include the name of the successful bidder and the
value of the winning bid. For lots with no bidder, print a message that indicates this.

Exercise 4.49 Add a getUnsold method to the Auction class with the following header:

public ArrayList<Lot> getUnsold()

This method should iterate over the lots field, storing unsold lots in a new ArrayList local
variable. What you are looking for is Lot objects whose highestBid field is null . At the end
of the method, return the list of unsold lots.