Collections, Maps and Iterators - Absolute Java - page 939

125 Suitor 4 eliminated; continue counting back from 2 and bounce from
front back to 2.
15 Suitor 2 eliminated; continue counting forward from 5.
1 Suitor 5 eliminated; 1 is the lucky winner.
Write a program that uses an ArrayList or Vector to determine which position
you should stand in to marry the princess if there are n suitors. Your program
should use the ListIterator interface to traverse the list of suitors and remove
a suitor. Be careful that your iterator references the proper object upon reversing
direction at the beginning or end of the list. The suitor at the beginning or end of
the list should only be counted once when the princess reverses the count.
7. In social networking websites, people link to their friends to form a social network.
Write a program that uses HashMaps to store the data for such a network. Your
program should read from a file that specifies the network connections for different
usernames. The file should have the following format to specify a link:
source_usernamefriend_username

There should be an entry for each link, one per line. Here is a sample file for five
usernames:
iba

java_guru

iba crisha

iba ducky

crisha java_guru

crisha iba

ducky java_guru

ducky iba

java_guru iba

java_guru crisha

java_guru ducky

wittless java_guru

In this network, everyone links to java_guru as a friend. iba is friends with java_
guru , crisha , and ducky . Note that links are not bidirectional; wittless links
with java_guru but java_guru does not link with wittless .

First, create a User class that has an instance variable to store the user's name
and another instance variable that is of type HashSet<User> . The HashSet<User>
variable should contain references to the User objects that the current user links
to. For example, for the user iba there would be three entries, for java_guru ,
crisha , and ducky . Second, create a HashMap<String,User> instance variable
in your main class that is used to map from a username to the corresponding User
object. Your program should do the following:
• Upon startup, read the data file and populate the HashMap and HashSet data
structures according to the links specified in the file.