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Exercise

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_guruiba	crishaiba	duckycrisha	java_gurucrisha	ibaducky	java_guruducky	ibajava_guru	ibajava_guru	crishaj ava_guru	duckywitless	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. The HashSet 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 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.

• Allow the user to enter a name.

• If the name exists in the map, then output all usernames that are one link away from the user entered.

• If the name exists in the map, then output all usernames that are two links away from the user entered. To accomplish this in a general way, you might consider writing a recursive subroutine.

Do not forget that your User class must override the hashCode and equals methods.