Improving ParticipantDirectory Using Multiple TreeMaps

Expected time required: 10 min

In this exercise you will add functionality to the ParticipantDirectory example to be able to:

• look up participants by ID number and
• obtain a roster of participants sorted by ID number.

This extra functionality will be accomplished by adding a second TreeMap to the ParticipantDirectory class.

Remember, a TreeMap associates a key with an object. The keys are stored in a balanced binary search tree based on the sorted order of the keys and provides efficient access using the keys. The values that are associated with each key in a TreeMap are actually object references. Multiple TreeMaps using different keys do not require multiple copies of the objects. Thus, multiple TreeMaps can provide multiple ways of efficiently accessing and organizing the same set of objects.

Figure 1 shows an example with three objects that each contain a character attribute and an integer attribute. The objects are A/2, B/3, and C/1. One TreeMap using the character attribute as the keys is shown on the left with B as the root, A as the left child, and C as the right child. A second TreeMap using the integer attribute as the keys is shown in the right with 2 as the root, 1 as the left child, and 3 as the right child.

Figure 1: Example showing multiple TreeMaps indexing the same objects.

ParticipantDirectory Example From Reading

Before we begin, examine the provided code and note the differences from the code as it was presented in your reading:

• The ParticipantDirectory class has an addParticipant method that accepts String parameters for the participant's username and full name. This method then creates a new Participant object and stores it in the TreeMap.

• The previous printRosterByUsername method has been re-written as getRosterByUsername and instead of printing directly to System.out, it builds and returns a String with the participant roster. This will allow for more flexible use of the roster information by a client application. (That is, it may still be printed to the console, or it could be displayed in a GUI component, or it may be sent directly to a file, to name just a few possibilities.)

• Some new test data is now loaded in the main method using addParticipant.

Note: Completing this exercise does not require making any changes to the Participant class.

Exercise

Your task for this exercise is as follows:

1. Add a second TreeMap to the ParticipantDirectory class to map Integer keys to Participant values.

   A) Declare the second TreeMap as an instance variable, just under the declaration of the participants instance variable.

   B) Instantiate the new TreeMap in the ParticipantDirectory() constructor.

2. Add a line to the addParticipant method to add the newly created Participant object to the new TreeMap using its idNumber as the key. Note: Be sure to only create one Participant object and add that same object to both maps using the appropriate key for each map.

3. Complete the getParticipantByID method that takes an Integer id number as a parameter and returns the Participant object associated with that id number by the newly added TreeMap. Note: This method will be very similar to the getParticipantByUsername method.

4. Complete the getRosterByID method that builds and returns a string containing the current roster of participants sorted by ID number. Note: This method will be very similar to the getRosterByUsername method.

5. (Optional) Before running the unit tests, add some code to the main method in the ParticipantDirectory class to test your new TreeMap similar to the tests of getParticipantByUsername and getRosterByUsername.

HINTS:

• Two separate TreeMaps?