Multiple TreeMaps Using a Comparator

Expected time required: 30 min

In this exercise you will reinforce your knowledge of the TreeMap by creating a TollRoad application that is very similar to the earlier ParticipantDirectory application. Specifically, you will create a TollRoad application that keeps track of how many tolls a set of vehicles owe. The application will receive information from automated toll cameras placed along the toll road. The application provides reports sorted by vehicle description or by vehicle state of registration.

For example, consider the following list of Vehicle description strings:

"CCO123ABC", "CIA432LMN", "TCO789XYZ", "SFL456DEF", "TCO789XYZ", "CIA432LMN",
"TCO789XYZ", "TIA765QRS", "CCO123ABC", "SFL456DEF", "CIA432LMN", "TCO789XYZ"

Notice several of the vehicle description strings appear twice. This is not a mistake as it indicates the vehicle passed multiple toll cameras.

With the above data, the TollRoad application would be able to produce the following two reports:

Vehicle Tolls By Description:
Description: CCO123ABC, Toll Count: 2
Description: CIA432LMN, Toll Count: 3
Description: SFL456DEF, Toll Count: 2
Description: TCO789XYZ, Toll Count: 4
Description: TIA765QRS, Toll Count: 1

Vehicle Tolls By State:
Description: CCO123ABC, Toll Count: 2
Description: TCO789XYZ, Toll Count: 4
Description: SFL456DEF, Toll Count: 2
Description: CIA432LMN, Toll Count: 3
Description: TIA765QRS, Toll Count: 1

The first report is sorted by vehicle type (the first character in the vehicle description string) and the second report is sorted by the state of registration for each vehicle (the second and third characters in the vehicle description string.)

Vehicle Class

To begin, you will use an existing Vehicle class. The first attribute of a Vehicle is a String description. In this description, the first character indicates the vehicle's type; C = car, T = truck, S = semi-trailer, etc. The next two characters are the vehicle's state code; CO = Colorado, FL = Florida, IA = Iowa, etc. The remaining characters are the vehicle's license plate. This vehicle description code is the information obtained by the toll road's automated cameras and passed to the TollRoad application. Your code does not generate these.

The second attribute of a Vehicle is a count of how many tolls the vehicle owes. The Vehicle object will not be created until it passes through a toll road camera for the first time, so the count is initialized to 1. The addToll() method in the Vehicle class increments the vehicle's toll count. Take a few minutes now to familiarize yourself with the Vehicle class. Note: Completing this exercise does not require making any changes to the Vehicle class.

TollRoad Class

The TollRoad class will keep two separate TreeMaps, very similar to the previous ParticipantDirectory exercise. The first TreeMap will use the String description attribute in the Vehicle class as the key. This will produce an ordering of the keys based on vehicle type since that character is the first in the string.

The second TreeMap will also need to use the String description attribute in the Vehicle class as the key. However, this TreeMap will need to use a Comparator to sort the keys based on the second and third characters, which represent the state code.

With these two TreeMaps, implementing the getVehicleReportByDescription() and getVehicleReportByState() methods in the TollRoad application will be nearly identical to the getRoster methods in the ParticipantDirectory class from the previous exercise.

The addToll() method in the TollRoad class will be called by the automated cameras placed on the toll road (your code does not need to worry about when or how this actually happens.) The String parameter is a Vehicle description. If a vehicle with this description has already passed a toll camera, then its toll count should be incremented. If this is the first time this vehicle has passed a toll camera, a new Vehicle object is created and placed in the toll road's two TreeMaps.

Exercise

Your task to complete the TollRoad class for this exercise is as follows:

• Declare two TreeMap variables to associate a String key with a Vehicle value.
• Complete the TollRoad constructor:
  • Create the first TreeMap - nothing out of the ordinary with this one.
  • Create a Comparator that will compare vehicle description strings starting with the second character as that is the beginning of the state abbreviation. (See this hint, if necessary.)
  • Create the second TreeMap, passing the Comparator object to the TreeMap constructor so it will be used when sorting keys.
• Complete the addToll() method:
  • Lookup the given vehicle description string in a TreeMap.
  • If there is not currently a Vehicle associated with this description string, create one and put it in each of the two TreeMaps.
  • If there is a Vehicle associated with this description string, call that vehicle's addToll() method to increment its toll counter.
• Complete the getVehicleReportByDescription() method (very similar to the getRoster methods in the ParticipantDirectory exercise.)
• Complete the getVehicleReportByState() method (very similar to the getRoster methods in the ParticipantDirectory exercise.)

HINTS:

• How to use a comparator?