A post office needs to keep track of letters that have to be delivered to various cities. For each city (identified by its name, a string), the letters to be delivered to that city are stored, in the order of delivery, in a LetterQueue data structure. Each letter in the LetterQueue is represented simply by a string (denoting the name of the recipient).

The LetterQueue objects support the following functionalities:

• creation of a LetterQueue, given the name of the city; the created LetterQueue is initially empty (i.e., it contains no letters);
• getCity: that returns the name of the city;
• newLetter: that, given a string representing a new letter, inserts the new letter into the queue, respecting the order of arrival;
• firstLetter: that returns a string containing the name of the first letter in the queue (the one that was inserted first in the queue); if the queue is empty, an exception is thrown;
• removeLetter: that eliminates the first letter from the queue; if the queue is empty, an exception is thrown;
• numLetters: that returns the number of letters in the queue;
• letter: that, given an integer i that represents the position in the queue of a letter, returns the letter (i.e., the name of the recipient) in that position; if the integer does not correspond to a valid position in the queue, then an exception is thrown. The position in the queue of a letter is equal to the number of letters that precede it in the queue (e.g., the first letter in the queue has position 0, the following one has position 1, etc.);
• shortLetters: that, given an integer len, returns an array of integers containing the positions in the queue of all letters whose string is shorter than len.

Part 1. (20 points) Realize a Java class LetterQueue to represent LetterQueue objects. In the proposed solution you have to take into account that there is no bound on the number of letters that can be in a LetterQueue. If you use arrays to represent the collection of letters, you should handle the recovery of unused memory.

Solution: representation of the objects, skeleton of the class, class LetterQueue, class LetterQueueException

Part 2. (6 points) Realize a static method lettersShortString client of the class LetterQueue (i.e., external to such a class) that, given a LetterQueue object q, a positive integer len, and the name f of a file, writes on the file f the position in the queue and the string (i.e., the name of the recipient) for all those letters for which the string is shorter than len, one letter per line. The method should handle exceptions due to input/output errors that may occur.

Solution

Part 3. (4 points) Discuss the concepts of overloading and of overriding in Java. Give for both a short example.