A parcel delivery service needs to manage parcels that have to be sent to various countries. For a country (identified by its name, a string), the sequence of parcels to be sent to the country are stored, in the order in which they have to be sent, in a ParcelsToSend data structure. Each parcel in the ParcelsToSend data structure is represented simply by a string (denoting the address to which the parcel has to be sent).

The ParcelsToSend objects support the following functionalities:

• creation of a ParcelsToSend object, given the name of the country; the created ParcelsToSend object is initially empty (i.e., it contains no parcels);
• getCountry: that returns the name of the country;
• insertParcel: that, given a string representing a new parcel, inserts the new parcel into the sequence of parcels to be sent, respecting the proper order (i.e., parcels inserted earlier will be sent earlier);
• nextParcel: that returns (the address of) the next parcel to be sent (the one that was inserted first in the sequence); if the sequence is empty, an exception is thrown;
• sendParcel: that returns (the address of) the next parcel to be sent and removes it from the sequence; if the sequence is empty, an exception is thrown;
• numParcels: that returns the number of parcels in the sequence;
• parcel: that, given an integer i returns (the address of) the i-th parcel from the sequence that has to be sent (starting to count from 0, i.e., the first parcel to be sent corresponds to i=0, the second parcel to i=1, etc.); if the integer does not correspond to any parcel in the sequence, then an exception is thrown.
• parcelsForAddress: that, given a string s, returns an array of integers containing the positions in the sequence of all parcels whose address string starts with s.

Part 1. (20 points)

Solution: representation of the objects, skeleton of the class, class ParcelsToSend, class ParcelsException

Part 2. (6 points) Realize a static method getParcelsByAddress client of the class ParcelsToSend (i.e., external to such a class) that, given a ParcelsToSend object ps, a string s, and the name f of a file, writes to the file f the position in the sequence and the address string for all those parcels for which the address string starts with s, one parcel per line. The method should handle exceptions due to input/output errors that may occur.

Solution

Part 3. (4 points) Explain the concept of recursion, discussing also the issue of termination of a recursive method invocation. Give a simple example of a recursive method in Java and show an example of the evolution of the stack of activation records when the method is executed.