Bill Teter
Office:          149 Redcay
Telephone:    2782
Office Hours:  Tuesday, Thursday 9:00-12:00

email:  william.teter@plattsburgh.edu

Lab 7--Comparators and sorting arrays.

The problem:   Points in the Cartesian plane are specified as a pair of doubles.  For example, (2.1, 3.0).  There are a variety of orderings that can be defined for these pairs.  You will create two classes that implement the Comparator interface that implement two of these ordering specifications.  Then you will a write test program that reads a file of pairs and stores the pairs in one ArrayList.  Then use the sort routine in the Array class to sort the pairs using the two ordering listed below.  Your test program will print the sorted pairs in each ordering.  The classes you will write are specified below.

Pair class

    Two fields of type double
    A constructor with two double parameters
    A toString() method that prints the pair in a format like (1.2, 3.4).

ByX class implements Comparator<Pair>

    The pair (a,b) is less than (c,d) if a<c or if a=c and b<d.

ByY class implements Comparator<Pair>
    The pair (a,b) is less than (c,d) if b<d or if b=d and a<c.

Test class
    The main method will open a file specified by a single command line argument.  The file will contain many lines with two doubles per line representing a Pair.  Your program will read each line and create a Pair which will then be added to an ArrayList.

To process the file you will need a BufferedReader and a FileReader objects.  After you have read a line that string will have to be split in two using the split method in the String class.  A regular expression that works in this case is the string of one blank: " ".  Now  you should have an array of strings of length 2.  Use the Double.parseDouble() static method to turn these strings into doubles so that you can create a Pair.

Look in the Arrays class and find a sort() method that will sort an array of a parameterized type T  using a specified comparator<T>.  Look in the ArrayList class and find a method called toArray() that returns an array of a parameterized type T.  Your test program will convert the ArrayList of Pairs into an array of Pairs, sort it using a ByX comparator.  Your program should now save the smallest and largest x values that occur among all pairs. Also,  print the resulting list of pairs. Now  sort the array  again using a ByY comparator.  Again remember the smallest and largest y values among all pairs, print the Pairs in order ByY.  Print the pair that is the lower left corner and the pair that is the  upper right corner of the smallest rectangle containing all the pairs.

This is too much code for one main method.  You can modularize the tasks many ways.  You can be creative and find your own design. Or, you can follow my suggested design.  I suggest you break down the problem into steps each with its own method:
    read the file data and build the arraylist and leave the data in a global array of pairs.

    sort the array by X coordinate, print results and set the global fields smallestX and largestX

    sort the array by Y coordinate, print results and set the global fields smallestY and largestY