Description
MATERIALCOVERED
Basic objects – instance variables, instance methods, constructors
Class variables andc (i.e. static)
Sorting objects using compareTo (Gold exercise only)
Notes:
In the lab, all exercises are to be done using Dr. Java. (At home, use anything, as usual.)
Make sure your TA has recorded your mark before leaving.
The three questions are sequential – each builds on the previous one.
Always try to complete as many as you can. The Gold exercise uses some material that has not yet
been fully covered, but it is explained briefly here.
A simple object class
1. Create a file HockeyTeam.java which will implement a class of objects that will store some
information about a hockey team. In your file, implement the following variables and methods.
a. private instance variables that will store the team’s name (a String), and the number
of wins, losses, and overtime losses (three int values).
b. a constructor that will accept those four data values, in the order listed above. A new
HockeyTeam object will always be created by using an expression such as
new HockeyTeam(“Winnipeg”,22,14,8)
c. three public methods void won(), void lost(), and void lostOvertime(),
each of which will add one to the corresponding integer value.
d. a public method int points() which will calculate and return the number of points
that the team has. A team receives 2 points for each win, 0 points for each loss, and 1
point for each overtime loss.
e. a public method String toString() which will return a String giving all of the
information stored about a team, and the number of points that the team has, in the
format “Winnipeg(22,14,8=52)” where 52 is the number of points (22*2+8). This
method should call the points() method.
2. Run the supplied file TestLab1Bronze.java to test your HockeyTeam class. Look at the
contents of that file to see how it is creating, updating, and printing team data. If your class is
working correctly the output should be:
Initial teams:
Winnipeg(22,14,8=52)
Chicago(28,13,2=58)
Colorado(18,17,8=44)
St. Louis(27,13,3=57)
Dallas(19,16,7=45)
Minnesota(18,19,5=41)
Nashville(29,9,4=62)
Final teams:
Winnipeg(28,14,8=64)
Chicago(28,15,2=58)
Colorado(19,17,9=47)
St. Louis(28,13,4=60)
Dallas(19,18,7=45)
Minnesota(18,20,5=41)
Nashville(29,9,5=63)
Adding class variables and a class method (static variables and method)
1. Modify your HockeyTeam class by adding the following class (static) variables and methods:
a. A few private class variables to keep a list of all of the HockeyTeam objects that have
been created, as a partially-full array with a maximum capacity of 64 teams. You will
need an array of HockeyTeam objects, an int to keep track of the current number of
teams, and a constant for the maximum capacity (64). (A separate HockeyTeamList class
would be another way to do this – don’t do that this time. If you know how to use an
ArrayList, go ahead and use one of those instead of an array if you want to.)
b. Modify the constructor, so that every time a new HockeyTeam object is created it will
be added to the list of HockeyTeam objects.
c. Add a public class method void listTeams() which will print all of the teams in
the list, one per line. You can simply use System.out.println to print a
HockeyTeam, and the toString method you wrote in the Bronze exercise should take
care of the rest automatically.
2. Modify the supplied file TestLab1Silver.java to test your new HockeyTeam class. In two
places, marked with ****ADD ONE LINE HERE****, add a line that will call your
listTeams class method.
3. Run the TestLab1Silver.java file to test your modified class. The output should be
identical to the Bronze exercise output, except that now it is your listTeams method that is
doing the printing, not the main method.
Sorting the standings
When the list of teams is printed, it should be sorted into descending order according to the number of
points each team has, as standings always are.
The built-in method Arrays.sort(a, 0, n) will sort elements a[0] to a[n-1] of any array of
objects a, provided that a compareTo method has been supplied that will compare those objects. You
will need the import statement import java.util.Arrays; to access this method.
Modify your HockeyTeam class to use this method to sort the teams.
1. Add a public instance method int compareTo(Object other) which will compare a
HockeyTeam object to some other object. The type Object is a generic type that can refer to
any object. Use the ordinary cast (HockeyTeam)other to convert it into a HockeyTeam object
before using it. This method should return a negative number if the other team is “larger” and
should appear later in the list, a positive number if the other team is “smaller” and should
appear earlier in the list, and a zero if the two teams are equal and can appear in either order.
2. Add the words implements Comparable to your class header, just before the {. This means “I
hereby promise to implement a compareTo method.” and allows Arrays.sort to be used.
3. Write a public class method void sortTeams() which uses the Arrays.sort method to sort
the teams in your list of teams into the proper order.
4. Modify the supplied file TestLab1Gold.java to test your new HockeyTeam class. In the
places marked with ****ADD ONE LINE HERE****, add lines that will first call your
sortTeams class method, and then call your listTeams class method.
5. Run the TestLab1Gold.java file to test your modified class. The output should be identical
to the Bronze exercise output, except that now the teams should be listed in descending order
according to points.
