Assignment 5: Inheritance and Using Our Own Classes
Objectives:
- Overriding inherited instance methods
- Using a class you wrote in another class
Due: Before the class on Friday.
Set Up
Cloning the Repository
In your terminal, go to the directory where you are keeping your repositories for CSCI209. Clone the repository for this assignment. Add your name to the README file.
Birthday.java
Copy
your Birthday.java file from assignment 4 into this
repository. Add Birthday.java to this repository.
Part 1: Updating the Birthday Class
Overriding Methods
Add appropriate toString and equals
methods. Make sure you use the appropriate signature for each
method, i.e., the same as the parent class's.
You will probably be tempted to have a Birthday object
as a parameter to the equals method you're writing.
However, the parameter should be an Object object, to
match the parent Object class's method.
Follow the procedure below for writing the equals method:
- Use the == operator to check if the argument is a reference to this object. (If the variables are references to the same object, they're clearly equal!)
- Use the
instanceofoperator to check if the argument has the correct type. (Note: if a variable is a null reference, theninstanceofwill befalse, so we don't need to check if the other object is null separately.) - Cast the argument to the correct type.
- For each "significant" field in the class, check if that field
of the argument matches the corresponding field of this object.
Note: for doubles, use
Double.compareand for floats useFloat.compare
As always, make sure to test your methods appropriately.
Overloading Constructors
Create a new constructor for the Birthday class. This
constructor takes no parameters and randomly generates the
birthday (the month and date).
To create the random birthday, you may need a static, final array
of integers representing the number of days in a month.
(Why?) For this assignment, assume that February has 29 days.
Also, you want
only
one random
number generator object for the whole Birthday class
to ensure the best random number generation. (We can debate about
whether a uniform
distribution is the correct way to generate birthdays, but for
simplicity, that is our assumption.)
Test your other methods, using this constructor.
Save the output from all your tests in birthday.out.
Part 2: Using the Birthday Class
Are you familiar with the birthday paradox? In a group of 23 randomly chosen people, there is more than 50% probability that some pair of them will both have been born on the same day. For 57 people, the probability is more than 99%.
Write a Java class (separate from the Birthday class) that runs
an experiment that tests/verifies these probabilities. Name the file
BirthdayParadox.java. Since Birthday.java
and BirthdayParadox.java are in the same directory,
BirthdayParadox.java does not need to
import Birthday.
For a set of people with randomly generated birthdays, determine if any of them have the same birthday. Because there is randomness involved, you need to run the experiment several times, each time with a new set of people with randomly generated birthdays. (30 is usually the magic number of experiments, but when you're first testing, you'll want to perform fewer experiments.)
Run the experiment with different numbers of people: from 5 to 100 people, in 5-person increments.
Note that a positive trial means that there was at least one pair of people with the same birthday. There could be more than one pair with the same birthday in a trial, but that only counts as one positive trial.
Stop here. Start thinking about what methods/functionality would be helpful in implementing the solution. How can you break this problem into smaller, easily testable pieces? If you don't break the problem into small, testable parts, you will undoubtedly struggle because you're trying to do too much at once and you're new to Java.
Print out the results of your experiment in a table. Recall that
you can use the printf method of System.out.
(One reference
for printf.)
Example output:
# People # Trials # Positive Pct
-------- -------- ---------- ------
5 10 1 10.0
10 10 1 10.0
15 10 2 20.0
20 10 4 40.0
25 10 6 60.0
30 10 7 70.0
35 10 5 50.0
40 10 9 90.0
45 10 10 100.0
50 10 10 100.0
55 10 10 100.0
60 10 10 100.0
65 10 10 100.0
70 10 10 100.0
75 10 10 100.0
80 10 10 100.0
85 10 10 100.0
90 10 10 100.0
95 10 10 100.0
100 10 10 100.0
# People # Trials # Positive Pct
-------- -------- ---------- -----
5 30 1 3.3
10 30 5 16.7
15 30 8 26.7
20 30 12 40.0
25 30 17 56.7
30 30 21 70.0
35 30 23 76.7
40 30 27 90.0
45 30 30 100.0
50 30 27 90.0
55 30 30 100.0
60 30 30 100.0
65 30 30 100.0
70 30 30 100.0
75 30 30 100.0
80 30 30 100.0
85 30 30 100.0
90 30 30 100.0
95 30 30 100.0
100 30 30 100.0
In the above table, # Positive is the number of times
at least two people had the same birthday out of all the times the
experiment was run.
Note: your new Java class should "see" your
Birthday class if you used the public
keyword before class in Birthday.java and if
the two .class files are in the same directory.
Use good program organization, e.g., write methods to make your code easier to read/understand.
Save the output from one run of your program using 30 trials in a
file named paradox.out. Add the file to your
repository.
Students sometimes get overwhelmed by this problem. It is important that you break it into small steps to make sure that it works, then move on to the next part. (Recall good development processes.)
Extra Credit (3 pts)
Have your program take as a command-line argument the number of trials to perform. If the command-line argument is not given, default to 30 experiments. You may find Java's Integer class helpful.
Submission
As usual, put all of your code in your GitHub repository that both you and I can access.
Grading (90 pts)
You will be evaluated based on the correctness, efficiency, testing, and style of your programs:
- (30 pts) Part 1: Updating the Birthday class
- (60 pts) Part 2: Birthday paradox