©2008 Felleisen, Proulx, et. al.
The goal of this lab is to learn to use the professional test harness
JUnit. It is completely separated from the application code. It is designed
to report not only the cases when the result of the test differs from
the expected value, but also to report any exceptions the program
would throw. The slight disadvantage is that it uses the Java
equals method that by default only checks for the
instance identity. To use the JUnit for the method tests similar to
those we have done before we need to override the
equals any time we wish to complare two instances of a
class in a manner different from the strict instance identity.
However, each time we override the equals method we should
make sure that the hashCode method is changed in a compatible
way. That means that if two instances are equal under our
definition of equals then the hashCode method for
both instances must produce the same value.
We start with learning to use HashMap class. We then see how
we can override the needed hashCode method. Finally, we also
override the equals method to implement the equality
comparison that best suits our problem.
The last part of the lab shows you how you can measure the algorithm performance (timing) to see concretely the differences between the running times of different algorithms that have been designed to perform the same tasks.
Our goal is to design a program that would show us on a map the locations of the capitals of all 48 contiguous US states and show us how we can travel from any capital to another.
This problem can be abstracted to finding a path in a network of nodes connected with links — known in the combinatorial mathematics as a graph traversal problem. You have already seen this problem in your assignments at least once.
To provide real examples of data the provided code includes the
(incomplete) definitions of the class City and the class
State.
Download the code for Lab 11 and build the project USmap.
Download the file of state capitals caps.txt.
The project contains an implementations of the Traversal
interface by the class InFileCityTraversal that allows
you to read a file of City data. The code in the
Examples class saves the city data generated by the
InFileCityTraversal into an ArrayList.
Run the code with some of the city data files.
The Examples class contains examples of the data
for three New England states (ME, VT, MA) and their
capitals. Add the data for the remaining three states: CT, NH,
RI. Initialize the lists of neighboring states for each of
these states. Do not include the neighbors outside of the New
England region.
Finally, look at the definition of the method toString
both in the City class and in the State class. The
class Object defines such method for all classes, but it is
of little use. Comment out the toString method in the class
City and see what happens when you run the code.
From now on, you should define a toString method for every
class you define, making sure the resulting String is
readable and the fields are clearly identifiable.
We now have all the data we need to proceed with learning about hash
codes, equals, and JUnit.
The class USmap contains only one field and a constructor. The
field is defined as:
HashMap<City, State> states = new HashMap<City, State>();
The HashMap is designed to store the values of the type
State, each corresponding to a unique key, an instance of a
City — its capital.
Note: In reality this would not be a good choice to the keys for a HashMap — we do it to illustrate the problems that may come up.
Go to Java documentation and read what is says about
HashMap. The two methods you will use the most are put and
get.
Define the method initMap in the class
Examples that will add to the given HashMap the six New
England states.
Test the effects by verifying the size of the HashMap
and by checking that it contains at least three of the items you
have added. Consult Javadocs to find the methods that allow you to
inspect the contents and the size of the HashMap.
We will now experiment with HashMap to understand how changes in the
equals method and the hashCode method affect its
behavior.
Define a new City instance boston2 initialized
with the
same values as the original boston. Now put the state
MA again into the table, using boston2 as the
key. The size of the HashMap should now be 7.
Now define the equals method in the class City
that makes sure the two cities have the same name, state, zip code,
and the same latitude and longitude. Use the given helper method
sameDouble to compare the last two fields. Start the method
with:
public boolean equals(Object obj){
City temp = (City)obj; ...
If the given object is of the type that cannot be cast to
City the method will fail at runtime with the
ClassCastException.
Now run the same experiment as above. The resulting HashMap
still has size seven. Even though we think the two cities are equal,
they produce a different hash code.
Now hide the equals method (comment it out) and define
a new hashCode method by producing an integer that is the
sum of the hash codes of all the fields in the City class.
Now run the same experiment as above. The resulting HashMap
still has size seven. Even though the two cities produce the same hash
code, the HashMap sees that they are not equal and
does not confuse the two values.
Now un-hide the equals method so that two City
objects that we consider to be the same produce the same hash code.
When you run the experiment again you will see that the size of the
HashMap remains the same after we inserted Massachusetts with
the boston2 key.
Note: Read in "Effective Java" a detailed tutorial on overriding equals and hashCode.
You will now rewrite all your tests using the JUnit. In the
File menu select New then JUnitTestCase. When the
wizard comes up, select to
include the main method, the constructor, and the setup method. The
tests for each of the methods will then become one test case similar
to this one:
/**
* Testing the method toString
*/
public void testToString(){
assertEquals("Hello: 1\n", this.hello1.toString());
assertEquals("Hello: 3\n", this.hello3.toString());
}
We see that assertEquals calls are basically the same as the
test methods for
our test harnesses, they just don’t include the names of the tests. Try
to see what happens when some of the tests fail, when a test throws an
exception, and finally, make sure that at the end all tests succeed.
Add a method that determines whether the city is South of the given latitude. Run the tests using the JUnit.
Add a method that determines whether this city is in the same state as the given city. Run the tests using the JUnit.
Ask for help, try things — make sure you can use JUnit, so you will not run into problems when working on the assignment and the final project.
Try to get as much as possible during the lab. Ask questions when you do not understand something. Everything that you do in this lab will be used in the next assignment or in the final project.