Classes, Methods, and Testing
There is a PDF version of these instructions, if you would prefer
to have that instead.
The purpose of this lab is to get you used to writing methods, and to introduce
you to the basics of testing. As a warning, we will tell you right now:
The Java file we give you has errors in it; do not look for
them and test them in the beginning. You should only correct it when you are
told. The point of this lab is get you in the habit of testing your programs.
Adopting this testing habit will prove to be unbelievably useful, particularly
for the first assignment.
Requirements For This Lab
The very first thing that you should do in this lab is to download the file
IntPoint3d.java. This is a variation of the file Point3d.java
demonstrated in class. The primary difference is that the coordinates are of type int instead of
double. You will be working on this file, so save it in a new directory. When you are done with
this lab, you may want to save this file; email it to yourself or put it on a USB storage key.
This lab will involve three different components. First, you will need to write answers to written
questions on these sheet. Second, you will need to modify the contents of IntPoint3d.java to
add new methods. Finally, you will need to make a JUnit test case. When you are done, you should
show all three to your lab instructor, who will record that you did it. You do not need
to submit the actual paper, and you do not need to submit the computer files anywhere.
As always, if you do not finish during the lab, you have until next Monday during consultant
hours to finish it. You should always do your best to finish during lab hours. Remember that
labs are graded on effort, not correctness.
Create a Test Class and Test the Constructor
For this part of the lab, you will create a JUnit test that tests both the constructor and
the three getters. You are to continue testing, until you get no error messages. You
are to go step-by-step as follows:
1. Create the JUnit Class
A JUnit test is a special class that is used to test other classes. In DrJava, select the menu
File item New JUnit Test Case.... You should save it, at which point
you wlll be asked to give a name for the new class. You should call it IntPoint3dTester.java
and save it in the same directory that your IntPoint3d.java is in.
2. Change the Name of Method textX
DrJava always creates an initial test method for you called testX. We like to be more
descriptive with our method names. Rename testX to testConstructor, and change
the JavaDoc comments appropriately.
3. Implement the Test Cases
In the body of method testConstructor, write Java statements that do the following:
-
Create an instance of
IntPoint3d.java and save its name in a variable.
Note that there is only one constructor, and it requires three integer arguments.
-
Insert tests to verify that the constructor sets the fields properly.
For help on how to set up the tests, look at this example on how to test the constructor of
the class Worker.java shown in lecture.
Remember, the special procedure assertEquals(expected, computed) stops all testing and
outputs an error message if expected does not equal computed. Also
remember that, unlike other methods, assertEquals is not proceded by an object. We will
talk about why this is later.
4. Compile the Two Classes
Save the file, and click your mouse on the button Compile.
5. Test
Click the button Test. If an error message appears, study the message, the
constructor and the relevant getter function to determine what is wrong. The error
could be anywhere.
6. Fix and Repeat
You now have permission to fix the code in IntPoint3d.java. However, you should
restrict your fixes to the constructor and getter methods only, as this is the only thing
that you are testing. Do not fix any other methods yet.
After you fix the code, you should repeat steps 4 to 6
until there are no more error messages.
Test Function asAZero
This function should return true if at least one of the x, y, or z coordinates is 0. If
none of them are zero, it returns false. That is what the specification says, and you
should not make any changes to the function before testing it.
In class IntPoint3dTester, you should make up another test function,
testHasAZero, that will test the function hasAZero. A
possible test case is any set of values (x, y, z).
Because this method can be ``true in more than one way'', you are going to want more than
one test case. For example, you will want a test case in which x is 0, but none of the
other coordinates are. You will also want a test case in which y is 0, but none of the
other coordinates are. This is because maybe you might have a variable, and the method
works for field x, but not for field y (and vice versa).
How many test cases do you think that you need in order to make sure that the method is
correct? Perhaps 6 or 7 or 8? Write down a list of test cases that you think will
suffice to assure that the function is correct:
Implement Your Test Cases
In test procedure testHasAZero, implement all the test cases that you think you
need. The test procedure may have to create more than one instance of IntPoint3d
in order to implement all of your test cases.
Test and Fix
Now compile the test program (click the button Compile) and run it
(click the button Test). If you get an error message, look at the program and
fix errors. Continue testing and debugging in this fashion until running the test program
does not produce an error.
Add A Function Method to IntPoint3d
You may notice that there is a specification for a function called length in the
class definition for IntPoint3d. However, there is no code for this function; only
comments. You should write this function.
The specification of this function is that it should return the distance between this point
and zero. For a point $(x,y,z)$, this distance is $\sqrt{x^2+y^2+z^2}$. Fix this method
so that it does just that.
Once you are done fixing the function length, compile your class and try out the
method using these statements in the Interactions pane:
IntPoint3d d;
d = new IntPoint3d(3, 4, 5);
d.length()
Write your answer here:
Now do the same thing using this single statement:
new IntPoint3d(3,4,5)).length()
Write your answer here:
Note that the second way, just above, contains a new-expression that is not
assigned to a variable. This is legal, and there is nothing wrong with it. The new object
(folder) is created and stored in class (file-drawer) IntPoint3d, and the name
of the folder is the result of the new-expression in the parenthesis. The function length()
is called for the object in the parenthesis.
Add A Procedural Method to IntPoint3d
You may notice that there is a procedure called averageWith in the class
definition for IntPoint3d. However, there is no code for this procedure;
only comments. You should write this procedure.
The specification of this procedure is that it should average the coordinates of this point
with those of the point q, and store the result in the fields for this point. For example,
suppose the current object is (2,4,4) and q is the name for the object (4,6,-8). In that
case, this procedure should average the x-coordinates and assign x = (2+4)/2. The procedure
should do the same thing for the y and z coordinates.
Test Your Code
Once again, you should test and fix your procedure. To create a test case for a method like
this, you work out the correct answer on a sheet of paper. The JUnit test should then verify
that the program gives you the same answer that you worked out on paper.
Finding good test cases for a method like this is tricky, so we are giving you some
specific ones to try out:
-
The average of (2,4,4) and (4,6,-8) is (3,5,-2)
-
The average of (1,1,1) and (2,2,2) is (1,1,1) [Remember: type int]
-
The average of (0,0,0) and (0,0,0) is (0,0,0)
You should implement the three test cases above. Make a new test procedure called
testAverageWith and add the test cases above using the special
procedure assertEquals. If you do not get any error messages, then you are
done.
Generate Javadoc Specifications
Now that you have finished working with IntPoint3d, there is one thing left to do.
You are going to create a webpage for IntPoint3d that looks exactly like what you
might find in the Java API.
Click the Javadoc button in DrJava. You will be asked where to save the result. When done,
you should see a browser window open with a description of class IntPoint3d.
More importantly, the comments in your class definition that are surrounded by "/** ... */"
should appear in the generated webpage. You should see your class specification
"An instance is a coordinate in three dimensions"; in the Method Summary and Method Detail
sections you should see the specifications for your methods.
Check to see that this is indeed the case.
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