CS100J       Lab 03. Writing and testing classes      Spring 2008
Playing with static variables

Name _______________________________       NetId ____________________________      

Section day/time _______________________     Section instructor ___________________

In this lab, we give you practice with writing a class and using the JUnit tester to test its methods.

Task 1. Download. Download file ThreeDimPoint.java from here:

http://www.cs.cornell.edu/courses/cs100j/2008sp/handouts/labs/lab03program/ThreeDimPoint.java.

Save it in a new directory. Each instance of class ThreeDimPoint is a three-dimensional point (x, y, z). When you are done with this lab, you may want to copy this file —and a JUnit file that you will construct— to your memory stick —or email the files to yourself.

1. In DrJava, select menu File item New JUnit Test Case.... This will create a second file; call it DimTester.java. Save it.
2. Change the name of method testX in class DimTester to testGetters.
3. In the body of method testGetters, write Java statements that:

(a) Create an instance of ThreeDimPoint and save its name in a variable. Note that the constructor call in the new expression requires three arguments --the x, y, and z coordinates of the point. (b) Test the three getter methods. Refer to the slides for the lecture of 5 February for help. Remember, method assertEquals(x,y) stops all testing, with an error message, if x not equal y.

4. Compile the two classes, by clicking your mouse on button Compile All.
5. Click button Test.
6. If an error message results, study the message and the constructor and the relevant getter function to determine what is wrong and fix the program.

Repeat steps 4, 5, and 6 until no error message results.

Congratulations! You have used a JUnit tester to debug your first program.

Task 4. Test function hasAZero. This function is supposed to return true if at least one of the x-coordinate, y-coordinate, and z-coordinate is 0. If none of them are zero, it returns false. That's what the specification says. Do not make any changes to the function before testing it. In class DimTester, make up another test function, testHasAZero, that will test function hasAZero. A possible test case is any set of values (x, y, z). Think about it: how many test cases do you need in order to be sure that the method is correct? Perhaps 6 or 7 or 8? Below, write down a list of test cases that you think will suffice to provide some assurance that the function is correct:

In test function testHasAZero, implement all the test cases that you think you need. The test function may have to create more than one instance of ThreeDimPoint in order to implement all your test cases.

Now run the test program (click on button Test). If you get error messages, look at the program and fix it. Continue testing and debugging in this fashion until running the test program does not produce an error.

Test 5. Trying to make a method static. Consider the line from point (0, 0, 0) to point (x, y, x). The length of this line is the square root of x*x + y*y + z*z. In Java, function call Math.sqrt(n) finds the square root of n, as you can see from method length in class ThreeDimPoint. Method length computes the length of the line given by the instance in which it appears. Try it out using these two statements in the interactions pane:

d= new ThreeDimPoint(3, 4, 5);
d.length()                                         // Write the answer here:

Now do the same thing using this single statement.

(new ThreeDimPoint(3,4,5)).length()       // Write the answer here:

The second way, just above, contains a new-expression that is NOT assigned to a variable. This is legal. There is nothing wrong with it. The new folder (object) is created and stored in file-drawer ThreeDimPoint, and the name of the folder is the result of the new-expression; then, function length() of that folder is called.

Now, make the function static by placing the word static right after public and compile. What happens? Write here the error message that is printed:

What is the problem? Explain in your own words what the problem is:

Then remove the word static and compile again.

Test 6. Making a method static. Now consider function length1. It has the heading

public double length1(ThreeDimPoint c)

so when one calls it, one has to give it an argument that is the name of a ThreeDimPoint folder. We'll show you this in a minute.

Make this method static, by placing keyword static after public, and compile. It works? Can you explain why?

Yes, the method does not refer directly to any fields of the folder in which the method occurs, so it does not have to be in each folder of the class, so it can be static. There is ONE copy of it, and the copy is in file-drawer ThreeDimPoint.

Now do these two lines in the interactions pane, one at a time. The first creates a ThreeDimPoint folder and stores its name in folder f. The second gets its length by calling function length in object f. The third gets it length by calling static function ThreeDimPoint.length1.

f= new ThreeDimPoint(3,4,5);
f.length()
ThreeDimPoint.length1(f)

This illustrates how one can use a static method. If a method does not refer to any fields, place it in the file-drawer by making it static. If you still have questions about this, ASK THE TA OR CONSULTANT!

Task 7. Class Math. Class Math has lots of static functions that are mathematical in nature. Class Math, which is always available (can always be used), is used mainly as a file-drawer for static functions and variables. It has no fields that you can change. It has no methods. Everything in it is static. Here are some function calls and what they mean:

In addition, static variable Math.PI gives the best double approximation to pi, the ratio of the diameter of a circle to its circumference. Class Math has other static functions. Later, we will tell you where to find their descriptions.

Fill in the following table by placing the calls in the interactions pane and seeing what value DrJava gives: