Name _______________________________ NetId ____________________________
This lab gives you practice with writing a class, using a JUnit tester, and working with static methods.
Task 1. Download. Download file ThreeDimPoint.java from here:
http://www.cs.cornell.edu/courses/cs1110/2010fa/handouts/labs/lab03program/ThreeDimPoint.java.
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.
ThreeDimPoint
is a 3dimensional
point (x, y, z). The class has 3 fields, x
, y
, and z
; a constructor; and 3 getter functions.
Understand what those four methods are supposed to do FROM THEIR SPECIFICATIONS.
Don't look at the bodies yet.
File
item New
JUnit Test Case...
. This will create a second file; call it DimTester.java
.
Save it to the same directory that your ThreeDimPoint.java
file is in. testX
in
class DimTester
to testConstructor
, and change the javadoc comments appropriately.testConstructor
, 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 integer arguments —the x, y, and z coordinates of the point.(b) Insert tests to verify that the constructor sets the fields properly. Refer to this partial example of testing a constructor (http://www.cs.cornell.edu/courses/cs1110/2010fa/handouts/labs/lab03junitconstructorTest.pdf) for help (the first page is a sample class; the second page shows a partial test method for that class's constructor). Remember, inherited procedure
assertEquals(expected, computed)
stops all testing and outputs an error message ifexpected
does not equalcomputed
.
Compile
.Test
.ThreeDimPoint
class definition.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 should return true if at least one of the x,
y, and z coordinates 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 procedure testHasAZero
, implement
all the test cases that you think you need. The test procedure may have to create
more than one instance of ThreeDimPoint
in order
to implement all your test cases.
Now compile the test program (click button Compile
) and run it (click button Test
).
If you get error messages, look at the program and fix errors. Continue testing
and debugging in this fashion until running the test program does not produce
an error.
Task 5. Trying to make a method static.
The length of the line from point (0, 0, 0) to point (x, y, z) 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 from the origin given by the instance in which it appears. Try it out
using these statements in the Interactions pane:
ThreeDimPoint d;
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 newexpression 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 filedrawer ThreeDimPoint
, and the name of the folder is the result
of the newexpression; 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.
Task 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 object 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 filedrawer ThreeDimPoint
.
Now do these lines in the Interactions pane, one at a time. The first creates
a ThreeDimPoint
folder and stores its name in variable f. The second gets its
length by calling function length in object f
. The third gets its length by calling
static function ThreeDimPoint.length1
.
ThreeDimPoint 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 filedrawer 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 without using an import statement, is used mainly as a filedrawer for
static functions and variables. It has no fields that you can change. It has
no instance methods. Everything in it is static. Below are some functions and what
they mean; later, we will tell you where to find their descriptions.
Math.abs(x)  absolute value of x  Math.abs(4) is 4 
Math.sqrt(x)  square root of x  Math.sqrt(25) is 5.0 
Math.min(b,c)  minimum of b and c  Math.min(5, 4) is 4 
Math.max(b,c)  maximum of b and c  Math.max(–6,4) is 4 
Math.floor(x)  largest integer that is not larger than x  Math.floor(–3.2) is –4.0 Math.floor(3.2) is 3.0 
Math.ceil(x)  smallest integer that is not smaller than x  Math.ceil(–3.2) is –3.0 Math.ceil(3.2) is 4.0 
Math.PI  best double approximation to pi, the ratio of the diameter of a circle to its circumference 
Fill in the following table by placing the calls in the Interactions pane and seeing what value DrJava gives:
call  value 
Math.min(7, 4)  
Math.min(Math.min(3,4), 5)  
Math.sqrt(5)  
Math.sqrt(5)  
Math.floor(3.7)  
Math.ceil(3.7) 

Math.ceil(3.7) 

Math.abs(3.7) 

Math.abs(3.7) 

Math.abs(3) 

Math.PI 