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...

Lab

...

06

...

-

...

Simple

...

Java

...

Classes

...

and

...

JUnit

...

Testing

1.

...

Writing

...

Simple

...

Java

...

Classes

...

(45

...

min

...

)

...

First steps

  1. If you have not done it already, download the latest DrJava to your Desktop. Use this link.
  2. Create a lab06 directory within your Comp211 directory.
  3. Set your DrJava "Language Level" to "Elementary".  The Elementary language level simplifies the real Java syntax to facilitate the transition from Scheme to Java.
  4. Create a class to calculate the areas of rectangles in DrJava by following the steps below. We will begin the process of writing a class that represents a rectangle given its width and height. We will write the class in several small steps. At each step, we will compile the code to ensure that everything is syntactically correct. By compiling the code at each small step, we hope to avoid seeing a large number of error messages that can be rather intimidating.
    • In the Definitions pane (upper right pane), type the following:
      Code Block
      
class Rectangle {
    double width;
    double height;
}
      Note the placement of the curly braces: the opening brace is on the same line as the class name, while the closing brace lines up with the beginning of the class definition on a new line. This is the de-facto Java coding style. The two lines beginning with double define fields within the class, much like the fields of a structure in Scheme.
      The word double is a the name for the principal real number type in Java. So the width and height fields of a Rectangle have type double. The Rectangle class is not very interesting because we cannot do much with it. However, it is syntactically correct and we can compile it.
      Aside: The equivalent Scheme code for class Rectangle is something like:
      Code Block
      
define-struct Rectangle(width height)
    • Save the file in your Comp211/lab06 folder under the default name Rectangle. The file will be saved as a .dj0 file, which is a DrJava Elementary level file.
    • Click the "Compile All" button in DrJava. The file should compile with no errors.
    • In the Interactions pane (bottom pane), type the following:
      Code Block
      
Rectangle r = new Rectangle(5, 10);
      This code defines a variable called r of type Rectangle and binds it to a new object belonging to class Rectangle.
    • There is not much we can do with the Rectangel class yet, but we can print the string representations of Rectangle objets. In the Interactions pane, type r and hit Enter. This action will print the string representation of object bound to calc. In the DrJava Elementary language level, the string representation of the Rectangle object r is {"Rectanlge(5.,10.)"}}. DrJava automatically strips the quotation marks off string representations when it prints them. Conventional Java would return a much more cryptic string representation for a Rectangle object.
    • Take a look at your Comp211/lab07 directory and you will see several new files. The compiler automatically created a file called Rectangle.class containing the compiled cJava bytecode ready execution on the Java Virtual Machine (JVM). The DrJava Interactions Pane is a convenient user interface to a Java Virtual Machine. When you refer to a Java class in the Interactions pane, DrJava automatically loads the byte code for that class. When you compile the file Rectangle.dj0 using DrJava, DrJava generates a corresponding conventional Java source file called Rectangle.java and invokes the Java compiler to translate this Java source file to bytecode.
  5. Now we are ready to add to Rectangle a method to compute the area of a rectangle. Change the definition of the Rectangle class to read:
    Code Block
    
class Rectangle {
    double width;
    double height;
    double area() { return width * height; } // The only new line
}
  • Compile All: the Rectangle file is automatically saved and recompiled.
  • To create an instance of class Rectangle, type the following in the Interactions pane:
    Code Block
    
Rectangle r = newRectangle(4,5);
    Note: the "Compile All" command resets the Interactions pane erasing all extant
    definitions of names bound to objects. So when we compiled our new definition for the Rectangle class,
    we destroyed our first binding of r in the Interactions pane.
  • To "ask" the Rectangle object bound to r2 to compute its area, type
    Code Block
    
r2.area();
    in the Interactions pane.
  • Experiment with the DrJava Interactions pane to create some new rectangles and computer their areas.

Defining Right Triangles and Circles

Suppose we want to define a class representing a right triangle given the length of the two sides forming the right angle, which we call base and height . What code should we write? Use the DrJava New command to
create a new document. In this document define a class called RightTriangle with fields of type double called base and height, and a method

Code Block

double area()

to compute the area of this right triangle. Save and compile this file. Interact with it in the Interactions pane.

  • Now suppose we are asked to define a class that can compute the area of a circle. How would you do it?
    Write a class called Circle that has one double field, radius, and a method to compute the area.

2. Unit Testing with JUnit (35 min)

Extreme Programming (XP) is a software development methodology that is "very hot" these days and is currently embraced by the software industry. One key element of XP, called "unit testing", is the idea of writing test code for functions (or procedures or object methods) before the functions (or procedures or object methods) are even written. (Don't ask me how to write test code for the test code themselves! The whole thing is "kinda" suspiciously recursive with no well-defined base cases.)

The test code becomes in effect the "documentation and specification" of the behavior of the functions (or procedures or object methods) in code form!

Each time a function (or procedure or object method) is modified, we require that it passes its existing test suite. Each time a test code for a function (or procedure or object method) is modified, the corresponding function (or procedure or object method) may have to be revised to satisfy the new specification. Tests and code must be developed in tandem.

JUnit (http:-www.junit.org) (www.junit.org) is an open source framework developed to support the above concept of unit testing for Java programs. This tutorial will lead you through a simple example of how to write unit test code in developing a (simple) Java program with only one class.

Step 0.

Run DrJava with Elementary Language level. We will do all development in DrJava since it has very nicely integrated JUnit with its development environment.

Step 1.

Suppose we want to model the notion of a smart person who knows his/her birthday and can compute the number of months till his/her next birthday given the current month. For our purpose, a month can simply be represented by an integer, which in Java is called int. We start by writing "stub" code for the class Person that is to represent our smart person.

Code Block

class Person {
  /**
 steps:*

# If you have not done it already, download the latest DrJava to your Desktop.  [Use this link!|http://drjava.org.}
# Create a {{lab07}} directory within your Comp211 directory.
# Set your DrJava "Language Level" to "Elementary"
# Create a class to calculate rectangle areas in DrJava by following the steps below. We will begin the process of writing a class that can compute the area of any rectangle given a width and a height.  We will write the class in several small steps.  At each step, we will compile the code to ensure that everything is syntactically correct.  By compiling the code at each small step, we hope to avoid seeing a large number of error messages that can be rather intimidating.
# *In the Definitions pane \(upper right pane\), type the following: 
{code}
class AreaCalc { 
}
{code}
Note the placement of the curly braces: the opening brace is on the same line as the class name, while the closing brace lines up with the beginning of the class definition on a new line.  This is the de-facto Java coding style.
This class is not very interesting because we cannot do much with it.  However, it is syntactically correct and can
be compiled.
* Save the file in your Comp211\lab07 folder under the default name AreaCalc.  he file will be saved as  a {.dj0} file, which is a DrJava Elementary level file. I
* Click the "Compile All" button in DrJava.  The file should compile with no errors.
* In the Interactions pane \(bottom pane\), type the following:
{code}
AreaCalc calc = new AreaCalc();
{code}
This code defines a variable called {{calc}} of type {{AreaCalc}} and binds it to a new object belonging to class
AreaCalc.
* There isn't much we can do with this class yet, but we can interact with it.   In the Interactions pane, type {{calc}} and hit Enter.  This action will print the string reprentation of object referred to by {{calc}}.  In the DrJava Elementary language level, the string representation of an {{AreaCalc}} object is {{"AreaCalc \(\)"}}. 
Conventional Java would return something much more cryptic.
* Take a look at your {{Comp211/lab07}} directory and you will see several new files.  
* The compiler automatically created a file called {{AreaCalc.class}} containing the compiled code expressed as "Java bytecode" ready execution on the Java Virtual Machine \(JVM\).
* DrJava's Elementary language level has automatically generated the "real" Java file, AreaCalc .java which has all the full-blown Java code in it that we don't want you worrying about at this stage.   Feel free to open the .java file in DrJava.    Close it when you are done so as not to confuse anything.

The purpose of this step is to give you some idea of what DrJava is doing "behind the scene".  You do not have to concern yourself with all the code that is automatically generated at all.

7. Now you are ready to add to AreaCalc a method to compute the area of a rectangle.  Edit the above AreaCalc class as follows:

{code}
class AreaCalc { 
    double rectArea(double width, double height) {
        return width * height;
    }
}
{code}

8. Compile All: the AreaCalc file is automatically saved.

9. To create a new instance of an AreaCalc , type the following in the Interactions pane:

{code}AreaCalc calc = new AreaCalc();{code}

To "ask" the AreaCalc object calc to compute the area of a 4 by 5 rectangle, type in

{code}calc.rectArea(4, 5){code}

11. Experiment with using the object calc to compute the area of a few rectangles of different sizes.

12. How do you instantiate another AreaCalc and name it robot?

*Computing the area of a right triangle*

Suppose you want to compute the area of a right triangle given its base and its height.  What code should you write?  You basically have two options.

13. Write another class called AreaCalc2 with a method to compute the area of a right triangle given as input parameters the base and the height.

14. Add a method to class AreaCalc to compute the area of a right triangle given as input parameters the base and the height.

Now, suppose now you are asked to compute the area of a circle.  What approach would you choose?  What are the pros and cons of each of the possible approaches?

*Creating classes with fields*

The following are classes with fields similar to class Entry discussed in lecture #18.

15. Write a class called Rectangle that has two double fields, width and height, and a method to compute the area.

16. Write a class called RightTriangle that has two double fields, base and height, and a method to compute the area.

17. Write a class called Circle that has one double field, radius, and a method to compute the area.

h2.  2. Unit Testing with JUnit \(35 min\)

Extreme Programming \(XP\) is a software development methodology that is "very hot" these days and is currently embraced by the software industry.  One key element of XP, called "unit testing", is the idea of writing test code for functions \(or procedures or object methods\) before the functions \(or procedures or object methods\) are even written.  \(Don't ask me how to write test code for the test code themselves!  The whole thing is "kinda" suspiciously recursive with no well-defined base cases.\) 
 
_The test code becomes in effect the "documentation and specification" of the behavior of the functions \(or procedures or object methods\) in code form!_

Each time a function \(or procedure or object method\) is modified, it must pass its existing test code.  Each time a test code for a function \(or procedure or object method\) is modified, the corresponding function \(or procedure or object method\) may have to be revised to satisfy the new specification.

[JUnit|http:-www.junit.org] \(www.junit.org\) is an open source framework developed to support the above concept of unit testing for Java programs.  This tutorial will lead you through a simple example of how to write unit test code in developing a \(simple\) Java program with only one class.

*Step 0.*

Run DrJava with Elementary Language level.  We will do all development in DrJava since it has very nicely integrated JUnit with its development environment. 

*Step 1.*

Suppose we want to model the notion of a smart person who knows his/her birthday and can compute the number of months till his/her next birthday given the current month.  For our purpose, a month can simply be represented by an integer, which in Java  is called int.  We start by writing "stub" code for the class Person  that is to represent our smart person.
{code}   
class Person {

    /**
    * Computes the number of months till the next birthday given the current month.
    */
    int nMonthTillBD(int currentMonth) {
        // to dotodo
    }

}
{code}

Notice

...

in

...

the

...

above

...

there

...

is

...

really

...

no

...

concrete

...

code.

...

As

...

a

...

matter

...

of

...

fact,

...

the

...

above

...

would

...

not

...

compile.

...

Now

...

we

...

must

...

abandon

...

everything

...

and

...

start

...

writing

...

test

...

code

...

for

...

nMonthTillBD

...

(...

...

).

...

Step

...

2.

...

DrJava

...

is

...

very

...

nice

...

to

...

you

...

and

...

will

...

create

...

a

...

test

...

stub

...

class

...

for

...

you

...

if

...

you

...

know

...

what

...

to

...

click:

...

  • Go

...

  • to

...

  • the

...

  • File

...

  • menu

...

  • and

...

  • select

...

  • New

...

  • Junit

...

  • Test

...

  • Case...

...

  • Create

...

  • a

...

  • stub

...

  • test

...

  • case

...

  • called

...

  • Test_Person.

...

  • DrJava

...

  • will

...

  • create

...

  • a

...

  • stub

...

  • class

...

  • that

...

  • looks

...

  • like

...

  • the

...

  • following

...

  • Code Block

...

  • 
/**

...

  •  * A JUnit

...

  • testcaseclass.
 * Every method starting with the word

...

  • "test"

...

  • will be called when running
 * the test with JUnit.

...

  •  */
class Test_

...

  • PersonextendsTestCase {

...

  •   /**

...

  •

...

  • * A test method.

...

  •    * (Replace

...

  • "X"

...

  • with a name describing the test.

...

  • You may write as

...

  • * many

...

  • "testSomething"

...

  • methods inthisclass as you wish, and each

...

  •    * one will be called when running JUnit

...

  • overthisclass.)

...

  •    */

...

  • void testX() {

...

  •   }
}

...

  •  At 
...
  •  this 
...
  •  point, 
...
  •  do 
...
  •  not 
...
  •  worry 
...
  •  about 
...
  •  what 
...
  •  "extends 
...
  •  TestCase" 
...
  •  means 
...
  •  in 
...
  •  the 
...
  •  above. 
...
  •  DrJava 
...
  •  is 
...
  •  re-using 
...
  •  the 
...
  •  class 
...
  •  TestCase 
...
  •  from 
...
  •  the 
...
  •  JUnit 
...
  •  framework 
...
  •  in 
...
  •  some 
...
  •  clever 
...
  •  way 
...
  •  without 
...
  •  you 
...
  •  having 
...
  •  to 
...
  •  deal 
...
  •  with 
...
  •  all 
...
  •  the 
...
  •  details 
...
  •  of 
...
  •  how 
...
  •  to 
...
  •  use 
...
  •  an 
...
  •  existing 
...
  •  class 
...
  •  from 
...
  •  some 
...
  •  other 
...
  •  files.
...
  • Compile 
...
  •  this 
...
  •  test 
...
  •  class 
...
  •  only 
...
  •  (by 
...
  •  clicking 
...
  •  on 
...
  •  Tools/Compile 
...
  •  current 
...
  •  document
...
  • ), 
...
  •  and 
...
  •  save 
...
  •  the 
...
  •  class 
...
  •  in 
...
  •  the 
...
  •  same 
...
  •  directory 
...
  •  as 
...
  •  Person. 
...
  •  The 
...
  •  Test_Person 
...
  •  class 
...
  •  should 
...
  •  compile, 
...
  •  though 
...
  •  the 
...
  •  Person 
...
  •  class 
...
  •  does 
...
  •  not.
...
  • Change 
...
  •  the 
...
  •  name 
...
  •  of 
...
  •  the 
...
  •  stub 
...
  •  method 
...
  •  testX
...
  • (
...
  • ) 
...
  •  in 
...
  •  the 
...
  •  above 
...
  •  to 
...
  •  test_nMonthTillBD
...
  • (
...
  • )and 
...
  •  add 
...
  •  code 
...
  •  to 
...
  •  make 
...
  •  it 
...
  •  look 
...
  •  like 
...
  •  the 
...
  •  code 
...
  •  below. 
...
  •  Code Block
...
  • 
class Test_
...
  • PersonextendsTestCase 
...
  • {
...
  • 

...
  •  void test_nMonthTillBD() {
...
  • 

...
  •    Person peter =
...
  • newPerson(9);
...
  • // a person born in September.

...
  •    
...
  • assertEquals("Calling nMonthTillBD(2).", 7, peter.nMonthTillBD(2));

...
  •    
...
  • assertEquals("Calling nMonthTillBD(9).", 0, peter.nMonthTillBD(9))
...
  • ;
   assertEquals("Calling nMonthTillBD(12).", 9, peter.nMonthTillBD(12));

...
  •  
...
  • }
...
  • 

...
  • }

     Note 
...
  •  that 
...
  •  in 
...
  •  the 
...
  •  code 
...
  •  for 
...
  •  test_nMonthTillBD
...
  • (
...
  • ), 
...
  •  we 
...
  •  have 
...
  •  decided 
...
  •  that 
...
  •  we 
...
  •  only 
...
  •  need 
...
  •  to 
...
  •  know 
...
  •  the 
...
  •  birth 
...
  •  month 
...
  •  of 
...
  •  a 
...
  •  person 
...
  •  in 
...
  •  order 
...
  •  to 
...
  •  compute 
...
  •  the 
...
  •  number 
...
  •  of 
...
  •  months 
...
  •  till 
...
  •  the 
...
  •  next 
...
  •  birth 
...
  •  day. 
...
  •  As 
...
  •  a 
...
  •  result, 
...
  •  we 
...
  •  instantiate 
...
  •  a 
...
  •  Person 
...
  •  object 
...
  •  by 
...
  •  passing 
...
  •  only 
...
  •  the 
...
  •  birth 
...
  •  month: 
...
  •  new 
...
  •  Person
...
  • (9
...
  • ).
...
Also 
...
 the 
...
 test 
...
 covers 
...
 three 
...
 cases:
...
  • the 
...
  •  current 
...
  •  month 
...
  •  is 
...
  •  less 
...
  •  than 
...
  •  the 
...
  •  birth 
...
  •  month
...
  • the 
...
  •  current 
...
  •  month 
...
  •  is 
...
  •  equal 
...
  •  to 
...
  •  the 
...
  •  birth 
...
  •  month
...
  • the 
...
  •  current 
...
  •  month 
...
  •  is 
...
  •  greater 
...
  •  than 
...
  •  the 
...
  •  birth 
...
  •  month.
...
The 
 {
 Code Block

assertEquals
method comes from the class TestCase and takes in three parameters:
  • the first parameter is a String
  • the second parameter is the expected result
  • the third parameter is the actual result of the computation you are testing.
Most of the time, your test code will call on the assertEquals method to test for equality between the result of the computation you are testing and the expected result.
When you compile the above code in DrJava, it won't compile. You will have to go in and fix the code for Person to make the test code compile.
Step 3.
Fix the code for Person until Test_Person compiles! (See the code below.)
First add a int field called _bMonth and compile. What happens?
Now add the statement return 0; to the body of the method nMonthTillBD(...) and compile.
 Code Block
}assertEquals{code} method comes from the class TestCase and takes in three parameters:
* the first parameter is a String
* the second parameter is the expected result
* the third parameter is the actual result of the computation you are testing. 

Most of the time, your test code will call on the assertEquals method to test for equality between the result of the computation you are testing and the expected result.

When you compile the above code in DrJava, it won't compile.  You will have to go in and fix the code for Person to make the test code compile.

*Step 3.*

Fix the code for Person  until Test_Person compiles!  \(See the code below.\)

First add a int field called _bMonth and compile.  What happens?

Now add the statement return 0; to the body of the method nMonthTillBD\(...\) and compile.
{code}
class Person {
    int _bMonth;

    /**
    * Computes the number of months till the next birthday.
    */
    int nMonthTillBD(int currentMonthintcurrentMonth) {
        return 0return0; // to dotodo
    }
}
{code}

*Step 
Step 4.
...
After 
...
 you 
...
 have 
...
 cleaned 
...
 up 
...
 your 
...
 code 
...
 for 
...
 Person 
...
 as 
...
 shown 
...
 in 
...
 the 
...
 above, 
...
 you 
...
 should 
...
 be 
...
 able 
...
 to 
...
 compile 
...
 Test_Person. 
...
 With 
...
 Test_Person 
...
 open, 
...
 click 
...
 on 
...
 the 
...
 Test 
...
 button 
...
 in 
...
 DrJava 
...
 tool 
...
 bar.
...
What 
...
 do 
...
 you 
...
 see? 
...
 Something 
...
 has 
...
 failed! 
...
 The 
...
 formula 
...
 for 
...
 the 
...
 number 
...
 of 
...
 months 
...
 till 
...
 the 
...
 next 
...
 birth 
...
 day 
...
 seems 
...
 to 
...
 be 
...
 the 
...
 culprit. 
...
 We 
...
 will 
...
 pretend 
...
 ignorance 
...
 and 
...
 fix 
...
 the 
...
 "bug" 
...
 in 
...
 two 
...
 steps.
...
Step 
...
 5.1
...
Change 
...
 the 
...
 formula 
...
 to

{
 Code Block
}
    int nMonthTillBD(int currentMonth) {
        return _bMonth - currentMonth; // to dotodo
    }
{code}

Compile 
...
 all 
...
 and 
...
 test 
...
 again.
...
Still 
...
 we 
...
 have 
...
 errors.
...
Step 
...
 5.2
...
Change 
...
 the 
...
 formula 
...
 to

{
 Code Block
}
    int nMonthTillBD(int currentMonth) {
        return (_bMonth - currentMonth + 12) % 12; // to dotodo
    }
{code}


Compile 
...
 all 
...
 and 
...
 test 
...
 again. 
...
 Now 
...
 everything 
...
 should 
...
 pass! 
...
 You 
...
 may 
...
 now 
...
 remove 
...
 the 
...
 TO 
...
 DO 
...
 comment 
...
 from 
...
 the 
...
 code 
...
 of 
...
 nMonthTillBD.
...
In 
...
 XP 
...
 programming, 
...
 only 
...
 after 
...
 a 
...
 method 
...
 has 
...
 passed 
...
 its 
...
 unit 
...
 test 
...
 that 
...
 you 
...
 are 
...
 allowed 
...
 to 
...
 proceed 
...
 to 
...
 another 
...
 one.
...
3. 
...
 Additional 
...
 Reading:
...
Lecture 
...
 notes 
...
 on 
...
 Object-Oriented 
...
 Programming 
...
 using 
...
 Java 
...
 by 
...
 Dung 
...
 X. 
...
 Nguyen 
...
 and 
...
 Stephen 
...
 B. 
...
 Wong: 
...
http://cnx.org/content/col10213/latest
...

 Here 
...
 is 
...
 another 
...
 link 
...
 for 
...
 a 
...
 more 
...
 detailed 
...
 discussion 
...
 of 
...
 JUnit 
...
 testing 
...
 uisng 
...
 DrJava: 
...
http://cnx.rice.edu/content/m11707/latest/
...

 The 
...
 following 
...
 link 
...
 contains 
...
 a 
...
 more 
...
 elaborate 
...
 discussion 
...
 on 
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 JUnit 
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 testting 
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 testing: 
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 http://junit.sourceforge.net/doc/testinfected/testing.htm.
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