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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.
  2. 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; } 
...
  1. // 
...
  1. The 
...
  1. only 
...
  1. 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
{code}
* To "ask" the {{Rectangle}} object bound to {{r2}} to compute its area, type
{code}
r2.area();
{code}
in the Interactions pane.
* Experiment with using the object calc to compute the area of a few rectangles of different sizes.
* How do you instantiate another {{AreaCalc}} and bind it to the name {{robot}}?

*Computing the area of a right triangle*

Suppose we want to define a class representing a right triangle given the length of the two sides forming the right answer, which we call {{base}} and {{height}} .  What code should we write?  Write another class called {{RightTriangle}} with fields of type {{double}} called {{base}} and {{height}}, and a method 
{code}
double area()
{code}

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

{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 
...
 JUnit 
...
 testting 
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 and 
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 a 
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 more 
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 involved 
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 example 
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 of 
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 testing: 
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 http://junit.sourceforge.net/doc/testinfected/testing.htm.
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