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1. Writing Simple Java Classes (45 min)

First steps

1. Wiki MarkupIf you have not done it already, download the latest DrJava to your Desktop. \[Use this link\!\|http://drjava.org.}
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 {doublewidth;doubleheight; 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/lab07lab06 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 =newRectangle 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 {doublewidth;doubleheight;doublearea double width; double height; double area() {returnwidth return width * height; } // The onlynewlineonly 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.

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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

doubleareadouble area()

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

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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.

class Person {
  /**
   * Computes the number of months till the next birthday given the current month.
   */intnMonthTillBD(intcurrentMonth
  int nMonthTillBD(int currentMonth) {
    // todo
  } 
}

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(...).

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• 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.

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• 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).

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Now add the statement return 0; to the body of the method nMonthTillBD(...) and compile.

class Person {
   int _bMonth;

   /**
    * Computes the number of months till the next birthday.
    */intnMonthTillBD
   int nMonthTillBD(intcurrentMonth) {
      return0;// todo
   } 
}

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.

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Change the formula to

intnMonthTillBD(intcurrentMonth   int nMonthTillBD(int currentMonth) {
       return_bMonth - currentMonth;// todo
   }

Compile all and test again.

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Change the formula to

intnMonthTillBD(intcurrentMonth   int nMonthTillBD(int currentMonth) {
      return(_bMonth - currentMonth + 12) % 12;// todo
   }

Compile all and test again. Now everything should pass! You may now remove the TO DO comment from the code of nMonthTillBD.

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