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1. Writing Simple Java Classes (45 min)
First steps
If you have not done it already, download the latest DrJava to your Desktop. \[Use this link\!\|http://drjava.org.}Wiki Markup - Create a
lab06
directory within your Comp211 directory. - Set your DrJava "Language Level" to "Elementary". The Elementary language level simplifies the real Java syntax to facilitate the transition from Scheme to Java.
- 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:
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 withCode Block class Rectangle { double width; double height; }
double
define fields within the class, much like the fields of a structure in Scheme.
The worddouble
is a the name for the principal real number type in Java. So thewidth
andheight
fields of aRectangle
have typedouble
. TheRectangle
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 nameRectangle
. 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:
This code defines a variable calledCode Block Rectangle r = new Rectangle(5, 10);
r
of typeRectangle
and binds it to a new object belonging to classRectangle
. - There is not much we can do with the
Rectangel
class yet, but we can print the string representations ofRectangle
objets. In the Interactions pane, typer
and hit Enter. This action will print the string representation of object bound tocalc
. In the DrJava Elementary language level, the string representation of theRectangle
objectr
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 aRectangle
object. - Take a look at your
Comp211/lab07
directory and you will see several new files. The compiler automatically created a file calledRectangle.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 fileRectangle.dj0
using DrJava, DrJava generates a corresponding conventional Java source file calledRectangle.java
and invokes the Java compiler to translate this Java source file to bytecode.
- In the Definitions pane (upper right pane), type the following:
- Now we are ready to add to
Rectangle
a method to compute the area of a rectangle. Change the definition of theRectangle
class to read:Code Block class Rectangle { double width; double height; double area() { return width * height; } // The only new line }
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