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1. If you have not done it already, download the latest DrJava to your Desktop. [Use this link!|http://drjava.org.}
2. Create a lab07 directory within your Comp211 directory.
3. Set your DrJava "Language Level" to "Elementary"
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.
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   • 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. This class is not very interesting because we cannot do much with it. However, it is syntactically correct and can
     be compiled.
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   • Save the file in your Comp211/lab07}}folder under the default name {{Rectangle. he file will be saved as a .dj0 file, which is a DrJava Elementary level file.
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   • Click the "Compile All" button in DrJava. The file should compile with no errors.
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   • 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.
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   • There isn't 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 which "Rectanlge(5.,10.)". Conventional Java would return something much more cryptic.
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   • 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 code expressed as "Java bytecode" ready execution on the Java Virtual Machine (JVM). DrJava automatically generated the file Rectangle.java which we will ignore for now.
   • Now we are ready to add to Rectangle a method to compute the area of a rectangle. Change the defintion of the Rectangle class to read:

     Code Block
     class Rectangle { double width; double height; double area() { return width * height; } // This line is the only change }

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