More-sophisticated behavior - Objects First with Java: A Practical Introduction Using BlueJ - page 187

or from another object. The Canvas class should not need any modification. It is probably best
to treat it as a library class: open the editor and switch to the documentation view. This displays
the class's interface with the javadoc documentation.
The Pen class provides a pen object that can be used to produce drawings on the canvas by
moving the pen across the screen. The pen itself is invisible, but it will draw a line when moved
on the canvas.
The DrawDemo class provides a few small examples of how to use a pen object to produce a
drawing on screen.
The best starting point for understanding and experimenting with this project is the DrawDemo
class.
Exercise 5.50 Create a DrawDemo object and experiment with its various methods. Read
the DrawDemo source code and describe (in writing) how each method works.
Exercise 5.51 Create a Pen object interactively using its default constructor (the construc-
tor without parameters). Experiment with its methods. While you do this, make sure to have
a window open showing you the documentation of the Pen class (either the editor window in
Documentation view or a web-browser window showing the project documentation). Refer to
the documentation to be certain what each method does.
Exercise 5.52 Interactively create an instance of class Canvas and try some of its meth-
ods. Again, refer to the class's documentation while you do this.
Some of the methods in the classes Pen and Canvas refer to parameters of type Color . This
type is defined in class Color in the java.awt package (thus, its fully qualified name is java.
awt.Color ). The Color class defines some color constants, which we can refer to as follows:
Color.RED
Using these constants requires the Color class to be imported in the using class.
Exercise 5.53 Find some uses of the color constants in the code of class DrawDemo .
Exercise 5.54 Write down four more color constants that are available in the Color class.
Refer to the class's documentation to find out what they are.
When calling methods interactively that expect parameters of the Color class, we have to refer
to the class slightly differently. Because the interactive dialog has no import statement (and thus
the Color class is not automatically known), we have to write the fully qualified class name to
refer to the class (Figure 5.5). This enables the Java system to find the class without using an
import statement.