Java How to Program Fourth Edition - DCC

534 Object-Oriented Programming Chapter 9
Class MyShape in Fig. 9.45 must be abstract. The only data representing the coordinates
of the shapes in the hierarchy should be defined in class MyShape. Lines, rectangles and ovals can
all be drawn if you know two points in space. Lines require x1, y1, x2 and y2 coordinates. The
drawLine method of the Graphics class will connect the two points supplied with a line. If you
have the same four coordinate values (x1, y1, x2 and y2) for ovals and rectangles, you can calculate
the four arguments needed to draw them. Each requires an upper-left x-coordinate value (minimum
of the two x-coordinate values), an upper-left y-coordinate value (minimum of the two y coordinate
values), a width (difference between the two x-coordinate values; must be nonnegative) and a height
(difference between the two y-coordinate values; must be nonnegative). [Note: In Chapter 12, each
x,y pair will be captured by using mouse events from mouse interactions between the user and the
program’s background. These coordinates will be stored in an appropriate shape object as selected
by the user. As you begin the exercise, you will use random coordinate values as arguments to the
constructor.]
In addition to the data for the hierarchy, class MyShape should define at least the following
methods:
a) A constructor with no arguments that sets the coordinates to 0.
b) A constructor with arguments that sets the coordinates to the supplied values.
c) Set methods for each individual piece of data that allow the programmer to independently
set any piece of data for a shape in the hierarchy (e.g., if you have an instance variable
x1, you should have a method setX1).
d) Get methods for each individual piece of data that allow the programmer to independently
retrieve any piece of data for a shape in the hierarchy (e.g., if you have an instance variable
x1, you should have a method getX1).
e) The abstract method
public abstract void draw( Graphics g );
This method will be called from the program’s paint method to draw a shape onto the
screen.
The preceding methods are required. If you would like to provide more methods for flexibility,
please do so. However, be sure that any method you define in this class is a method that would be
used by all shapes in the hierarchy.
All data must be private to class MyShape in this exercise (this forces you to use proper
encapsulation of the data and provide proper set/get methods to manipulate the data). You are not
allowed to define new data that can be derived from existing information. As explained previously,
the upper-left x, upper-left y, width and height needed to draw an oval or rectangle can be calculated
if you already know two points in space. All subclasses of MyShape should provide two constructors
that mimic those provided by class MyShape.
Objects of the MyOval and MyRect classes should not calculate their upper-left x-coordinate,
upper-left y-coordinate, width and height until they are about to draw. Never modify the x1, y1, x2
and y2 coordinates of a MyOval or MyRect object to prepare to draw them. Instead, use the temporary
results of the calculations described above. This will help us enhance the program in Chapter 12
by allowing the user to select each shape’s coordinates with the mouse.
There should be no MyLine, MyOval or MyRect references in the program—only MyShape
references that refer to MyLine, MyOval and MyRect objects are allowed. The program should
keep an array of MyShape references containing all shapes. The program’s paint method should
walk through the array of MyShape references and draw every shape (i.e., call every shape’s draw
method).
Begin by defining class MyShape, class MyLine and an application to test your classes. The
application should have a MyShape instance variable that can refer to one MyLine object (created
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