Control Statements: Part 2; Logical Operators - Java: How to Program Early Objects - page 188

Line 6 begins the class declaration for Shapes , which extends JPanel . I nstance vari-
able choice determines whether paintComponent should draw rectangles or ovals. The
Shapes constructor initializes choice with the value passed in parameter userChoice .
Method paintComponent performs the actual drawing. Remember, the first statement
in every paintComponent method should be a call to super.paintComponent , as in line
19. Lines 21-35 loop 10 times to draw 10 shapes. The nested switch statement (lines 24-
34) chooses between drawing rectangles and drawing ovals.
If choice is 1, then the program draws rectangles. Lines 27-28 call Graphics method
drawRect . Method drawRect requires four arguments. The first two represent the x- and
y- coordinates of the upper-left corner of the rectangle; the next two represent the rect-
angle's width and height. In this example, we start at a position 10 pixels down and 10
pixels right of the top-left corner, and every iteration of the loop moves the upper-left
corner another 10 pixels down and to the right. The width and the height of the rectangle
start at 50 pixels and increase by 10 pixels in each iteration.
If choice is 2, the program draws ovals. It creates an imaginary rectangle called a
bounding rectangle and places inside it an oval that touches the midpoints of all four sides.
Method drawOval (lines 31-32) requires the same four arguments as method drawRect .
The arguments specify the position and size of the bounding rectangle for the oval. The
values passed to drawOval in this example are exactly the same as those passed to drawRect
in lines 27-28. Since the width and height of the bounding rectangle are identical in this
example, lines 27-28 draw a circle . As an exercise, try modifying the program to draw both
rectangles and ovals to see how drawOval and drawRect are related.

Class ShapesTest
Figure 5.28 is responsible for handling input from the user and creating a window to dis-
play the appropriate drawing based on the user's response. Line 3 imports JFrame to han-
dle the display, and line 4 imports JOptionPane to handle the input. Lines 11-13 prompt
the user with an input dialog and store the user's response in variable input . Notice that
when displaying multiple lines of prompt text in a JOptionPane , you must use \n to begin
a new line of text, rather than %n . Line 15 uses Integer method parseInt to convert the
String entered by the user to an int and stores the result in variable choice . Line 18 cre-
ates a Shapes object and passes the user's choice to the constructor. Lines 20-25 perform
the standard operations that create and set up a window in this case study—create a frame ,
set it to exit the application when closed, add the drawing to the frame, set the frame size
and make it visible.

1 // Fig. 5.28: ShapesTest.java

2 // Obtaining user input and creating a JFrame to display Shapes.

3 import javax.swing.JFrame; //handle the display

4 import javax.swing.JOptionPane;

5
6 public class ShapesTest

7 {

8 public static void main(String[] args)

9 {

Fig. 5.28 | Obtaining user input and creating a JFrame to display Shapes . (Part 1 of 2.)