Section 5.10. (Optional) GUI and Graphics Case Study: Drawing Rectangles and Ovals

Line 6 begins the class declaration for Shapes , which extends JPanel.Shapes contains one instance variable, choice , declared on line 8, that determines whether paintComponent should draw rectangles or ovals. The Shapes constructor at lines 1114 initializes choice with the value passed in parameter userChoice .

Method paintComponent (lines 1736) performs the actual drawing. Remember, the first statement in every paintComponent method should be a call to super.paintComponent , as in line 19. The for statement (lines 2135) loops 10 times to draw 10 shapes. The switch statement (Lines 2434) chooses between drawing rectangles and drawing ovals.

If choice is 1, then the program draws a rectangle. Lines 2728 call Graphics method drawRect . Method drawRect requires four arguments. The first two arguments represent the x- and y- coordinates of the upper-left corner of the rectangle. The next two represent the width and the height of the rectangle. 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, then the program performs a similar operation, drawing an oval instead of a rectangle. When drawing an oval, an imaginary rectangle called a bounding rectangle is created, and an oval that touches the midpoints of all four sides of the bounding rectangle is placed inside. Method drawOval (lines 3132) 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 the values passed to drawRect in lines 2728. Since the width and height of the bounding rectangle are identical in this example, lines 2728 draw a circle. You may modify the program to draw both rectangles and ovals to see how drawOval and drawRect are related .

Figure 5.27 is responsible for handling input from the user and creating a window to display the appropriate drawing based on the user's response. Line 3 imports JFrame to handle the display, and Line 4 imports JOptionPane to handle the input.

Figure 5.27. Obtaining user input and creating a JFrame to display Shapes .

1

// Fig. 5.27: ShapesTest.java

2

// Test application that displays class Shapes.

3

import

javax.swing.JFrame;
 4

import

javax.swing.JOptionPane;
 5
 6

public class

ShapesTest
 7  {
 8

public static void

main( String args[] )
 9     {
10

// obtain user's choice

11        String input = JOptionPane.showInputDialog(
12

"Enter 1 to draw rectangles\n"

+
13

"Enter 2 to draw ovals"

);
14
15

int

choice = Integer.parseInt( input );

// convert input to int

16
17

// create the panel with the user's input

18        Shapes panel =

new

Shapes( choice );
19
20        JFrame application =

new

JFrame();

// creates a new JFrame

21
22        application.setDefaultCloseOperation(

JFrame.EXIT_ON_CLOSE

);
23        application.add( panel );

// add the panel to the frame

24        application.setSize(

300

,

300

);

// set the desired size

25        application.setVisible(

true

);

// show the frame

26     }

// end main

27  }

// end class ShapesTest