Graphical User Interfaces - Building Java Programs: A Back to Basics Approach - page 869

14.6 Case Study: Implementing DrawingPanel
In previous chapters, we used the DrawingPanel class to create simple two-dimensional
graphics. In this final section, we'll analyze DrawingPanel to determine how it works
and implement a basic version of the class.
Initial Version without Events
When we created the DrawingPanel class, we had two major design goals. The first
was to provide an easy way for client code to access a Graphics object. The second
was to provide a simple interface so that the client did not need to know about calling
paintComponent , repaint , and other complex methods.
The graphics drawn by the client onto a DrawingPanel are actually drawn onto a
BufferedImage object stored inside it. The DrawingPanel declares a
BufferedImage as a field and initializes it in its constructor. When the client calls
getGraphics on the DrawingPanel , the program returns a reference to the buffered
image's graphics pen. To place the buffered image onto the screen, we set it as the
icon for a JLabel .
The start of the DrawingPanel class looks a lot like the code for the GUIs that we
developed in this chapter. It begins by declaring various graphical components as
fields. The fields are the overall window frame, the Graphics object for the onscreen
buffered image, and a panel to hold the image:
public class DrawingPanel {
private JFrame frame; // overall window frame
private JPanel panel; // drawing surface
private Graphics g; // drawing pen
...
}
The constructor of the DrawingPanel accepts two parameters that represent the
panel's width and height. It initializes the fields and constructs the BufferedImage
to serve as the persistent buffer where shapes and lines can be drawn. The class
also adds a few methods for the client, such as getGraphics , setBackground ,
and setVisible . The following lines of code form an initial version of its com-
plete code:
1 // A simple interface for drawing persistent images.
2 // Initial version without events.
3
4 import java.awt.*;
5 import java.awt.image.*;
6 import javax.swing.*;
7
8 public class DrawingPanel {