Class 27: Swing (3): Drawing

Held: Wednesday, 3 November 2004

Summary: Today we consider how to draw things in Swing.

Related Pages:

• EBoard.
• Sample Code.
• API for java.awt.Graphics.
• Creating a GUI with JFC/Swing.

Notes:

• The exam is now due on Monday. Sorry for the confusion.
• Are there questions on the exam?

Overview:

• Graphics basics
• Normal steps
• Example

Drawing Basics

• Drawing and painting are complex UI tasks. Why?
  • There are many "events" that can require a redraw. You might uncover part of a window by moving another window. You might want to explicitly redraw part of a window. You may restart the window after a screen saver is turned off.
  • When you have nested objects, the order in which you redraw things is important.
  • For efficiency, you often only want to redraw the portion of the window that was affected.
  • ...
• Swing helps programmers handle this complexity by telling programmers that they should focus on the paintComponent(java.awt.Graphics) method.
  • Swing will call that method when appropriate.
  • To force that method to be called, you can also call repaint().
  • The actual drawing gets done with calls to the Graphics object.
• The Graphics object does some clever things with clipping.
  • If you draw stuff outside of the clipping area, it doesn't really get drawn.
  • You can look at the clipping area, so that you only redraw what needs to be drawn.

Typical Drawing Steps

• You can get the bounds of your component with this.getBounds().
  • You can also get them which this.getX(), this.getY(), this.getWidth(), this.getHeight()
  • Which is preferable?
• Once you have the bounds, you can call a variety of methods in Graphics.
• See the documentation for the useful ones.

An Example

• We'll try an example, but I don't guarantee that everything will work quite as expected.