Swing and threads - An Introduction to Graphical User Interfaces with Java Swing - page 245

analysed and displayed. One might experience the problem that the drawing is in-
completely displayed. We use a simple example to demonstrate this phenomenon.
We define a class Drawing as an abstract representation of the drawing. The
drawing shows three parallel lines. Initially the lines are horizontal. Method flip-
Lines will change the drawing to three vertical lines. Calling flipLines again will
make the lines horizontal again. To simulate a complex modification of a drawing,
the lines are flipped one after the other with a one second pause after each flip.
Class UpdatePanel is defined to display a drawing. To this end it has access to an
instance of class Drawing .Wedonot list these classes; they can be downloaded
from the topic's home page.
Class BadUpdateFrame is the main class of the application. It has a variable
drawing of class Drawing .Italso has an UpdatePanel to display this drawing.
Initially the lines are horizontal. Then the BadUpdateFrame calls the flipLines
method of the drawing, which flips the lines one after the other in roughly three
seconds. This cannot be seen in the display because no event is created to trigger
a repaint. Therefore we add a repaint command for the panel after the lines have
been flipped. Then flipLines and repaint are called again to flip the lines to
horizontal and display this. We only list the program BadUpdateFrame ; the actual
demonstration is placed in method badDemo .

File: its/GraphicsUpdate/BadUpdateFrame.java

package its.GraphicsUpdate; 1.

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import javax.swing.JFrame;

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import its.SimpleFrame.SimpleFrame;

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public class BadUpdateFrame extends SimpleFrame{

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private UpdatePanel uppane;

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private Drawing drawing;

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public BadUpdateFrame(){

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drawing = new Drawing();

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uppane = new UpdatePanel(drawing);

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getContentPane().add(uppane);

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pack();

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}

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19. public void badDemo(){

uppane.repaint(); // paint the horizontal lines 20.

drawing.flipLines(); // flip them to vertical 21.

uppane.repaint(); // paint the vertical lines 22.

drawing.flipLines(); // flip them back to horizontal 23.

uppane.repaint (); // paint the horizontal lines 24.