Displaying a drawing - An Introduction to Graphical User Interfaces with Java Swing - page 48

not want to lose the original function of paintComponent , namely to draw the
panel itself, especially its background. Hence we call the original method using

!

the super command. This has to be the first command in our implementation
of paintComponent . Then our own graphic commands follow. Hence we have the
following generic structure:

public void paintComponent(Graphics g) {
super .paintComponent(g);
// Own graphics command
// which draws on the panel .

}

We are then sure that first the empty panel is drawn, and then our drawing is
displayed. In particular, any existing old drawing is erased before our drawing is
begun.

5.2

■

The graphics commands

The class Graphics supplies a number of commands for drawing. We present only
a few - further commands can be found in the Java documentation.

drawLine( int xstart, int ystart, int xend, int yend)
drawRect( int xleft, int ytop, int width, int height)
drawOval( int xleft, int ytop, int width, int height)
drawString(String text, int xleft, int ybottom)
fillRect( int xleft, int ytop, int width, int height)
fillOval( int xleft, int ytop, int width, int height)
setColor(Color col)
We explain the commands below; see also Figure 5.1. The coordinates are in pixels
in the Java coordinate system. The shapes are drawn in the order in which the
graphic commands are executed. If shapes overlap, the one drawn later covers
those drawn previously.

drawLine(xstart,ystart,xend,yend) draws a line segment between the points
with coordinates ( xstart , ystart ) and ( xend , yend ).

drawRect(xleft,ytop,width,height) draws the contour of an axes-aligned
rectangle. The upper left corner of the rectangle is at (xleft, ytop) .Itis w idth

pixels wide and height pixels tall.

drawOval(xleft,ytop,width,height) draws the contour of an ellipse. The axes
of the ellipse are parallel to the coordinate axes. The upper left corner of the
bounding rectangle of the ellipse is at (xleft, ytop) and is called the reference
point . The horizontal axis of the ellipse has length

w idth , the vertical one

height .