Java Reference
In-Depth Information
Printing Pages
The class object that you pass to the
setPrintable()
method is responsible for all the detail of the
printing process. This class must implement the
Printable
interface, which implies defining the
print()
method in the same class. We can make the
SketchView
class implement the
Printable
interface like this:
import javax.swing.*; // For various Swing components
import java.awt.*;
import java.util.*; // For Observer, Observer, Iterator
import java.awt.event.*; // For events
import javax.swing.event.MouseInputAdapter;
import java.awt.geom.Line2D;
import java.awt.print.*;
class SketchView extends JComponent
implements Observer, Constants, ActionListener,
Printable {
public int print(Graphics g, // Graphics context for printing
PageFormat pageFormat, // The page format
int pageIndex) // Index number of current page
throws PrinterException {
// Code to do the printing
}
// Rest of the class definition as before...
}
The
import
statements here have been changed to import all the names in a package where this
significantly reduces the number of lines required. The
PrinterJob
object will call the
print()
method here for each page to be printed. This process starts when you call the
print()
method for the
PrinterJob
object that has overall control of the printing process. You can see that our
print()
method can throw an exception of type
PrinterException
. If you identify a problem within your
print()
method code, the way to signal the problem to the
PrinterJob
object is to throw an
exception of this type.
One point to keep in mind - you should not assume that the
PrinterJob
object will call the
print()
method for your
Printable
object just once per page. In general the
print()
method is likely to be
called several times for each page as the output destined for the printer is buffered within the Java
printing system and the buffer will not necessarily be large enough to hold a complete page. You don't
need to worry about this unduly. Just don't build any assumptions into your code about how often
print()
is called for a given page.
Of course, our
PrinterJob
object in the
actionPerformed()
method code has no way of knowing
how many pages need to be printed. When we call the
PrinterJob
object's
print()
method, it will
continue calling the
SketchView
object's
print()
method until the value returned indicates there are
no more pages to be printed. You can return one of two values from the
print()
method in the
Printable
interface -
PAGE
_
EXISTS
to indicate you have rendered a page, or
NO
_
SUCH
_
PAGE
if
there are no more pages to be printed. Both of these constants are defined in the
Printable
interface.
The
PrinterJob
object will continue calling
print()
until the
NO
_
SUCH
_
PAGE
value is returned.
