Creating Windows - Beginning Java - page 817

You obtain the Toolkit object, theKit , by calling the getToolkit() method for the JFrame object,
aWindow . This object represents the environment on your computer, so it encapsulates all the properties
and capabilities of that environment as far as Java is concerned, including the screen resolution and size.
NOTE Youcan'tcreatea Toolkit objectdirectlybecause Toolkit isan abstract
class. There is only one Toolkit object in an application — the one that you get a
reference for when you call getToolkit() for a component.
The getScreenSize() method that is a member of the Toolkit object returns an object of type Dimen-
sion containing data members width and height . These hold the number of pixels for the width and
height of your primary display. You use these values to set the coordinates for the position of the window
and the width and height of the window through the setBounds() method. Of course, you can use this
approach to position the application window anywhere relative to the screen.
This is not the only way of centering a window. A java.awt.GraphicsEnvironment object contains
information about the graphics devices attached to a system, including the display — or displays in sys-
tems with more than one. You can obtain a reference to a GraphicsEnvironment object that encapsulates
information about the graphics devices on the local machine by calling the static getLocalGraphicsEn-
vironment() method in the GraphicsEnvironment class, like this:
GraphicsEnvironment localGE =
GraphicsEnvironment.getLocalGraphicsEnvironment();
This requires the following import:
import java.awt.GraphicsEnvironment;
You can now call this object's getCenterPoint() method to obtain a java.awt.Point object contain-
ing the coordinates of the center of the screen:
Point center = localGE.getCenterPoint();
You can now pass the coordinates of center to the setBounds() method:
aWindow.setBounds(center.x-wndSize.width/2, center.y-
wndSize.height/2,
wndSize.width,
wndSize.height);
An even simpler mechanism is to call the setLocationRelativeTo() method that a JFrame object in-
herits from the Window class. This method centers the window at the center of another component that
you pass as the argument. If the argument is null , the method centers the window relative to the primary
display.
You could try this with a variation on the original version of the example.