GUIs - Multimedia Introduction to Programming Using Java - page 460

and and the horizontal box b that contain vertical boxes leftColumn and
rightColumn .
Now look at the constructor of class MouseDemo . After the superconstructor
is called, the following happens:

Obtain the class
in Fig. 17.16
from lesson page
17-4.

1. The left column of boxes is created and added to Box b .
2. The right column of boxes is created and added to Box b .
3. Horizontal box b and button jb are added to the content pane.
4. A button listener is registered with button jb (we look at this later).
5. The JFrame is packed.
6. The JFrame is fixed so that it cannot be resized.

Registering a button listener
Button jb , the initialize button, needs a listener. Recall that to handle a click
of a mouse button, a method actionPerformed has to be written and an object
that contains this method has to be registered with the button, using the button's
method addActionListener .
Class MouseDemo has an inner class, which provides method actionPer-
formed . This class is written as an inner class so that the method can call meth-
ods clearDisk of the four squares to remove any pink disks on them —notice
the four calls in the body of method actionPerformed .
Step 4 in the list above registers an instance of this class with button jb ,
using the statement:
jb.addActionListener( new ButtonListener());
This concludes our discussion of class MouseDemo . Looked at as a whole, it
can seem quite daunting. But broken down into its constituent pieces, it is quite
logical and reasonable. We urge you to get a copy of these classes from the CD
and experiment with them to see that the clicking of buttons really works.

Listening to the Squares in class MouseDemo
We placed the listener for mouse events on a square in class Square . This
was easier because, in the Square constructor, we had to register only one mouse
listener (although that registration happens whenever an instance is created).
Registering the mouse listeners in class MouseDemo is also possible. It has
the disadvantage that a listener for each Square must be registered separately. It
has the advantage that only class MouseDemo knows the precise location of each
Square , and this information may be necessary when processing some mouse
events, like mouse drags and releases. We do not explore this issue further.

17.4.4 Listening to other components

Listening to other components besides buttons and mouse clicks is similar to lis-
tening to buttons. One registers a listener with that component. Therefore, we do
not discuss listening to other components in this paper text.