Java Reference
In-Depth Information
Common Programming Error 12.3
If you extend an adapter class and misspell the name of the method you're overriding, and
you do not declare the method with
@Override
, your method simply becomes another
method in the class. This is a logic error that is difficult to detect, since the program will
call the empty version of the method inherited from the adapter class.
A user of a Java application may be on a system with a one-, two- or three-button
mouse. Class
MouseEvent
inherits several methods from class
InputEvent
that can distin-
guish among mouse buttons on a multibutton mouse or can mimic a multibutton mouse
with a combined keystroke and mouse-button click. Figure 12.33 shows the
InputEvent
methods used to distinguish among mouse-button clicks. Java assumes that every mouse
contains a left mouse button. Thus, it's simple to test for a left-mouse-button click. How-
ever, users with a one- or two-button mouse must use a combination of keystrokes and
mouse-button clicks at the same time to simulate the missing buttons on the mouse. In
the case of a one- or two-button mouse, a Java application assumes that the center mouse
button is clicked if the user holds down the
Alt
key and clicks the left mouse button on a
two-button mouse or the only mouse button on a one-button mouse. In the case of a one-
button mouse, a Java application assumes that the right mouse button is clicked if the user
holds down the
Meta
key (sometimes called the
Command
key or the “Apple” key on a
Mac) and clicks the mouse button.
InputEvent
method
Description
isMetaDown()
Returns
true
when the user clicks the
right mouse button
on a
mouse with two or three buttons. To simulate a right-mouse-
button click on a one-button mouse, the user can hold down
the
Meta
key on the keyboard and click the mouse button.
isAltDown()
Returns
true
when the user clicks the
middle mouse button
on a
mouse with three buttons. To simulate a middle-mouse-button
click on a one- or two-button mouse, the user can press the
Alt
key and click the only or left mouse button, respectively.
Fig. 12.33
|
InputEvent
methods that help determine whether the right or center mouse
button was clicked.
Line 21 of Fig. 12.31 registers a
MouseListener
for the
MouseDetailsFrame
. The
event listener is an object of class
MouseClickHandler
, which extends
MouseAdapter
. This
enables us to declare only method
mouseClicked
(lines 28-45). This method first captures
the coordinates where the event occurred and stores them in local variables
xPos
and
yPos
(lines 31-32). Lines 34-35 create a
String
called
details
containing the number of con-
secutive mouse clicks, which is returned by
MouseEvent
method
getClickCount
at line 35.
Lines 37-42 use methods
isMetaDown
and
isAltDown
to determine which mouse button
the user clicked and append an appropriate
String
to
details
in each case. The resulting
String
is displayed in the
statusBar
. Class
MouseDetails
(Fig. 12.32) contains the
main
method that executes the application. Try clicking with each of your mouse's buttons
repeatedly to see the click count increment.
