The purpose of this example is to show you how to
create your own EventListener interfaces and to help you
understand why it carries some real benefits.
The setup
Suppose we want to create this GUI:
... where changing "mass" or "acceleration" (either by hitting enter
or clicking the change button) causes "force" to be recalculated,
but changing "force" causes "acceleration" to be recalculated.
I'm sure you can imagine many ways to do this. What I want to
show you, is how we can do this really nicely if we follow the
approach of viewing each label/textfield/button combination as a
new GUI widget — one that acts like the builtin widgets.
Specifically, we will call this widget a
Field and it will have its own kind of event listener
FieldChangeListener, and the outside world call
register to be listeners for those events. What I want you to
do is:
- read the code closely and understand how it works, and
- think about how this embodies the ideas of encapsulation,
data hiding, and separation of interface from implementation,
as well as avoiding duplication of code, and
- notice how, in the process, we've created
this
Field class that we can imagine reusing in
other projects!