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Frame2.java
and the associated
Frame2App.java
is at the end of this module, or download the entire
archive When a user clicks on a button, an
ActionEvent
object is delivered to the button. In order for the button to perform an application specific task, we must register an
ActionEventListener
to the button and program this event listener to perform the task we want. The
ActionListener
class is an example of what is called a "
command " from the
Command Design Pattern . The code to add an
ActionListener
to a button looks like:
myButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {// code to perform a task goes here...
}});
The example
Frame2
(source code at end of the module) shows how to dynamically change the layout manager for a
JFrame
by calling its
setLayout(...)
method resulting in a dynamic re-arrangement of the GUI components in the
JFrame
.
Frame3.java
and the associated
IView2World.java
,
Frame3controller.java
and
Frame3App.java
is at the end of this module, or download the entire
archive
Frame3
illustrates how to decouple the view from the rest of the world by having the view communicate to an interface called "adapter". A controller object,
Frame3Controller
connects the view
Frame3
to the world by installing a concrete adapter into the view. The adapter is instantiated as anonymous inner object and has access to all of its outer object. The view does not and should not know anything about the world to which it is connected. This adds flexibility to the design.
In much of the current programming practice, the special value
null
is often used as a flag to represent a gamut of different and often disconnected concepts such as emptiness and falseness. This can result in a lot of confusion and complex control structures in the code. In our view,
null
should only be used to denote the non-existence of an object.
null
is not an object and has no "intelligence" (i.e. behavior) for other objects to interact with. Therefore, whenever we work with a
union of objects and discover that one or more of these objects may be referencing a
null
value, we almost always have to write code to distinguish
null
as a special case. To avoid this problem, we should think of adding a special object, called the
null object , to the
union with appropriate behavior that will allow us to treat all object references in a consistent and uniform way, devoid of special case consideration. This design pattern is called the null object pattern. We have used the null object pattern in one occasion: the
EmptyNode
to represent the empty state of a (mutable) list (
LRStruct
).
In
Frame3
, the view adapter,
_v2wAdapter
, is initialized to an (anonymous)
IView2World
object. It is there to guarantee that _
v2wAdapter
is always referencing a concrete
IView2World
object, and , since
setV2WAdapter(...)
only allows a non-null assignment, we can always call on _
v2WAdapter
to perform any method without checking for
null
.
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