Java Reference
In-Depth Information
The figure doesn't explain how V
ISITOR
works; the next section does that. The figure simply
shows some of the groundwork that lets you apply V
ISITOR
.
One aspect of allowing for later extensions is that the hierarchy classes make their attributes
accessible. In particular, the
getId()
method of the
MachineComponent
class and the
getComponents()
method of the
MachineComposite
class make it possible for visitors
to access these critical attri-butes of hierarchy objects. The mechanics of providing support
for V
ISITOR
also include adding an
accept()
operation to the hierarchy and adding a visitor
interface that developers can implement to create new extensions.
The
accept()
method in the
MachineComponent
class is abstract. Both subclasses
implement this method with exactly the same code:
public void accept(MachineVisitor v)
{
v.visit(this);
}
You might think that because this method is identical in the
Machine
and
MachineComposite
class, you could move the method up to the abstract
MachineComponent
class. However, a compiler will see a difference in these two
"identical" methods.
CHALLENGE 29.1
What difference will a Java compiler see between the
accept()
methods in the
Machine
and
MachineComposite
classes?
The
MachineVisitor
interface requires implementers to define methods for visiting
machines and machine composites:
package com.oozinoz.machine;
public interface MachineVisitor
{
void visit(Machine m);
void visit(MachineComposite mc);
}
The
accept()
methods in the
MachineComponent
, together with the visitor interface,
invite developers to provide new operations to the hierarchy.
Extending with Visitor
Suppose that you accept an assignment to help bring up a new Oozinoz factory in Dublin,
Ireland. The developers in Dublin have created an object model of the new factory's machine
composition and have made this model accessible as the static
dublin()
method of
the
OozinozFactory
class. To display this composite, the developers created
