Java Reference
In-Depth Information
Recall that the method
center
was defined in the class
Figure
and that the
definition of the method
center
included an invocation of the method
draw
. If,
contrary to fact, Java used early binding, then when the code for the method
center
compiles, the invocation of the method
draw
would be bound to the currently available
definition of
draw
, which is the one given in the definition of
Figure
. If early binding
were used, the method
center
would behave exactly the same for all derived classes of
the class
Figure
as it does for objects created using the class
Figure
. But, fortunately,
Java uses late binding, so when
center
is invoked by an object of the class
Triangle
,
the invocation of
draw
(inside the method
center
) is not bound to a definition of
draw until the invocation actually takes place. At this point in time, the run-time
system knows the calling object is an instance of the class
Triangle
and so uses the
definition of
draw
given in the definition of the class
Triangle
(even if the invocation
of
draw
is inside the definition of the method
center
). So, the method
center
behaves differently for an object of the class
Triangle
than it would for an object that
is just a plain old
Figure
. With late binding, as in Java, things automatically work out
the way you normally want them to.
Note that in order for late binding to work, each object must somehow know which
definition of each method applies to that object. So, when an object is created in a
system using late binding, the description of the object must include (either directly or
indirectly) a description of where the appropriate definition of each method is located.
This additional overhead is the penalty you pay for the convenience of late binding.
VideoNote
Late Binding
Example
Late Binding
With
late binding
, the definition of a method is not bound to an invocation of the method
until run time—in fact, not until the time at which the particular invocation takes place. Java
uses late binding (for all methods except those discussed in the Pitfall section entitled “No
Late Binding for Static Methods”).
polymorphism
The terms
polymorphism
and
late binding
are essentially just different words for
the same concept. The term
polymorphism
refers to the processes of assigning multiple
meanings to the same method name using late binding.
Polymorphism
Polymorphism
refers to the ability to associate many meanings to one method name
by means of the late binding mechanism. Thus, polymorphism and late binding are really
the same topic.
