Revathy,
A virtual function in Java is a method that is declared but not implemented
(except trivially). This can be accomplished in three ways.
First, in a class, a method can be declared as "abstract" (a declaration
keyword), and not implemented. No braces are used to indicate the method
body in a declaration, and the semicolon is used to terminate the
declaration (it being a statement). For a class containing a function
declared as "abstract" to compile, the class itself must be declared as
"abstract". An abstract class cannot be instantiated; therefore, there is
no harm in its abstract methods not being implemented. Instead, a
("concrete" - not a keyword) class extending an abstract class - and
necessarily implementing all of its abstract methods - would be
instantiated.
Second, a method can be declared in an interface. All methods declared in
interfaces are implicitly abstract; really, an interface is just a construct
for packaging declarations without packaging implementations. Class can be
declared as implementing interfaces (more than one, in fact), in which case
the class must either be declared as abstract, or implement all of the
methods in the interface. (Note that while a class can be declared as
implementing more than one interface - and while interfaces can be declared
as extending many other interfaces - a class cannot be declared as extending
more than one class. This means that Java supports multiple interface
inheritance, but not multiple class inheritance. This approach provides a
straightforward, problem-free form of multiple inheritance - for example,
there would be no conflict in a class implementing more than one interface
which declared the same method; the class would implement only one version
of the method. In languages supporting multiple class inheritance, there
would be an issue as to which version of the method were inherited.)
Third, a method that has a void return value can be implemented with an
"empty" method body (indicated with succesive opening and closing braces).
Note that the method must have a void return value; otherwise it would fail
to compile, as it does not return a value (or do anything else) in its
(empty) method body. Such methods are also called "no-op" (for "no
operation") methods; they are actually invoked, but do not execute any
statements themselves on invocation. No-op methods are often used in
support classes for listener interfaces declared in the java.awt.event
package. Extending these support classes allows a developer to implement
only the methods with which he is concerned, and to neglect other methods
declared in the interface.
I would refer you to the O'Reilly book, "Java in a Nutshell", for clear and
accurate information on interfaces, abstract classes & abstract methods, as
well as other aspects of Java.
Best wishes for your life & work with Java!
Sincerely,
Carl
Carl R. Castro
[EMAIL PROTECTED]
(415) 351-CARL
-----Original Message-----
From: Revathy Ravi <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED] <[EMAIL PROTECTED]>
Date: Monday, March 22, 1999 11:04 PM
Subject: virtual functions
>Sir,
>How Virtual function concept is implemented in Java.
>
>
> Revathy R.
>
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