Java Reference
In-Depth Information
}
...
}
8.5 Abstract Classes and Methods
Letusreexaminethe
FictionalCharacter
class. It stores the name of the fictional
character. Maybe we do not want anyone to be able to directly create fictional characters.
In other words, let us suppose that the only reason the class exists is so that we can inherit
from it and create the subclasses
Superhero
and
Villain
. In this case, we can make the
class
abstract
.
An abstract class is similar to a regular class. It can contain variables, methods,
and even constructors. However, an object cannot be directly instantiated from it. An
object of an abstract class can only be instantiated from a subclass using the
super
keyword.
Here is a rewrite of the
FictionalCharacter
class using the
abstract
keyword.
public abstract class
FictionalCharacter
{
private
String name;
public
FictionalCharacter ()
{
}
public
FictionalCharacter(String name)
{
this
.name = name;
}
public
String getName ()
{
return
name ;
}
public void
setName( String name)
{
this
.name = name;
}
}
We will add one more method to the
FictionalCharacter
class:
computeStrength
.The
method computes the strength of a fictional character. We assume that we can compute the
strength of any fictional character. However, the
FictionalCharacter
class does not store
enough information to compute this value. In other words, we will have this information
in every subclass of the
FictionalCharacter
class, but not in the class itself. Therefore,
we want to force every class that inherits from the
FictionalCharacter
class to redefine
the
computeStrength
method and include proper implementation. The
computeStrength
method inside the
FictionalCharacter
class will not have a body and the method will be
defined as
abstract
.
