Java Reference
In-Depth Information
Abstract Method
An
abstract method
serves as a placeholder for a method that will be fully defined in a
descendent class. An abstract method has a complete method heading with the addition of
the modifier
abstract
. It has no method body but does end with a semicolon in place of a
method body. An abstract method cannot be private.
EXAMPLES
public abstract double
getPay();
public abstract void
doSomething(
int
count);
abstract class
A class that has at least one abstract method is called an
abstract class
and, in Java,
must have the modifier
abstract
added to the class heading. The redefined, now
abstract, class
Employee
is shown in Display 8.7 .
An abstract class can have any number of abstract methods. In addition, it can have,
and typically does have, other regular (fully defined) methods. If a derived class of an
abstract class does not give full definitions to all the abstract methods, or if the derived
class adds an abstract method, then the derived class is also an abstract class and must
include the modifier
abstract
in its heading.
In contrast with the term
abstract class
, a class with no abstract methods is called a
concrete class
.
concrete class
Display 8.7
Employee
Class as an Abstract Class
(part 1 of 2)
1
/**
2 Class Invariant: All objects have a name string and hire date.
3
A name string of "No name" indicates no real name specified yet.
4
A hire date of January 1, 1000 indicates no real hire date specified
yet.
5
*/
6
public abstract class
Employee
7 {
8
The class
Date
is defined in Display 4.13, but the
details are not relevant to the current discussion
of abstract methods and classes. There is no
need to review the definition of the class
Date
.
private
String name;
9
private
Date hireDate;
10
public abstract double
getPay();
11
public
Employee()
12 {
13 name = "No name";
14
hireDate =
new
Date("January", 1, 1000);
//Just a placeholder.
15 }
16
public boolean
samePay(Employee other)
17 {
18
if
(other ==
null
)
(continued)
