Abstraction refers to the ability to make a class abstract
in OOP. An abstract class is one that cannot be instantiated. All other
functionality of the class still exists, and its fields, methods, and
constructors are all accessed in the same manner. You just cannot create an
instance of the abstract class.
If a class is abstract and cannot be instantiated, the class
does not have much use unless it is subclassed. This is typically how abstract
classes come about during the design phase. A parent class contains the common
functionality of a collection of child classes, but the parent class itself is
too abstract to be used on its own.
Abstract Class:
Use the abstract keyword to declare a class abstract. The
keyword appears in the class declaration somewhere before the class keyword.
/* File name : Employee.java */
public abstract class Employee
{
private String
name;
private String
address;
private int number;
public
Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name =
name;
this.address =
address;
this.number =
number;
}
public double
computePay()
{
System.out.println("Inside Employee computePay");
return 0.0;
}
public void
mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " "
+ this.address);
}
public String
toString()
{
return name +
" " + address + " " + number;
}
public String
getName()
{
return name;
}
public String
getAddress()
{
return address;
}
public void
setAddress(String newAddress)
{
address =
newAddress;
}
public int
getNumber()
{
return number;
}
}
Notice that nothing is different in this Employee class. The
class is now abstract, but it still has three fields, seven methods, and one
constructor.
Now if you would try as follows:
/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void
main(String [] args)
{
/* Following is
not allowed and would raise error */
Employee e = new
Employee("George W.", "Houston, TX", 43);
System.out.println("\n Call mailCheck using
Employee
reference--");
e.mailCheck();
}
}
When you would compile above class then you would get
following error:
Employee.java:46: Employee is abstract; cannot be
instantiated
Employee e = new
Employee("George W.", "Houston, TX", 43);
^
1 error1
Extending Abstract Class:
We can extend Employee class in normal way as follows:
/* File name : Salary.java */
public class Salary extends Employee
{
private double
salary; //Annual salary
public
Salary(String name, String address, int number, double
salary)
{
super(name,
address, number);
setSalary(salary);
}
public void
mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with
salary " + salary);
}
public double
getSalary()
{
return salary;
}
public void
setSalary(double newSalary)
{
if(newSalary
>= 0.0)
{
salary =
newSalary;
}
}
public double
computePay()
{
System.out.println("Computing salary pay for " + getName());
return
salary/52;
}
}
Here we cannot instantiate a new Employee, but if we
instantiate a new Salary object, the Salary object will inherit the three
fields and seven methods from Employee.
/* File name : AbstractDemo.java */
public class AbstractDemo
{
public static void
main(String [] args)
{
Salary s = new
Salary("Mohd Mohtashim", "Ambehta, UP",
3, 3600.00);
Salary e = new
Salary("John Adams", "Boston, MA",
2, 2400.00);
System.out.println("Call mailCheck using
Salary
reference --");
s.mailCheck();
System.out.println("\n Call mailCheck using
Employee
reference--");
e.mailCheck();
}
}
This would produce following result:
Constructing an Employee
Constructing an Employee
Call mailCheck using
Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0
Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.
Abstract Methods:
If you want a class to contain a particular method but you
want the actual implementation of that method to be determined by child
classes, you can declare the method in the parent class as abstract.
The abstract keyword is also used to declare a method as
abstract.An abstract methods consist of a method signature, but no method body.
Abstract method would have no definition, and its signature
is followed by a semicolon, not curly braces as follows:
public abstract class Employee
{
private String
name;
private String
address;
private int number;
public abstract
double computePay();
//Remainder of class
definition
}
Declaring a method as abstract has two results:
The class must also be declared abstract. If a class
contains an abstract method, the class must be abstract as well.
Any child class must either override the abstract method or
declare itself abstract.
A child class that inherits an abstract method must override
it. If they do not, they must be abstract,and any of their children must
override it.
Eventually, a descendant class has to implement the abstract
method; otherwise, you would have a hierarchy of abstract classes that cannot
be instantiated.
If Salary is extending Employee class then it is required to
implement computePay() method as follows:
/* File name : Salary.java */
public class Salary extends Employee
{
private double
salary; //Annual salary
public double
computePay()
{
System.out.println("Computing salary pay for " + getName());
return
salary/52;
}
//Remainder of
class definition
}
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