Chapter 8.1 : 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 the following error:

Employee.java:46: Employee is abstract; cannot be instantiated

      Employee e = new Employee("George W.", "Houston, TX", 43);

                   ^

1 error